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Intel Architecture Software Developer`s Manual

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1. LOCAL CONTROL DW ASSUME INITIALIZED REENTRANTPROC SAVE REGISTERS ALLOCATE STACK SPACE FOR FPU STATE IMAGE PUSH EBP MOV EBP ESP SUB ESP 108 ALLOCATES 108 BYTES 32 bit PROTECTED MODE SIZE SAVE STATE LOAD NEW CONTROL WORD RESTORE INTERRUPT ENABLE FLAG IF FNSAVE EBP 108 FLDCW LOCAL CONTROL PUSH OFFSET TO EFLAGS COPY OLD EFLAGS TO STACK TOP POPFD RESTORE IF TO VALUE BEFORE FPU EXCEPTION APPLICATION DEPENDENT EXCEPTION HANDLING CODE GOES HERE AN UNMASKED EXCEPTION GENERATED HERE WILL CAUSE THE EXCEPTION HANDLER TO BE REENTERED IF LOCAL STORAGE IS NEEDED IT MUST BE ALLOCATED ON THE STACK GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS intel CLEAR EXCEPTION FLAGS IN STATUS WORD WHICH IS IN MEMORY RESTORE MODIFIED STATE IMAGE MOV BYTE PTR EBP 104 OH FRSTOR EBP 108 DE ALLOCATE STACK SPACE RESTORE REGISTERS MOV ESP EBP POP EBP RETURN TO POINT OF INTERRUPTION IRETD REENTRANT ENDP D 3 5 Need for Storing State of IGNNE Circuit If Using FPU and SMM The recommended circuit see Figure D 1 for MS DOS compatibility FPU exception handling for Intel486 processors and beyond contains two flip flops When the FPU exception handler accesses I O port OFOH it clears the IRQ13 interrupt request output from F
2. 7 49 7 8 3 Denormal Operand Exception 7 50 7 8 4 Numeric Overflow Exception 7 50 7 8 5 Numeric Underflow Exception U 7 52 7 8 6 Inexact Result Precision Exception 7 53 7 8 7 Exception Priority e umm ea eae 7 53 7 9 FLOATING POINT EXCEPTION SYNCHRONIZATION 7 54 CHAPTER 8 PROGRAMMING WITH THE INTEL MMX TECHNOLOGY 8 1 OVERVIEW OF THE MMX TECHNOLOGY PROGRAMMING ENVIRONMENT 8 1 8 1 1 MMX Registers Lulu rr deret ed motore eels e A Ane yeh 8 2 8 1 2 MMX Data Types eee ede eee Rm x E 8 2 8 1 3 Single Instruction Multiple Data SIMD Execution Model 8 3 8 1 4 Memory Data 8 4 8 1 5 Data Formats for MMX Registers 8 4 8 2 MMX INSTRUCTION SET 8 4 8 2 1 Saturation Arithmetic and Wraparound Mode 8 5 8 2 2 Instruction Operands eee he ie Wt ea ee ee eae les 8 6 8 3 OVERVIEW OF THE MMX INSTRUCTION 8 6 8 3 1 Data Transfer Instructions 8 6 8 3 2 Arithmetic Instructions 0 2 lee HR eee 8 8
3. 1 5 1 4 3 Instruction Operands saina ah adna ia ala ene 1 6 1 4 4 Hexadecimal and Binary Numbers 1 6 1 4 5 Segmented Addressing lille 1 7 1 4 6 EXCODlIOnS ike fie IRA IE Bre nba BUR RC 1 7 1 5 RELATED LITERATURE neq DA eas be eee oe eee I d 1 7 CHAPTER 2 INTRODUCTION TO THE INTEL ARCHITECTURE 2 1 BRIEF HISTORY OF THE INTELARCHITECTURE 2 1 2 2 INCREASING INTEL ARCHITECTURE PERFORMANCE AND MOORE S LAW 2 4 2 3 BRIEF HISTORY OF THE INTEL ARGHITECTURE FLOATING POINT UNIT 2 5 2 4 INTRODUCTION TO THE PENTIUM PRO PROCESSOR S ADVANCED MICROARCHITECTURE OA UU a 2 5 2 5 DETAILED DESCRIPTION OF THE PENTIUM PRO PROCESSOR MICROARCHITECTURE 2 8 2 5 1 Memory Subsystem inti on suan ni dee FRE Y aos We ee RR z 2 8 2 5 2 The Fetch Decode 2 10 2 5 3 Instruction Pool Reorder 2 10 2 5 4 Dispatch Execute 2 11 2 5 5 Retirement Urt vote pi hohes epu EU XAR Mee ean 2 12 CHAPTER 3 BASIC EXECUTION ENVIRONMENT 3 1 MODES OF OPERATION Bite eae IRR DPUPLA VOPITIW Y Y va 3 1 3 2 OVERVIEW OF THE BASIC EXECUTION ENVIRONMENT 3 2 3 3 MEMORY ORGANIZATION a 3 2 3
4. NaN NaN Denormalized Finite Denormalized Finite oo Normalized Finite 0 0 A Normalized Finite HH Real Number and NaN Encodings For 32 Bit Floating Point Format SE F S E F 1 0 0 0 0 0 0 0 1 0 0002 e o rmalized 0 0 0000 1 1 254 Any Value Normalized Normalized 0 1 254 Any Value 1 255 0 oo 0 255 0 X 255 1 000 SNaN SNaN 255 1 0XX X 255 1 1XX QNaN QNaN 255 1 1XX NOTES 1 Sign bit ignored 2 Fractions must be non zero Figure 7 3 Real Numbers and NaNs 7 6 Intel FLOATING POINT UNIT When real numbers become very close to zero the normalized number format can no longer be used to represent the numbers This is because the range of the exponent is not large enough to compensate for shifting the binary point to the right to eliminate leading zeros When the biased exponent is zero smaller numbers can only be represented by making the integer bit and perhaps other leading bits of the significand zero The numbers in this range are called denormalized or tiny numbers The use of leading zeros with denormalized numbers allows smaller numbers to be represented However this denormalization causes a loss of preci sion the number of significant bits in the fraction is reduced by the leading zeros When performing normalized floating point computations an FPU normally operates on normalized numbers and produces normalized numbers
5. 7 41 Arithmetic and Non arithmetic Instructions 7 44 Invalid Arithmetic Operations and the Masked Responses to Them 7 49 Divide By Zero Conditions and the Masked Responses to Them 7 50 Masked Responses to Numeric OverfloW 7 51 Data Range Limits for 8 5 MMX Instruction Set 5 8 7 Effect of Prefixes on MMX Instructions 8 10 I O Instruction 9 7 EFLAGS A 1 EFLAGS Condition Codes B 1 Floating Point Exceptions 5 C 1 XV Intel 1 About This Manual CHAPTER 1 ABOUT THIS MANUAL The Intel Architecture Software Developer s Manual Volume 1 Basic Architecture Order Number 243190 is part of a three volume set that describes the architecture and programming environment of all Intel Architecture processors The other two volumes in this set are The Intel Architecture Software Developer s Manual Volume 2 Instruction Set Reference Order Number 243191 The Intel Architecture Software Developer s Manual Volume 3 System Programming Guide Order Number 243192 The Intel Architecture Software Developer
6. 6 19 6 3 2 1 Type Conversion Instructions 6 21 6 3 2 2 Simple Conversion 6 21 6 3 2 3 Move and cars nres aaa a r e 6 22 6 4 BINARY ARITHMETIC INSTRUCTIONS 6 22 6 4 1 Addition and Subtraction Instructions 6 22 6 4 2 Increment and Decrement Instructions 6 22 6 4 3 Comparison and Sign Change Instruction 6 23 6 4 4 Multiplication and Divide Instructions 6 23 6 5 DECIMAL ARITHMETIC INSTRUCTIONS 6 23 6 5 1 Packed BCD Adjustment Instructions 6 24 vii TABLE OF CONTENTS intel e PAGE 6 5 2 Unpacked BCD Adjustment Instructions 6 24 6 6 LOGICAL INSTRUCTIONS 0 0 0 0 cece rn 6 25 6 7 SHIFT AND ROTATE INSTRUCTIONS 6 25 6 7 1 Shift Instructl ns yn sete aed u cR ee 6 25 6 7 2 Double Shift 5 6 27 6 7 3 Rotate Instructions ils cree REGE doh Soe Pad M ee 6 27 6 8 BIT AND BYTE INSTRUCTIONS 0 0 0 e rn 6 29 6 8 1 Bit Test and Modify Instructions 6 29 6 8 2 Bit Scan Instr ctions cere es cae ntes rre
7. 7 38 FYL2XP1 instruction 7 38 INDEX 4 G General purpose registers 3 5 parameter passing 4 6 GS 3 7 3 9 H Hexadecimal numbers 1 6 ID identification flag EFLAGS register 3 13 IDIV instruction 6 23 IE invalid operation exception flag FPU status word 7 14 7 48 IEEE 754 and 854 standards for floating point arithmetic 7 1 IF interrupt enable flag EFLAGS register 3 13 4 11 9 5 Immediate operands 5 5 IMUL instruction 6 23 IN au t ooa qan duit pk 9 3 IN instruction 6 36 9 3 9 4 INC instruction a 6 22 Indefinite description 7 8 WML GSN o uos tet ua etse 7 27 packed BCD decimal 7 28 TO Als rr het dt pan coded e S RR Edu 7 26 Index operand addressing 5 8 5 9 Inexact result FPU 7 19 Inexact result precision exception P 7 53 Infinity control flag FPU control word 7 20 Infinity floating point format 7 8 INIT pisses ees Poor aa adr Boy 3 10 Input output see I O INS instruction 6 36 9 3 9 4 Instruction decoder 2 10 Instruction operand
8. 6 29 Conditional Jump Instructions 6 32 Information Provided by the CPUID Instruction 6 40 Real Number Notation 7 5 Denormalization 55 7 7 FPU Condition Code 1 7 14 Precision Control Field 7 17 Rounding Control Field RC 7 18 Rounding of Positive Numbers With Masked Overflow 7 19 Rounding of Negative Numbers With Masked Overflow 7 19 Length Precision and Range of FPU Data Types 7 25 Real Number NaN 7 26 Binary Integer 5 7 27 Packed Decimal Integer Encodings 7 29 Unsupported Extended Real Encodings 7 30 Data Transfer Instructions 7 31 Floating Point Conditional Move Instructions 7 32 Setting of FPU Condition Code Flags for Real Number Comparisons 7 35 Setting of EFLAGS Status Flags for Real Number Comparisons 7 35 TEST Instruction Constants for Conditional Branching 7 36 Rules for Generating
9. 3 9 Data types alignment of words doublewords and QUADWOFS eterne eer eedem 5 1 BCD integers 5 4 6 24 bit 8 vee cee ee ex US 5 4 DV exeo n ve eps 5 1 doubleword 5 1 FPU BCD decimal 7 28 FPU 7 27 FPU real number 7 25 fundamental data types 5 1 integers 5 2 6 22 6 23 packed bytes 8 2 packed doublewords 8 2 8 2 POIS cacas lay 5 4 quadword 5 1 8 2 SIMIO d e be uuu aa dite Da 5 4 unsigned integers 5 4 6 22 6 23 WONG att kwa kdy feta Piya iet 5 1 INDEX 2 DE denormal operand exception flag FPU status word 7 14 7 50 DEC 6 22 Decimal integers FPU descriptionof 7 28 encodings eects Ran 7 29 Denormal number see Denormalized finite number Denormal operand exception D 7 50 Denormalization process 7 7 Denormalized finite number 7 6 7 25 DF direction flag EFLAGS register 3 12 DH registers vs imei da emis 3 7 Dlrregister 3 7 Dispatch execute unit 2 11 Displacement
10. 3 12 6 37 CMOVcc instructions 6 1 6 16 INDEX 1 INDEX CMP instruction 6 23 CMPS instruction 3 12 6 35 CMPXCHG instruction 6 2 6 18 CMPXCHG8B instruction 6 2 6 18 10 1 Code segment 3 9 Compare compare and exchange 6 18 Integers soe em os enh Qe ead 6 23 real numbers FPU 7 35 SITIOS o EHE RED ENS 6 35 Compatibility SOMWAIE i u hee deni htl LIAE 1 5 Condition code flags FPU status word branchingon 7 15 conditional moves on 7 15 description of 7 12 interpretation 7 14 US nee Bet eS 7 35 Conditional moves FPU condition codes 7 15 Constants floating point descriptions of 7 33 Cosine FPU operation 7 37 CPUID instruction 6 2 6 40 10 1 10 3 CS register a s ear ma ees 3 7 3 9 CTlinstr ction ay uu tc 6 37 Current privilege level see CPL Currentstack 4 2 4 4 CWDiinstruction 6 22 CWDE instruction 6 21 CX 3 7 D DAA instruction 6 24 DAS instruction 6 24 Data pointer FPU 7 21 Data segment
11. 6 36 9 3 serialization 9 6 map 9 5 permission bit 9 5 I O ports addressing 9 1 foe uum vote RN ROPA 9 1 hardware 9 1 memory mapped 9 2 Ordering iii erede perde 9 6 protected mode O 9 4 INDEX I O privilege level see IOPL I O sensitive instructions 9 4 J JD Trato tt edite sto eee 7 3 Jccinstructions 3 12 3 14 6 32 JMP instruction 3 14 6 30 6 39 L L1 level1 cache 2 6 2 8 L2 level 2 cache 2 6 2 8 instruction 3 10 6 37 Last instruction opcode FPU 7 21 LDS instruction 0 000 6 39 LEA iinstruction 6 39 LEAVE instruction 4 16 4 21 6 36 LES iinstruction 6 39 LGS instruction 6 39 Linear address 3 3 Linear address space defined ssir sq sa ua IEEE 3 3 maximum size 3 3 LOCK signal ul repete bare 6 17 LODS instruction 3 12 6 35 Log epsilon FPU operation 7 38 Log base 2 FPU computation 7 38 Logical address 3 8 LOOP instructions 6 33 LOOPcc ins
12. 7 14 7 51 OF overflow flag EFLAGS register 3 11 4 15 Offset operand addressing 5 7 Operand FPU instructions 7 31 instruction 1 6 Operand addressing modes 5 5 Operand sizes 3 5 Operand size attribute code segment 3 14 description 3 14 Operating modes 3 4 ORCIDSIUCIHOD Ie eere tape EE e 6 25 Ordering VO 0 2 cee ee 9 6 OUT instruction 6 36 9 3 9 4 OUTS instruction 6 36 9 3 9 4 Overflow exception HOF 4 15 Overflow FPU exception see Numeric overflow exception Overflow FPU stack 7 47 7 48 P Packed BCD integers 5 4 Packed bytes datatype 8 2 Packed decimal indefinite 7 28 Packed doublewords data type 8 2 Packed words data 8 2 Parameter passing argument list 4 7 FPU register 7 11 on procedure stack 4 6 intel on the procedure stack 4 6 through general purpose registers 4 6 PC precision field FPU control word 7 17 PE inexact result exception flag FPU status word 7 13 7 14 7 19 7 53 Pentium Pro processor microarchitecture
13. 8 2 detecting MMX technology with CPUID instructions 2o bs do wee eme ceed 8 10 detecting with CPUID instruction 8 11 effect of instruction prefixes on MMX INSTrUCONS htc 8 10 EMMS instruction 8 9 exception handling in MMX code 8 15 instruction operands 8 6 instruction 8 4 8 6 interfacing with MMX code 8 12 introductionto 8 1 logical instructions 8 9 memory data formats 8 4 mixing MMX and floating point instr ctionS oer mtu 8 13 programming environment overview 8 1 register data formats 8 4 register mapping 8 15 IegisterS ior mee RARE PE e 8 2 saturation 8 5 shift instructions 8 9 SIMD execution environment 8 3 support for determing 10 2 using MMX code in a multitasking operating system environment 8 14 using the EMMS instruction 8 11 wraparound mode 8 5 Modes operating 3 4 MOV instruction 6 16 6 38 instruction 8 6 MOVQiinstruction 8 6 MOVS instruction 3 12 6 35 MOVSX instruction 6 22 MOVZX instruction
14. Figure 7 12 Contents of FPU Opcode Registers N o FPU Opcode Register The manner in which this information is stored in memory depends on the operating mode of the processor protected mode or real address mode and on the operand size attribute in effect 32 bit or 16 bit See Figures 7 13 through 7 16 In virtual 8086 mode or SMM the real address mode formats shown in Figure 7 16 is used See Using the FPU in SMM in Chapter 11 of the Intel Architecture Software Developer s Manual Volume 3 for special considerations for using the FPU while in SMM 32 Bit Protected Mode Format 31 1615 0 Control Word 0 Status Word 4 Tag Word 8 FPU Instruction Pointer Offset 12 0000 Opcode 10 00 FPU Instruction Pointer Selector 16 FPU Operand Pointer Offset 20 FPU Operand Pointer Selector 24 Reserved Figure 7 13 Protected Mode FPU State Image in Memory 32 Bit Format 7 22 intel FLOATING POINT UNIT 32 Bit Real Address Mode Format 31 1615 0 Control Word 0 Status Word 4 Tag Word 8 FPU Instruction Pointer 15 00 12 0000 FPU Instruction Pointer 31 16 0 Opcode 10 00 16 Reserved FPU Operand Pointer 15 00 20 0000 FPU Operand Pointer 31 16 000000000000 24 Reserved Figure 7 14 Real Mode FPU State Image in Memory 32 Bit Forma
15. POPF R R R R R R R R R R PUSH PUSHA PUSHF RCL RCR 1 M TM RCL RCR count TM A 3 EFLAGS CROSS REFERENCE Table A 1 EFLAGS Cross Reference Contd Instruction OF SF ZF AF PF CF TF DF NT RF RDMSR RDPMC RDTSC REP REPE REPNE RET ROL ROR 1 ROL ROR count RSM SAHF SAL SAR SHL SHR 1 SAL SAR SHL SHR count SBB lt m lt lt Z SCAS SETcc SGDT SIDT SLDT SMSW SHLD SHRD lt lt Z mz Z lt A Z lt E STC STD STI STOS STR SUB TEST UD2 VERR VERRW WAIT WBINVD WRMSR XADD XCHG XLAT XOR A 4 Intel EFLAGS Condition Codes APPENDIX B EFLAGS CONDITION CODES Table B 1 gives all the condition codes that can be tested for by the CMOVcc FCMOVcc Jcc and SETcc instructions The condition codes refer to the setting of one or more status flags CF OF SE ZF and PF in the EFLAGS register The Mnemonic column gives the suffix cc add ed to the instruction to specific the test condition The Condition Tested For column describes the condition specified in the Status Flags Setting column The Instruction Subcode column gives the opcode suffix added to the main opcode to specify a test condition Table B 1 EFLAGS Condition Codes Instruction Mnemonic
16. 6 1 6 1 1 New Instructions Introduced with the MMX Technology 6 1 6 1 2 New Instructions in the Pentium Pro Processor 6 1 6 1 3 New Instructions in the Pentium Processor 6 2 6 1 4 New Instructions in the Intel486 Processor 6 2 6 2 INSTRUCTION SET LIST 6 2 6 2 1 Integer Instr ctloris 2 7 Encre ERI bee Cee eae rer ey RE pue TR URS 6 3 6 2 1 1 Data Transfer Instructions 6 3 6 2 1 2 Binary Arithmetic 1 5 6 4 6 2 1 3 Decimal Arithmetic 6 4 6 2 1 4 Logic Instructions lul Rm ie Rr eee 6 5 6 2 1 5 Shift and Rotate Instructions 6 5 6 2 1 6 Bit and Byte InstructionSs 6 5 6 2 1 7 Control Transfer Instructions 6 6 6 2 1 8 String lnstr ctions cc ca Rl RE IR ER Sc ee 6 7 6 2 1 9 Flag Control 5 5 6 8 6 2 1 10 Segment Register Instructions 6 8 6 2 1 11 Miscellaneous Instructions 6 9 6 2 2 MMX Technology Instructions 6 9 6 2 2 1 MMX Data Transfer Instruc
17. 7 14 ete rrollo 7 50 PE flag emo Cette oh E E ERA 7 18 TOP Hielo o Ca 7 9 FPU tag sa 7 20 Fraction floating point number 7 3 FRNDINT instruction 7 33 FRSTOR instruction 7 15 7 20 7 24 7 40 FS 3 7 3 9 FSAVE FNSAVE instructions 7 12 7 15 7 20 7 21 7 40 FSCALE instruction 7 39 FSIN instruction 7 13 7 37 FSINCOS instruction 7 13 7 37 FSQRT instruction 7 33 FST instruction ai eme eter jara 7 81 FSTCW FNSTOW instructions 7 16 7 40 FSTENV FNSTENV instructions 7 12 7 20 7 21 7 40 FSTP instruction 7 32 FSTSW FNSTSW instructions 7 12 7 40 FSUB instruction 7 33 FSUBP instruction 7 33 FSUBR instruction 7 33 FSUBRP instruction 7 33 instruction 7 15 7 34 FUCOM 7 34 FUCOMI instruction 6 1 7 16 7 34 FUCOMIP instruction 6 1 7 16 7 34 FUCOMP instruction 7 34 FUCOMPP instruction 7 15 7 34 FXAM instruction 7 18 7 34 FXCH instruction 7 32 FXTRACT instruction 7 33 FYL2X instruction
18. 6 22 MS DOS compatibility mode D 1 MTRRs memory type range registers presenceof 10 1 MUL instruction 6 23 N NaN description of 7 5 7 8 encoding of 7 6 7 26 Operating on 7 41 SNaNs vs QNaNs 7 8 Near call description of 4 4 4 5 INDEX 6 Near pointer description of 5 4 Near return Operation esse cee I e 4 5 Near return operation 4 6 NEG instruction 6 23 Non arithmetic instructions FPU 7 43 Non number encodings FPU 7 5 Non waiting instructions 7 40 7 45 NOP instruction 6 40 Normalized finite number 7 4 7 6 NOT instruction 6 25 Notation bit and byte order 1 5 exceptions 1 7 hexadecimal and binary numbers 1 6 instruction operands 1 6 reserved DIS ce 1 5 segmented addressing 1 7 Notational conventions 1 5 NT nested task flag EFLAGS register 3 13 Numeric overflow exception O 7 13 7 50 Numeric underflow exception U 7 13 7 52 OE numeric overflow exception flag FPU status word
19. 7 14 Exception handler 4 10 Exception priority FPU exceptions 7 53 Exception flag masks FPU control word 7 17 Exceptions intel BOUND range exceeded BR 4 15 description of 4 10 implicit call to handler 4 1 in real address mode 4 15 IOLatl On c oo ioo natem Grace adeo epe ens 1 7 overflow exception OF 4 15 4 12 Veclot 4 11 Exponent floating point number 7 3 Exponential FPU operation 7 38 Extended real encodings unsupported 7 28 floating point format 7 25 F F2XM1 instruction 7 38 FABS instruction 7 33 FADD instruction 7 33 FADDP instruction 7 33 Far call description of 4 4 4 6 Far pointer 16 bit addressing 3 5 32 bit addressing 3 5 description of 3 3 5 4 FBSTP instruction 7 32 instruction 7 33 FCLEX FNCLEX instructions 7 15 FCMOVcc instructions 6 1 7 16 7 32 FCOM innstruction 7 15 7 34 FCOMI instruction 6 1 7 16 7 34 FCOMI
20. OF bit 11 Overflow flag Set if the integer result is too large a positive number or too small a negative number excluding the sign bit to fit in the destina tion operand cleared otherwise This flag indicates an overflow condition for signed integer two s complement arithmetic Of these status flags only the CF flag can be modified directly using the STC CLC and CMC instructions Also the bit instructions BT BTS BTR and BTC copy a specified bit into the CF flag The status flags allow a single arithmetic operation to produce results for three different data types unsigned integers signed integers and BCD integers If the result of an arithmetic oper ation is treated as an unsigned integer the CF flag indicates an out of range condition carry or a borrow if treated as a signed integer two s complement number the OF flag indicates a carry or borrow and if treated as a BCD digit the AF flag indicates a carry or borrow The SF flag indicates the sign of a signed integer The ZF flag indicates either a signed or an unsigned integer zero When performing multiple precision arithmetic on integers the CF flag is used in conjunction with the add with carry ADC and subtract with borrow SBB instructions to propagate a carry or borrow from one computation to the next The condition instructions Jcc jump on condition code cc SETcc byte set on condition code cc LOOPcc and CMOV cc conditional move use one or mo
21. 2 5 2 8 PF parity flag EFLAGS register 3 11 Physical address 3 2 Physical 3 2 Pi description of FPU constant 7 37 POS eec e ens 5 4 POP instruction 4 1 4 3 6 20 6 38 POPA instruction 4 7 6 20 POPF instruction 3 10 4 7 6 37 9 5 POPFD instruction 3 10 4 7 6 37 Privilege levels descriptionof e 4 8 inter privilege level calls 4 7 stack switching 4 11 Procedure calls description of 4 4 ia mam eee oh a aaa es 4 4 for block structured languages 4 16 inter privilege level call 4 9 TA A 4 3 near woe geek ees 4 4 OVOIV IOW Soak 0 4 1 procedure stack 4 1 return instruction pointer EIP register 4 4 saving procedure state information 4 7 stack switching 4 8 to exception handler procedure 4 11 to exception 5 4 14 to interrupt handler procedure 4 11 to interrupttask 4 14 to other privilege levelsS 4 7 TYPOS Oiii o S ERN I eens hed 4 1 Procedure stack address size attribute 4 3 alignment of stack pointer 4 3 currentstack 4 2 4 4 descr
22. Figure 5 5 Offset or Effective Address Computation The uses of general purpose registers as base or index components are restricted in the following manner The ESP register cannot be used as an index register When the ESP or EBP register is used as the base the SS segment is the default segment In all other cases the DS segment is the default segment The base index and displacement components can be used in any combination and any of these components can be null A scale factor may be used only when an index also is used Each possible combination is useful for data structures commonly used by programmers in high level languages and assembly language The following addressing modes suggest uses for common combinations of address components Displacement A displacement alone represents a direct uncomputed offset to the operand Because the displacement is encoded in the instruction this form of an address is sometimes called an abso lute or static address It is commonly used to access a statically allocated scalar operand 5 8 intel e DATA TYPES AND ADDRESSING MODES Base A base alone represents an indirect offset to the operand Since the value in the base register can change it can be used for dynamic storage of variables and data structures Base Displacement A base register and a displacement can be used together for two distinct purposes As an index into an array when the element size is not
23. intel e INTRODUCTION TO THE INTEL ARCHITECTURE 2 3 BRIEF HISTORY OF THE INTEL ARCHITECTURE FLOATING POINT UNIT The Intel Architecture Floating Point Units FPUs before the Intel486 lack the added efficiency of integration into the CPU but have provided the option of greatly enhanced floating point performance since the beginning of the family Since the earlier FPUs were on separate chips they were often referred to as numeric processor extensions NPXs or math coprocessors MCPs With each succeeding generation Intel has made significant increases in the power and flexibility of the FPU and yet has maintained complete upward compatibility The Pentium Pro Processor offers compatibility with object code for 8087 Intel 287 Intel 387 DX Intel 387 SX and Intel 487 DX math coprocessors and the Intel486 DX and Pentium processors The 8087 numeric processor extension NPX was designed for use in 8086 family systems The 8086 was the first microprocessor family to partition the processing unit to permit high perfor mance numeric capabilities The 8087 NPX for this processor family implemented a complete numeric processing environment in compliance with an early proposal for IEEE Standard 754 for Binary Floating Point Arithmetic With the Intel 287 coprocessor NPX high speed numeric computations were extended to 80286 high performance multitasking and multi user systems Multiple tasks using the numeric processor extension were afforded
24. 5 1 Bytes Words Doublewords and Quadwords in 5 2 Numeric Pointer and Bit Field Data 5 8 Memory Operand lt 5 6 Offset or Effective Address Computation 5 8 Operation of the PUSH Instruction 6 19 Operation of the PUSHA Instruction 6 20 Operation of the POP Instruction 6 20 Operation of the POPA Instruction 6 21 Sigm Extensi n ee eee nde eevee re SERO E RR 6 21 SHL SAL Instruction 6 25 SHR Instruction Operation 6 26 SAR Instruction Operation 6 26 SHLD and SHRD Instruction Operations 6 27 ROL ROR RCL RCR Instruction Operations 6 28 Flags Affected by the PUSHF POPF PUSHFD and POPFD instructions 6 38 Binary Real Number System 7 3 Binary Floating Point Format 7 4 Real Numbers 7 6 Relationship Between the Integer Unit and the FPU 7 9 FPU Execution 7 10 FPU
25. ADD ADC SUB SBB IMUL MUL IDIV DIV INC DEC NEG CMP Integer add Add with carry Subtract Subtract with borrow Signed multiply Unsigned multiply Signed divide Unsigned divide Increment Decrement Negate Compare 6 2 1 3 DECIMAL ARITHMETIC DAA DAS 6 4 Decimal adjust after addition Decimal adjust after subtraction Intel AAA AAS AAM AAD INSTRUCTION SET SUMMARY ASCII adjust after addition ASCII adjust after subtraction ASCII adjust after multiplication ASCII adjust before division 6 2 1 4 LOGIC INSTRUCTIONS AND OR XOR NOT And Or Exclusive or Not 6 2 1 5 SHIFT AND ROTATE INSTRUCTIONS SAR SHR SAL SHL SHRD SHLD ROR ROL RCR RCL Shift arithmetic right Shift logical right Shift arithmetic left Shift logical left Shift right double Shift left double Rotate right Rotate left Rotate through carry right Rotate through carry left 6 2 1 6 BIT AND BYTE INSTRUCTIONS BT BTS BTR BTC BSF BSR SETE SETZ SETNE SETNZ Bit test Bit test and set Bit test and reset Bit test and complement Bit scan forward Bit scan reverse Set byte if equal Set byte if zero Set byte if not equal Set byte if not zero 6 5 INSTRUCTION SET SUMMARY intel e SETA SETNBE SETAE SETNB SETNC SETB SETNAE SETC SETBE SETNA SETG SETNLE SETGE SETNL SETL SETNGE SETLE SETNG SETS SETNS SETO SETNO SETPE SETP SETPO SETNP TEST Set byte if above Set byte if not
26. 8 5 6 Exception Handling in MMX Code MMX instructions generate the same type of memory access exceptions as other Intel Architec ture instructions Some examples are page fault segment not present and limit violations Existing exception handlers can handle these types of exceptions They do not have to be modi fied Unless there is a pending floating point exception MMX instructions do not generate numeric exceptions Therefore there is no need to modify existing exception handlers or add new ones If a floating point exception is pending the subsequent MMX instruction generates a numeric error exception interrupt 16 and or FERR The MMX instruction resumes execution upon return from the exception handler 8 5 7 Register Mapping The MMX registers and their tags are mapped to physical locations of the floating point regis ters and their tags Register aliasing and mapping is described in more detail in Chapter 10 MMX Technology System Programming Model in the Intel Architecture Software Developer s Manual Volume 3 8 15 Intel Input Output CHAPTER 9 INPUT OUTPUT In addition to transferring data to and from external memory Intel Architecture processors can also transfer data to and from input output ports I O ports I O ports are created in system hard ware by circuity that decodes the control data and address pins on the processor These I O ports are then configured to communicate with peripheral dev
27. Comparison and Classification Instruc tions RDPMC Read performance monitoring counters see RDPMC Read Performance Monitoring Counters in Chapter 3 of the Intel Architecture Software Developer s Manual 6 1 INSTRUCTION SET SUMMARY intel e Volume 2 This instruction is also available in all Pentium processors that implement the MMX technology UD2 Undefined instruction see Section 6 15 4 No Operation and Undefined Instruc tions 6 1 3 New Instructions in the Pentium Processor The following instructions are new in the Pentium processor CMPXCHGSB compare and exchange 8 bytes instruction CPUID CPU identification instruction This instruction was introduced in the Pentium processor and added to later versions of the Intel486TM processor RDTSC read time stamp counter instruction RDMSR read model specific register instruction WRMSR write model specific register instruction RSM resume from SMM instruction The form of the MOV instruction used to access the test registers has been removed on the Pentium and future Intel Architecture processors 6 1 4 New Instructions in the Intel486 Processor The following instructions are new in the Intel486 processor BSWAP byte swap instruction XADD exchange and add instruction CMPXCHG compare and exchange instruction invalidate cache instruction WBINVD write back and inv
28. EBP POP EBP Y ETURN TO INTERRUPTED CALCULATION IRETD E ALL SAVI ENDP ERS ORY Example D 2 Reduced Latency Exception Handler SAV ENVIRONMENTPROC SAVE REGISTERS PUSH EBP ALLOCATE MOV EBP ESP SUB ESP 28 STACK SPACE ALLOCATES 28 BYTES FOR FPU ENVIRONMENT 32 bit PROT ECT ED MODE SIZ SAVE ENVIRONMENT RESTOR INT ERRUPT ENABLE FLAG IF FNSTENV EBP 28 PUSH OFFSET TO EF LAGS COPY OLD EFLAGS TO STACK TOP POPF RESTORE IF TO APPLICATION DEPENDENT EXCEP EAR EXCEPTION FLAGS IN STATUS WORD g ESTORE MODIFIED ENVIRONMEN MOV BYTE PTR EBP 24 m IMAGE OH ION HANDLI VALUE BEFOR FPU XCEPTIO G COD F GOES H WHICH IS IN M ORY Intel FLD ENV MOV ESP POP EBP EBP E ALLOCATE STACK SPACE 28 EBP GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS RESTORE EGIST ERS RET IRETD SAVE ENVIRONMENT ENDP URN TO INTERRUPTED CALCULATION Example D 3 Reentrant Exception Handler
29. Examples D 1 and D 2 assume that the exception handler itself will not cause an unmasked exception Where this is a possibility the general approach shown in Example D 3 can be employed The basic technique is to save the full FPU state and then to load a new control word in the prologue Note that considerable care should be taken when designing an exception handler of this type to prevent the handler from being reentered endlessly Example D 1 Full State Exception Handler SAVE ALL PROC SAVE REGISTERS ALLOCATE STACK SPACE FOR FPU STATE IMAGE PUSH EBP MOV EBP ESP SUB ESP 108 ALLOCATES 108 BYTES 32 bit PROTECTED MODE SIZE GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS intel SAVE FULL FPU STATE R ESTOR INT ERRUPT ENABL F FLAG I FNSAVE EBP 108 PUSH EBP OFFSET TO EFLAGS F COPY OLD EFLAGS TO STACK TOP POPFD RESTORE IF TO APPLICATION DEPENDEN EXC C EPTION HANDLI EAR EXCEPTION FLAGS IN STATUS WORD RESTORE MODIFIED STA IMAG m MOV BYTE PTR FRSTOR EBP 108 EBP 104 OH VALUE B F EFOR FPU EXC EPTION G COD F GOES H ER WHICH IS IN M E ALLOCATE STACK SPAC F R ESTOR EGIST MOV ESP
30. FNSTENV ENSAVE FNSTSW FNSTCW FNENI FNDISI or FNSETPM in the MS DOS compatibility mode in the following manner See Section 7 5 11 FPU Control Instructions and Section 7 5 12 Waiting Vs Non waiting Instructions for a discussion of the no wait instructions If an unmasked numeric exception is pending from a preceding FPU instruction a member of the no wait class of instructions will at the beginning of its execution assert the FERR pin in response to that exception just like other FPU instructions but then unlike the other FPU instructions FERR will be de asserted This de assertion was implemented to allow the no wait class of instructions to proceed without an interrupt due to any pending numeric exception However the brief assertion of FERR is sufficient to latch the FPU exception request into most hardware interface implementations including Intel s recommended circuit D 7 GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS intel All the FPU instructions are implemented such that during their execution there is a window in which the processor will sample and accept external interrupts If there is a pending interrupt the processor services the interrupt first before resuming the execution of the instruction Conse quently it is possible that the no wait floating point instruction may accept the external interrupt caused by it s own assertion of the FERR pin in the event of a pending unmasked numeric exception
31. FUCOMPP Unordered compare real and pop twice FICOM Compare integer FICOMP Compare integer and pop FCOMI Compare real and set EFLAGS FUCOMI Unordered compare real and set EFLAGS FCOMIP Compare real set EFLAGS and pop FUCOMIP Unordered compare real set EFLAGS and pop FTST Test real FXAM Examine real 6 2 3 4 TRANSCENDENTAL FSIN Sine FCOS Cosine FSINCOS Sine and cosine FPTAN Partial tangent FPATAN Partial arctangent F2XMI 2 1 FYL2X y log x FYL2XP1 y log x 1 6 2 3 5 LOAD CONSTANTS FLD1 Load 1 0 FLDZ Load 0 0 6 13 INSTRUCTION SET SUMMARY intel e FLDPI FLDL2E FLDLN2 FLDL2T FLDLG2 Load x Load log e Load log 2 Load log 10 Load 10212 6 2 3 6 FPU CONTROL FINCSTP FDECSTP FFREE FINIT FNINIT FCLEX FNCLEX FSTCW FNSTCW FLDCW FSTENV FNSTENV FLDENV FSAVE FNSAVE FRSTOR FSTSW FNSTSW WAIT FWAIT FNOP 6 14 Increment FPU register stack pointer Decrement FPU register stack pointer Free floating point register Initialize FPU after checking error conditions Initialize FPU without checking error conditions Clear floating point exception flags after checking for error conditions Clear floating point exception flags without checking for error conditions Store FPU control word after checking error conditions Store FPU control word without checking error conditions Load FPU control word Store FPU environment after checking error conditions Store FPU environment without checking err
32. MS DOS program is set up in a virtual machine that provides a virtualized interrupt table The MS DOS application hooks interrupt 16 in the virtual machine in the normal way A numeric exception will trap to the kernel via the real INT 16 residing in the kernel at ring 0 The INT 16 handler in the kernel then locates the correct MS DOS virtual machine and reflects the interrupt to the virtual machine monitor The virtual machine monitor then emulates an interrupt by jumping through the address in the virtualized interrupt table eventually reaching the applica tion s numeric exception handler D 4 DIFFERENCES FOR HANDLERS USING NATIVE MODE The 8087 has a pin INT which it asserts when an unmasked exception occurs But there is no interrupt input pin in the 8086 or 8088 dedicated to its attachment nor an interrupt vector number in the 8086 or 8088 specific for an FPU error assertion But beginning with the Intel 286 and Intel 287 hardware connections were dedicated to support the FPU exception and interrupt vector 16 assigned to it D 26 Intel GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS D 4 1 Origin With the Intel 286 and Intel 287 and Intel386 and Intel 387 Processors The Intel 286 and Intel 287 and Intel386 and Intel 387 processor coprocessor pairs are each provided with ERROR pins that are recommended to be connected between the processor and FPU If this is done when an unmasked FPU exception occurs the FPU records the exc
33. Move String CMPS Compare string SCAS Scan string LODS Load string and STOS Store string instructions permit large data structures such as alphanumeric char acter strings to be moved and examined in memory These instructions operate on individual elements in a string which can be a byte word or doubleword The string elements to be oper ated on are identified with the ESI source string element and EDI destination string element registers Both of these registers contain absolute addresses offsets into a segment that point to a string element By default the ESI register addresses the segment identified with the DS segment register A segment override prefix allows the ESI register to be associated with the CS SS ES FS or GS segment register The EDI register addresses the segment identified with the ES segment register no segment override is allowed for the EDI register The use of two different segment registers in the string instructions permits operations to be performed on strings located in different segments Or by associating the ESI register with the ES segment register both the 6 34 intel e INSTRUCTION SET SUMMARY source and destination strings can be located in the same segment This latter condition can also be achieved by loading the DS and ES segment registers with the same segment selector and allowing the ESI register to default to the DS register The MOVS instruction moves the string element a
34. The CALL instructions allows control transfers to procedures within the current code segment near call and in a different code segment far call Near calls usually provide access to local procedures within the currently running program or task Far calls are usually used to access operating system procedures or procedures in a different task See CALL Call Procedure in Chapter 3 of the Intel Architecture Software Developer s Manual Volume 2 for a detailed description of the CALL instruction The RET instruction also allows near and far returns to match the near and far versions of the CALL instruction In addition the RET instruction allows a program to increment the stack pointer on a return to release parameters from the stack The number of bytes released from the 4 4 intel stack is determined by an optional argument n to the RET instruction See RET Return from Procedure in Chapter 3 of the Intel Architecture Software Developer s Manual Volume 2 for a detailed description of the RET instruction 4 3 1 PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS Near CALL and RET Operation When executing a near call the processor does the following see Figure 4 4 Pushes the current value of the EIP register on the stack 2 Loads the offset of the called procedure in the EIP register 3 Begins execution of the called procedure When executing a near return the processor performs these actions Pops th
35. Tracking FPU Ownership interaction of FPU State Saves and Floating Point Exception Association Interrupt Routing From the Kernel TABLE OF CONTENTS xi TABLE OF CONTENTS intel e D 4 D 4 1 D 4 2 D 4 3 xii PAGE DIFFERENCES FOR HANDLERS USING NATIVE D 26 Origin With the Intel 286 and Intel 287 and Intel886 and Intel 387 50 5 D 27 Changes with Intel486 Pentium and Pentium Pro Processors WithixG RO NES T nei ERE w ER IC D 27 Considerations When FPU Shared Between Tasks Using Native Mode D 28 intel Figure 1 1 Figure 2 1 Figure 2 2 Figure 3 1 Figure 3 2 Figure 3 3 Figure 3 4 Figure 3 5 Figure 3 6 Figure 3 7 Figure 4 1 Figure 4 2 Figure 4 3 Figure 4 4 Figure 4 5 Figure 4 6 Figure 4 7 Figure 4 8 Figure 4 9 Figure 4 10 Figure 5 1 Figure 5 2 Figure 5 3 Figure 5 4 Figure 5 5 Figure 6 1 Figure 6 2 Figure 6 3 Figure 6 4 Figure 6 5 Figure 6 6 Figure 6 7 Figure 6 8 Figure 6 9 Figure 6 10 Figure 6 11 Figure 7 1 Figure 7 2 Figure 7 3 Figure 7 4 Figure 7 5 Figure 7 6 Figure 7 7 Figure 7 8 Figure 7 9 Figure 7 10 Figure 7 11 Figure 7 12 TABLE OF FIGURES PAGE Birand Byte Order Cid ar ta 1 5 The Processing Units in the Pentium Pro Processor Microarchitecture and Thei
36. a Soh up lel td Ms 7 20 7 3 7 The FPU Instruction and Operand Data Pointers 7 21 7 3 8 Last Instruction Opcode u a sakirira ne 7 21 7 3 9 Saving the FPU s State uuu u sls suala ama aa usis eee 7 21 7 4 FLOATING POINT DATA TYPES AND FORMATS 7 24 7 4 1 Real i 2d e RC I Ae eo DU RB E 7 25 7 4 2 Binary Integers x steht A pela wae SS 7 27 7 4 3 Decimal Integers or sacs ee ea ee aes 7 28 7 4 4 Unsupported Extended Real Encodings 7 28 7 5 INSTRUCTION SET irae e RR ye ran uei et 7 29 7 5 1 Escape ESC Instructions 7 30 7 5 2 FPU Instruction Operands 7 31 7 5 3 Data Transfer Instructions 7 31 7 5 4 Load Constant Instructions 7 33 7 5 5 Basic Arithmetic Instructions 7 33 7 5 6 Comparison and Classification Instructions 7 34 7 5 6 1 Branching on the FPU Condition 7 36 7 5 7 Trigonometric 5 5 7 37 7 5 8 Pl bh sa pie tend De en eite ib aka 7 37 7 5 9 Logarithmic Exponential and 7 38 7 5 10 Transcend
37. and shrinks up towards greater addresses when the items are popped from the stack A program or operating system executive can set up many stacks For example in multitasking systems each task can be given its own stack The number of stacks in a system is limited by the maximum number of segments and the available physical memory When a system sets up many 4 1 PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS intel stacks only one stack the current stack is available at a time The current stack is the one contained in the segment referenced by the SS register Stack Segment Bottom of Stack Initial ESP Value Local Variables oe d The Stack Can Be 16 or 32 Bits Wide Parameters Passed to The EBP register is Called typically set to point Procedure to the return instruction pointer Frame Boundary Henri eturn Instruction Pointer lt EBP Register lt ESP Register Top of Stack Pushes Move the Pops Move the Top Of Stack to Top Of Stack to Lower Addresses Higher Addresses Figure 4 1 Stack Structure The processor references the SS register automatically for all stack operations For example when the ESP register is used as a memory address it automatically points to an address in the current stack Also the CALL RET PUSH POP ENTER and LEAVE instructions all perform operations on the current stack 4 2 1 Setting Up a Stack To set a stac
38. processor and their functions intel 8 ABOUT THIS MANUAL 1 4 NOTATIONAL CONVENTIONS This manual uses special notation for data structure formats for symbolic representation of instructions and for hexadecimal numbers A review of this notation makes the manual easier to read 1 4 1 Bit and Byte Order In illustrations of data structures in memory smaller addresses appear toward the bottom of the figure addresses increase toward the top Bit positions are numbered from right to left The numerical value of a set bit is equal to two raised to the power of the bit position Intel Architec ture processors are little endian machines this means the bytes of a word are numbered starting from the least significant byte Figure 1 1 illustrates these conventions Data Structure Highest Address 2 24 23 16 15 87 0 Bit offset Lowest Address Byte Offset Figure 1 1 Bit and Byte Order 1 4 2 Reserved Bits and Software Compatibility In many register and memory layout descriptions certain bits are marked as reserved When bits are marked as reserved it is essential for compatibility with future processors that software treat these bits as having a future though unknown effect The behavior of reserved bits should be regarded as not only undefined but unpredictable Software should follow these guidelines in dealing with reserved bits Do not depend on the states of an
39. which is not an explicitly documented behavior of a no wait instruction This process is illustrated in Figure D 3 Exception Generating Floating Point Instruction Assertion of FERR Y by the Processor Start of the No Wait Floating Point Instruction System Dependent Delay Case 1 External Interrupt I A Sampling Window Assertion of INTR Pin by the System Window Closed Figure 0 3 Timing of Receipt of External Interrupt Figure D 3 assumes that a floating point instruction that generates a deferred error as defined in the Section D 2 1 1 Basic Rules When FERR Is Generated which asserts the FERR pin only on encountering the next floating point instruction causes an unmasked numeric exception Assume that the next floating point instruction following this instruction is one of the no wait floating point instructions The FERR pin is asserted by the processor to indicate the pending exception on encountering the no wait floating point instruction After the assertion of the FERR pin the no wait floating point instruction opens a window where the pending external interrupts are sampled Then there are two cases possible depending on the timing of the receipt of the interrupt via the INTR pin asserted by the system in response to the FERR pin by the processor Case 1 If the system responds to the assertion of FERR
40. 16 when the flag is set the address size attribute is 32 4 2 4 Procedure Linking Information The processor provides two pointers for linking of procedures the stack frame base pointer and the return instruction pointer When used in conjunction with a standard software procedure call technique these pointers permit reliable and coherent linking of procedures 4 3 PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS intel 4 2 4 1 STACK FRAME BASE POINTER The stack is typically divided into frames Each stack frame can then contain local variables parameters to be passed to another procedure and procedure linking information The stack frame base pointer contained in the EBP register identifies a fixed reference point within the stack frame for the called procedure To use the stack frame base pointer the called procedure typically copies the contents of the ESP register into the EBP register prior to pushing any local variables on the stack The stack frame base pointer then permits easy access to data structures passed on the stack to the return instruction pointer and to local variables added to the stack by the called procedure Like the ESP register the EBP register automatically points to an address in the current stack segment that is the segment specified by the current contents of the SS register 4 2 4 2 RETURN INSTRUCTION POINTER Prior to branching to the first instruction of the called procedure the CALL instructi
41. 2 Pushes the current value of the EIP register on the stack 3 Loads the segment selector of the segment that contains the called procedure in the CS register 4 Loads the offset of the called procedure in the EIP register 5 Begins execution of the called procedure When executing a far return the processor does the following 1 Pops the top of stack value the return instruction pointer into EIP register 2 Pops the top of stack value the segment selector for the code segment being returned to into the CS register 3 If the RET instruction has an optional n argument Increments the stack pointer by the number of bytes specified with the n operand to release parameters from the stack 4 Resumes execution of the calling procedure 4 3 8 Parameter Passing Parameters can be passed between procedures in any of three ways through general purpose registers in an argument list or on the stack 4 3 3 1 PASSING PARAMETERS THROUGH THE GENERAL PURPOSE REGISTERS The processor does not save the state of the general purpose registers on procedure calls A calling procedure can thus pass up to six parameter to the called procedure by copying the parameters into any of these registers except the ESP and EBP registers prior to executing the CALL instruction The called procedure can likewise pass parameters back to the calling proce dure through general purpose registers 4 3 3 2 PASSING PARAMETERS ON THE STACK To
42. 25 SHLD instruction 6 27 SHR instruction 6 25 SHRD instruction 6 27 Sl register eae ee Rm en 3 7 Signaling NaN see SNaN Signed infinily 7 8 Signed zero 7 6 Significand of floating point number 7 3 Sign floating point number 7 3 SIMD single instruction multiple data execution Model 8 3 Sine FPU operation 7 37 Single precision IEEE floating point format 7 25 INDEX 8 Single real floating point format 7 25 SNaN description 7 8 operating 7 41 typical uses 7 41 SP register 3 7 Speculative execution 2 6 SS register 3 7 3 9 4 1 Stack alignment 4 3 Stack fault 7 15 Stack overflow and underflow exceptions IS aoa b on hd 7 48 Stack overflow exception FPU 7 13 7 47 Stack pointer ESP register 4 1 Stack segment 3 9 Stack switching on calls to interrupt and exception handlers 4 11 on inter privilege level calls 4 9 4 14 Stack underflow exception FPU 7 18 7 47 Stack see Pr
43. 28 EBP n 32 ESI n 36 EDI lt ESP Figure 6 4 Operation of the POPA Instruction 6 3 2 1 TYPE CONVERSION INSTRUCTIONS The type conversion instructions convert bytes into words words into doublewords and double words into quadwords These instructions are especially useful for converting integers to larger integer formats because they perform sign extension see Figure 6 5 Two kinds of type conversion instructions are provided simple conversion and move and convert 15 0 SINININININ NININININ Nin Before Sign Extension 31 15 0 After Sign sssssssssssssssSsSNNNNNNNNNNNNNNN E C nson Figure 6 5 Sign Extension 6 3 2 2 SIMPLE CONVERSION The CBW convert byte to word CWDE convert word to doubleword extended CWD convert word to doubleword and CDQ convert doubleword to quadword instructions perform sign extension to double the size of the source operand The CBW instruction copies the sign bit 7 of the byte in the AL register into every bit position of the upper byte of the AX register The CWDE instruction copies the sign bit 15 of the word in the AX register into every bit position of the high word of the EAX register 6 21 INSTRUCTION SET SUMMARY intel e The CWD instruction copies the sign bit 15 of the word in the AX register into every bit posi tion in the D
44. 4 MODES OF OPERATION 5222 ner a a a Red 3 4 3 5 32 BIT VS 16 BIT ADDRESS AND OPERAND 514 5 3 5 3 6 REGISTERS 3 ee Sed Se eae Ge ye ee el VE Ses 3 5 3 6 1 General Purpose Data Registers 3 5 3 6 2 Segment Registers 3 7 3 6 3 EFLAGS Registera lai enne EE LER le as ae bie ete 3 10 3 6 3 1 AUS AA a Gears Y PIA ERR Er 3 11 3 6 3 2 DE Flag a Baa OIM viu v I PEERS 3 12 3 6 4 System Flags and IOPL 3 13 3 7 INSTRUCTION POINTER 3 14 3 8 OPERAND SIZE AND ADDRESS SIZE ATTRIBUTES 3 14 TABLE OF CONTENTS intel e PAGE CHAPTER 4 PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS 4 1 PROCEDURE CALL TYPES ne 4 1 4 2 apo CRM thea mace hale suu te wane O 4 1 4 2 1 Setting Upa Stack u Wehen e ate 4 2 4 2 2 Stack Alignment noni Seong are See uma ee ee et ites s 4 3 4 2 3 Address Size Attributes for Stack 4 3 4 2 4 Procedure Linking Information 4 3 4 2 4 1 Stack Frame Base 4 4 4 2 4 2 Return Instruction 4 4 4 3 CALL
45. 6 13 3 EFLAGS Transfer Instructions 6 37 6 13 4 Interrupt Flag Instructions 6 38 6 14 SEGMENT REGISTER INSTRUCTIONS 6 38 6 14 1 Segment Register Load and Store Instructions 6 38 6 14 2 Far Control Transfer Instructions 6 39 6 14 3 Software Interrupt Instructions 6 39 6 14 4 Load Far Pointer Instructions 6 39 6 15 MISCELLANEOUS INSTRUCTIONS 6 39 6 15 1 Address Computation Instruction 6 39 6 15 2 Table Lookup 1 5 6 40 6 15 3 Processor Identification 1 5 6 40 6 15 4 No Operation and Undefined Instructions 6 40 CHAPTER 7 FLOATING POINT UNIT 7 1 COMPATIBILITY AND EASE OF USE OF THE INTEL ARCHITECTURE 7 1 7 2 REAL NUMBERS AND FLOATING POINT FORMATS 7 2 7 2 1 Real Number System 7 3 7 2 2 Floating Point Format E AE E EE E EE E EAE hn 7 3 7 2 2 1 Normalized Numbers 7 4 7 2 2 2 Biased EXPONENT i cic oss u Aid ety et AE hat ed 7 5 7 2 3 Rea
46. 6 4 4 Multiplication and Divide Instructions The processor provides two multiply instructions MUL unsigned multiply and IMUL signed multiply and two divide instructions DIV unsigned divide and IDIV signed divide The MUL instruction multiplies two unsigned integer operands The result is computed to twice the size of the source operands for example if word operands are being multiplied the result is a doubleword The IMUL instruction multiplies two signed integer operands The result is computed to twice the size of the source operands however in some cases the result is truncated to the size of the source operands see IMUL Signed Multiply in Chapter 3 of the Intel Architecture Software Developer s Manual Volume 2 The DIV instruction divides one unsigned operand by another unsigned operand and returns a quotient and a remainder The IDIV instruction is identical to the DIV instruction except that IDIV performs a signed division 6 5 DECIMAL ARITHMETIC INSTRUCTIONS Decimal arithmetic can be performed by combining the binary arithmetic instructions ADD SUB MUL and DIV discussed in Section 6 4 Binary Arithmetic Instructions with the decimal arithmetic instructions The decimal arithmetic instructions are provided to carry out the following operations adjust the results of a previous binary arithmetic operation to produce a valid BCD result To adjust the operands of a subsequent binary a
47. Architecture Software Devel oper s Manual Volume 3 Detailed descriptions of all the Intel Architecture instructions are given in Intel Architecture Software Developer s Manual Volume 2 Included in this volume are a description of each instruction s encoding and operation the effect of an instruction on the EFLAGS flags and the exceptions an instruction may generate 6 1 NEW INTEL ARCHITECTURE INSTRUCTIONS The following sections give the Intel Architecture instructions that were new in the MMX Tech nology and in the Pentium Pro Pentium and Intel486 processors 6 1 1 New Instructions Introduced with the MMX Technology The Intel MMX technology introduced a new set of instructions to the Intel Architecture designed to enhance the performance of multimedia applications These instructions are recog nized by all Intel Architecture processors that implement the MMX technology The MMX instructions are listed in Section 6 2 2 MMX Technology Instructions 6 1 2 New Instructions in the Pentium Pro Processor The following instructions are new in the Pentium Pro processor CMOVcc Conditional move see Section 6 3 1 2 Conditional Move Instructions FCMOVcc Floating point conditional move on condition code flags in EFLAGS register see Section 7 5 3 Data Transfer Instructions FCOMI FCOMIP FUCOMI FUCOMIP Floating point compare and set condition code flags in EFLAGS register see Section 7 5 6
48. BCD digits per byte The value of an unpacked BCD integer is the binary value of the low half byte bits 0 through 3 The high half byte bits 4 through 7 can be any value during addition and subtraction but must be zero during multiplication and division Packed BCD integers allow two BCD digits to be contained in one byte Here the digit in the high half byte is more significant than the digit in the low half byte 5 2 4 Pointers Pointers are addresses of locations in memory The Pentium Pro processor recognizes two types of pointers a near pointer 32 bits and a far pointer 48 bits A near pointer is a 32 bit offset also called an effective address within a segment Near pointers are used for all memory refer ences in a flat memory model or for references in a segmented model where the identity of the segment being accessed is implied A far pointer is a 48 bit logical address consisting of a 16 bit segment selector and a 32 bit offset Far pointers are used for memory references in a segmented memory model where the identity of a segment being accessed must be specified explicitly 5 2 5 Bit Fields A bit field is a contiguous sequence of bits It can begin at any bit position of any byte in memory and can contain up to 32 bits 5 2 6 Strings Strings are continuous sequences of bits bytes words or doublewords A bit string can begin at any bit position of any byte and can contain up to 2 1 bits A byte string can cont
49. Data Register 1 7 10 Example FPU Dot Product Computation 7 12 FPU Stat s WOrd 2 Ram tela 7 13 Moving the FPU Condition Codes to the EFLAGS Register 7 16 Control Word cesset er Oa M RR a 7 17 FPU Tag Word meses ai eee bee UU IARE UN RUE P VES 7 20 Contents of FPU Opcode Registers 7 22 TABLE OF FIGURES intel Figure 7 13 Figure 7 14 Figure 7 15 Figure 7 16 Figure 7 17 Figure 8 1 Figure 8 2 Figure 8 3 Figure 9 1 Figure 9 2 Figure D 1 Figure D 2 Figure D 3 Figure D 4 Figure D 5 Figure D 6 xiv PAGE Protected Mode FPU State Image in Memory 32 Bit Format 7 22 Real Mode FPU State Image in Memory 32 Bit Format 7 23 Protected Mode FPU State Image in Memory 16 Bit Format 7 23 Real Mode FPU State Image in Memory 16 Bit Format 7 23 Floating Point Unit Data Type Formats 7 24 MMXIM Register Set 2 2 2 a XAR ERR 8 2 MMX M Data ypes iens oe mus nay bin EG vases eu E puras XAR Ros 8 3 Eight Packed Bytes in Memory at address 1000H 8 4 Memory Mapped 3 uaa 9 3 Permission Bit 9 5 Recommended Circuit for MS DOS C
50. FP_code e leave the FPU stack empty MMX code EMMS mark the FPU tag word as empty FP code 1 leave the FPU stack empty 8 5 5 Using MMX Code in a Multitasking Operating System Environment An application needs to identify the nature of the multitasking operating system on which it runs Each task retains its own state which must be saved when a task switch occurs The processor state context consists of the general purpose registers and the floating point and MMX registers Operating systems can be classified into two types Cooperative multitasking operating system Preemptive multitasking operating system The behavior of the two operating system types in context switching is described in Context Switching in Chapter 10 of the Intel Architecture Software Developer s Manual Volume 3 8 5 5 1 COOPERATIVE MULTITASKING OPERATING SYSTEM Cooperative multitasking operating systems do not save the FPU or MMX state when performing a context switch Therefore the application needs to save the relevant state before relinquishing direct or indirect control to the operating system 8 5 5 2 PREEMPTIVE MULTITASKING OPERATING SYSTEM Preemptive multitasking operating systems are responsible for saving and restoring the FPU and MMX state when performing a context switch Therefore the application does not have to save or restore the FPU and MMX state 8 14 Intel PROGRAMMING WITH THE INTEL MMX TECHNOLOGY
51. JECXZ instruction when the 16 bit address size attribute is used Here the CX register is tested for zero 6 9 3 Software Interrupts The INT n software interrupt INTO interrupt on overflow and BOUND detect value out of range instructions allow a program to explicitly raise a specified interrupt or exception which in turn causes the handler routine for the interrupt or exception to be called The INT n instruction can raise any of the processor s interrupts or exceptions by encoding the vector number or the interrupt or exception in the instruction This instruction can be used to support software generated interrupts or to test the operation of interrupt and exception handlers The IRET instruction see Section 6 9 1 3 Return From Interrupt Instruction allows returns from interrupt handling routines The INTO instruction raises the overflow exception if the OF flag is set If the flag is clear execution continues without raising the exception This instruction allows software to access the overflow exception handler explicitly to check for overflow conditions The BOUND instruction compares a signed value against upper and lower bounds and raises the BOUND range exceeded exception if the value is less than the lower bound or greater than the upper bound This instruction is useful for operations such as checking an array index to make sure it falls within the range defined for the array 6 10 STRING OPERATIONS The MOVS
52. NaN values that are available provide the sophisticated programmer with a tool that can be applied to a variety of special situations For example a compiler can use signaling NaNs as references to uninitialized real array elements The compiler can preinitialize each array element with a signaling NaN whose signif icand contained the index relative position of the element Then if an application program attempts to access an element that it had not initialized it can use the NaN placed there by the compiler If the invalid operation exception is unmasked an interrupt will occur and the excep tion handler will be invoked The exception handler can determine which element has been accessed since the operand address field of the exception pointers will point to the NaN and the NaN will contain the index number of the array element 7 6 2 Uses for Quiet NANs Quiet NaNs are often used to speed up debugging In its early testing phase a program often contains multiple errors An exception handler can be written to save diagnostic information in memory whenever it was invoked After storing the diagnostic data it can supply a quiet NaN as the result of the erroneous instruction and that NaN can point to its associated diagnostic area in memory The program will then continue creating a different NaN for each error When the program ends the NaN results can be used to access the diagnostic data saved at the time the errors occurred Many e
53. PUSHA push all registers and POPA pop all registers instructions move data to and from the stack The PUSH instruction decrements the stack pointer contained in the ESP register then copies the source operand to the top of stack see Figure 6 1 It operates on memory operands immediate operands and register operands including segment registers The PUSH instruction is commonly used to place parameters on the stack before calling a proce dure It can also be used to reserve space on the stack for temporary variables Stack Before Pushing Doubleword After Pushing Doubleword Growth 31 0 31 0 n lt ESP n 4 Doubleword Value 8 Figure 6 1 Operation of the PUSH Instruction The PUSHA instruction saves the contents of the eight general purpose registers on the stack see Figure 6 2 This instruction simplifies procedure calls by reducing the number of instruc tions required to save the contents of the general purpose registers The registers are pushed on the stack in the following order EAX ECX EDX EBX the initial value of ESP before EAX was pushed EBP ESI and EDI 6 19 INSTRUCTION SET SUMMARY intel e Stack Before Pushing Registers After Pushing Registers Growth 131 i n n 4 ESP n 8 EAX n 12 ECX n 16 EDX n 20 EBX n 24 Old ESP n 28 EBP n 32 ESI n 36 EDI ESP Figure 6 2 Op
54. RET instructions Both of these procedure call mechanisms use the procedure stack commonly referred to simply as the stack to save the state of the calling procedure pass parameters to the called procedure and store local variables for the currently executing procedure The processor s facilities for handling interrupts and exceptions are similar to those used by the CALL and RET instructions 4 2 STACK The stack see Figure 4 1 is a contiguous array of memory locations It is contained in a segment and identified by the segment selector in the SS register When using the flat memory model the stack can be located anywhere in the linear address space for the program A stack can be up to 4 gigabytes long the maximum size of a segment The next available memory location on the stack is called the top of stack At any given time the stack pointer contained in the ESP register gives the address that is the offset from the base of the SS segment of the top of the stack Items are placed on the stack using the PUSH instruction and removed from the stack using the POP instruction When an item is pushed onto the stack the processor decrements the ESP register then writes the item at the new top of stack When an item is popped off the stack the processor reads the item from the top of stack then increments the ESP register In this manner the stack grows down in memory towards lesser addresses when items are pushed on the stack
55. RUE ERA MEAM 6 29 6 8 3 Byte Set On Condition 5 6 29 6 8 4 TestInstruction 2e uu s a uku Saldana og ig UA ROM REN a 6 30 6 9 CONTROL TRANSFER INSTRUCTIONS 6 30 6 9 1 Unconditional Transfer Instructions 6 30 6 9 1 1 Jump InstEUctlon c soc gu yuku em Ta ee ew ie meg Rs puma s Rats 6 30 6 9 1 2 Call and Return Instructions 6 31 6 9 1 3 Return From Interrupt Instruction 6 31 6 9 2 Conditional Transfer Instructions 6 31 6 9 2 1 Conditional Jump Instructions 6 32 6 9 2 2 Loop Instr ctions oe ee Bat wee 6 33 6 9 2 3 Jump If Zero Instructions 6 33 6 9 3 Software Interrupts gt neige u canman a ee 6 34 6 10 STRING OPERATIONS ooccccccccc eh mr 6 34 6 10 1 Repeating String Operations 6 35 6 11 VOANSTRUGTIONS A e Soe bebe y E e PETER R 6 36 6 12 ENTER AND LEAVE INSTRUCTIONS 6 36 6 13 EFLAGS INSTRUCTIONS pia ae e tees 6 37 6 13 1 Carry and Direction Flag Instructions 6 37 6 13 2 Interrupt Flag Instructions 6 37
56. SCASB SCAS SCASW SCAS SCASD LODS LODSB LODS LODSW INSTRUCTION SET SUMMARY Jump if overflow Jump if not overflow Jump if sign negative Jump if not sign non negative Jump if parity odd Jump if not parity Jump if parity even Jump if parity Jump register CX zero Jump register ECX zero Loop with ECX counter Loop with ECX and zero Loop with ECX and equal Loop with ECX and not zero Loop with ECX and not equal Call procedure Return Return from interrupt Software interrupt Interrupt on overflow Detect value out of range High level procedure entry High level procedure exit STRING INSTRUCTIONS Move string Move byte string Move string Move word string Move string Move doubleword string Compare string Compare byte string Compare string Compare word string Compare string Compare doubleword string Scan string Scan byte string Scan string Scan word string Scan string Scan doubleword string Load string Load byte string Load string Load word string 6 7 INSTRUCTION SET SUMMARY intel e LODS LODSD STOS STOSB STOS STOSW STOS STOSD REP REPE REPZ REPNE REPNZ INS INSB INS INSW INS INSD OUTS OUTSB OUTS OUTSW OUTS OUTSD Load string Load doubleword string Store string Store byte string Store string Store word string Store string Store doubleword string Repeat while ECX not zero Repeat while equal Repeat while zero Repeat while not equal Repeat while not zero Input string from port Input byte string f
57. The temporal regions within the FPU exception handler activity are described as follows 1 The FERR signal is activated by an FPU exception and sends an interrupt request through the PIC to the processor s INTR pin 2 During the FPU interrupt service routine exception handler the processor will need to clear the interrupt request latch Flip Flop 1 It may also want to execute non control FPU instructions before the exception is cleared from the FPU For this purpose the IGNNE must be driven low Typically in the PC environment an I O access to Port OFOH clears the external FPU exception interrupt request FP IRQ In the recommended circuit this access also is used to activate IGNNE With IGNNE active the FPU exception handler may execute any FPU instruction without being blocked by an active FPU exception 3 Clearing the exception within the FPU will cause the FERR signal to be deactivated and then there is no further need for IGNNE to be active In the recommended circuit the deactivation of FERR is used to deactivate IGNNE If another circuit is used the software and circuit together must assure that IGNNE is deactivated no later than the exit from the FPU exception handler D 5 GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS In RESET I O Port OFOh Address Decode CLR 5V FERR gt O O 5 Intel486 Y
58. This means that the floating point code loses its pointer to its floating point registers if the code mixes MMX instructions within a floating point routine An MMX instruction write to an MMX register writes ones 115 to the exponent part of the corresponding floating point register Floating point code that uses register contents that were generated by the MMX instruc tions may cause floating point exceptions or incorrect results These floating point exceptions are related to undefined floating point values and floating point stack usage MMX instructions except EMMS set the entire tag word to the valid state 00s in all tag fields without preserving the previous floating point state Frequent transitions between the MMX and floating point instructions may result in significant performance degradation in some implementations If the application contains floating point and MMX instructions follow these guidelines Partition the MMX technology module and the floating point module into separate instruction streams separate loops or subroutines so that they contain only instructions of one type Do not rely on register contents across transitions When the MMX state is not required empty the MMX state using the EMMS instruction Exit the floating point code section with an empty stack 8 13 PROGRAMMING WITH THE INTEL MMX TECHNOLOGY intel Example 8 2 Floating point FP and MMX Code
59. a mask bit in the FPU control word If an exception is masked the corre sponding mask bit in the control word 1 the processor takes an appropriate default action and continues with the computation The processor has a default fix up activity for every possible exception condition it may encounter These masked exception responses are designed to be safe and are generally acceptable for most numeric applications For example if the Inexact result Precision exception is masked the system can specify whether the FPU should handle a result that cannot be represented exactly by one of four modes of rounding rounding it normally chopping it toward zero always rounding it up or always down If the Underflow exception is masked the FPU will store a number that is too small to be represented in normalized form as a denormal or zero if it s smaller than the smallest denormal Note that when exceptions are masked the FPU may detect multiple exceptions in a GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS intel single instruction because it continues executing the instruction after performing its masked response For example the FPU could detect a denormalized operand perform its masked response to this exception and then detect an underflow As an example of how even severe exceptions can be handled safely and automatically using the default exception responses consider a calculation of the parallel resistance of several values using only t
60. and control registers A subsequent waiting floating point instruction can then handle any pending exceptions 7 55 Intel Programming With the Intel MMX Technology CHAPTER 8 PROGRAMMING WITH THE INTEL MMX TECHNOLOGY The Intel MMX technology comprises a set of extensions to the Intel Architecture that are designed to greatly enhance the performance of advanced media and communications applica tions These extensions which include new registers data types and instructions are combined with a single instruction multiple data SIMD execution model to accelerate the performance of applications such as motion video combined graphics with video image processing audio synthesis speech synthesis and compression telephony video conferencing and 2D and 3D graphics which typically use compute intensive algorithms to perform repetitive operations on large arrays of simple native data elements The MMX technology defines a simple and flexible software model with no new mode or oper ating system visible state All existing software will continue to run correctly without modifi cation on Intel Architecture processors that incorporate the MMX technology even in the presence of existing and new applications that incorporate this technology The following sections of this chapter describe the MMX technology s basic programming envi ronment including the MMX register set data types and instruction set Detailed
61. below Conditional move if not above or equal CMOVBE CMOVNA Conditional move if below or equal Conditional move if not above CMOVG CMOVNLE Conditional move if greater Conditional move if not less or equal CMOVGE CMOVNL Conditional move if greater or equal Conditional move if not less CMOVL CMOVNGE Conditional move if less Conditional move if not greater or equal CMOVLE CMOVNG Conditional move if less or equal Conditional move if not greater CMOVC Conditional move if carry CMOVNC Conditional move if not carry CMOVO Conditional move if overflow CMOVNO Conditional move if not overflow CMOVS Conditional move if sign negative CMOVNS Conditional move if not sign non negative CMOVP CMOVPE Conditional move if parity Conditional move if parity even CMOVNP CMOVPO Conditional move if not parity Conditional move if parity odd XCHG Exchange 6 3 INSTRUCTION SET SUMMARY intel e BSWAP XADD CMPXCHG CMPXCHG8B PUSH POP PUSHA PUSHAD POPA POPAD IN OUT CWD CDQ CBW CWDE MOVSX MOVZX Byte swap Exchange and add Compare and exchange Compare and exchange 8 bytes Push onto stack Pop off of stack Push general purpose registers onto stack Pop general purpose registers from stack Read from a port Write to a port Convert word to doubleword Convert doubleword to quadword Convert byte to word Convert word to doubleword in EAX register Move and sign extend Move and zero extend 6 2 1 2 BINARY ARITHMETIC INSTRUCTIONS
62. below or equal Set byte if above or equal Set byte if not below Set byte if not carry Set byte if below Set byte if not above or equal Set byte if carry Set byte if below or equal Set byte if not above Set byte if greater Set byte if not less or equal Set byte if greater or equal Set byte if not less Set byte if less Set byte if not greater or equal Set byte if less or equal Set byte if not greater Set byte if sign negative Set byte if not sign non negative Set byte if overflow Set byte if not overflow Set byte if parity even Set byte if parity Set byte if parity odd Set byte if not parity Logical compare 6 2 1 7 CONTROL TRANSFER INSTRUCTIONS JMP JE JZ JNE JNZ JA JNBE JAE JNB JB JNAE JBE JNA JG INLE JGE JNL JL INGE JLE ING JC JNC 6 6 Jump Jump if equal Jump if zero Jump if not equal Jump if not zero Jump if above Jump if not below or equal Jump if above or equal Jump if not below Jump if below Jump if not above or equal Jump if below or equal Jump if not above Jump if greater Jump if not less or equal Jump if greater or equal Jump if not less Jump if less Jump if not greater or equal Jump if less or equal Jump if not greater Jump if carry Jump if not carry Intel JO JNO JS JNS JPO JNP JPE JP JCXZ JECXZ LOOP LOOPZ LOOPE LOOPNZ LOOPNE CALL RET IRET INT INTO BOUND ENTER LEAVE 6 2 1 8 MOVS MOVSB MOVS MOVSW MOVS MOVSD CMPS CMPSB CMPS CMPSW CMPS CMPSD SCAS
63. beyond branches to keep the instruction pipeline full In the Pentium Pro processor the instruction fetch decode unit uses a highly opti mized branch prediction algorithm to predict the direction of the instruction stream through multiple levels of branches procedure calls and returns Dynamic data flow analysis involves real time analysis of the flow of data through the processor to determine data and register dependencies and to detect opportunities for out of order instruc tion execution The Pentium Pro processor dispatch execute unit can simultaneously monitor many instructions and execute these instructions in the order that optimizes the use of the processor s multiple execution units while maintaining the integrity of the data being operated on This out of order execution keeps the execution units busy even when cache misses and data dependencies among instructions occur Speculative execution refers to the processor s ability to execute instructions ahead of the program counter but ultimately to commit the results in the order of the original instruction stream To make speculative execution possible the Pentium Pro processor microarchitecture decouples the dispatching and executing of instructions from the commitment of results The processor s dispatch execute unit uses data flow analysis to execute all available instructions in the instruction pool and store the results in temporary registers The retirement unit then linearly s
64. both of the values is a NaN or in an undefined format a floating point invalid operation exception is generated The pop versions of the instruction pop the FPU register stack once or twice after the comparison operation is complete The FUCOM FUCOMP and FUCOMPP instructions operate the same as the FCOM FCOMP and FCOMPP instructions The only difference is that with the FUCOM FUCOMB and FUCOMPP instructions if an unordered condition is detected because one or both of the oper ands is a QNaN the floating point invalid operation exception is not generated Table 7 15 Setting of FPU Condition Code Flags for Real Number Comparisons Condition C3 C2 co ST 0 gt Source Operand 0 0 0 ST 0 Source Operand 0 0 1 ST 0 Source Operand 1 0 0 Unordered 1 1 1 The FICOM and FICOMP instructions also operate the same as the FCOM and FCOMP instruc tions except that the source operand is an integer value in memory The integer value is auto matically converted into an extended real value prior to making the comparison The FICOMP instruction pops the FPU register stack following the comparison operation The FTST instruction performs the same operation as the FCOM instruction except that the value in register ST 0 is always compared with the value 0 0 The FCOMI and FCOMIP instructions are new in the Intel Pentium Pro processor They perform the same comparison as the FCOM and FCOMP instructions except
65. control from the current or calling procedure to another procedure the called procedure To allow a subsequent return to the calling procedure the CALL instruction saves the current contents of the EIP register on the stack before jumping to the called procedure The EIP register prior to transferring program control contains the address of the instruction following the CALL instruction When this address is pushed on the stack it is referred to as the return instruction pointer or return address The address of the called procedure the address of the first instruction in the procedure being jumped to is specified in a CALL instruction the same way as it is in a JMP instruction see Section 6 9 1 1 Jump Instruction The address can be specified as a relative address or an absolute address If an absolute address is specified it can be either a near or a far pointer The RET instruction transfers program control from the procedure currently being executed the called procedure back to the procedure that called it the calling procedure Transfer of control is accomplished by copying the return instruction pointer from the stack into the EIP register Program execution then continues with the instruction pointed to by the EIP register The RET instruction has an optional operand the value of which is added to the contents of the ESP register as part of the return operation This operand allows the stack pointer to be incre mented to
66. designed it The no wait instruc tions were intended to be used inside the FPU exception handler to allow manipulation of the FPU before the error condition is cleared without hanging the processor because of the FPU er ror condition and without the need to assert IGNNE They will perform this function correctly since before the error condition is cleared the assertion of FERR that caused the FPU error handler to be invoked is still active Thus the logic that would assert FERR briefly at a no wait instruction causes no change since FERR is already asserted The no wait instructions may also be used without problem in the handler after the error condition is cleared since now they will not cause FERR to be asserted at all If a no wait instruction is used outside of the FPU exception handler it may malfunction as ex plained above depending on the details of the hardware interface implementation and which particular processor is involved The actual interrupt inside the window in the no wait instruc tion may be blocked by surrounding it with the instructions PUSHFD CLI no wait then POPFD CLI blocks interrupts and the push and pop of flags preserves and restores the original value of the interrupt flag However if FERR was triggered by the no wait its latched value and the PIC response will still be in effect Further code can be used to check for and correct such a condition if needed Section D 3 6 Considerations When FPU Sha
67. destination operand and the source operand is not modified The CF flag is loaded with the last bit shifted out of the desti nation operand The SHRD instruction operates the same as the SHLD instruction except bits are shifted to the left in the destination operand with the empty bit positions filled with bits shifted out of the source operand 6 7 3 Rotate Instructions The ROL rotate left ROR rotate right RCL rotate through carry left and RCR rotate through carry right instructions rotate the bits in the destination operand out of one end and back through the other end see Figure 6 10 Unlike a shift no bits are lost during a rotation The rotate count can range from 0 to 31 6 27 INSTRUCTION SET SUMMARY intel e ROL Instruction 31 0 CF lt Destination Memory or Register lt 31 ROR Instruction 0 gt Destination Memory or Register CF 31 RCL Instruction 0 CF lt Destination Memory or Register lt 31 RCR Instruction 0 Destination Memory or Register gt CF Figure 6 10 ROL ROR RCL and RCR Instruction Operations The ROL instruction rotates the bits in the operand to the left toward more significant bit loca tions The ROR instruction rotates the operand right toward less significant bit locations The RCL instruction rotates the bits in the operand to the lef
68. explicitly cleared They can be cleared by executing the FCLEX FNCLEX clear exceptions instructions by reinitializing the FPU with the FINIT ENINIT or FSAVE ENSAVE instructions or by over writing the flags with an FRSTOR or FLDENV instruction The B bit bit 15 is included for 8087 compatibility only It reflects the contents of the ES flag 7 3 2 4 STACK FAULT FLAG The stack fault flag bit 6 of the FPU status word indicates that stack overflow or stack under flow has occurred The FPU explicitly sets the SF flag when it detects a stack overflow or under flow condition but it does not explicitly clear the flag when it detects an invalid arithmetic operand condition When this flag is set the condition code flag C1 indicates the nature of the fault overflow C1 1 and underflow C1 0 The SF flag is a sticky flag meaning that after it is set the processor does not clear it until it is explicitly instructed to do so for example by an FINIT FNINIT FCLEX FNCLEX or FSAVE FNSAVE instruction See Section 7 3 6 FPU Tag Word for more information on FPU stack faults 7 3 8 Branching and Conditional Moves on FPU Condition Codes The Intel Architecture FPU beginning with the Pentium Pro processor supports two mecha nisms for branching and performing conditional moves according to comparisons of two floating point values These mechanism are referred to here as the old mechanism and the new mechanism Th
69. floating point compare and set EFLAGS instructions FCOMI FCOMIP FUCOMI and FUCOMIP compare two floating point values and set the ZF PF and CF flags in the EFLAGS register directly A single instruction thus replaces the three instructions required by the old mechanism Note also that the FCMOVcc instructions also new in the Pentium Pro processor allow condi tional moves of floating point values values in the FPU data registers based on the setting of the status flags ZF PF and CF in the EFLAGS register These instructions eliminate the need for an IF statement to perform conditional moves of floating point values 7 3 4 Control Word The 16 bit FPU control word see in Figure 7 10 controls the precision of the FPU and rounding method used It also contains the exception flag mask bits The control word is cached in the FPU control register The contents of this register can be loaded with the FLDCW instruction and stored in memory with the FSTCW FNSTCW instructions When the FPU is initialized with either an FINIT FNINIT or FSAVE ENS instruction the FPU control word is set to 037FH which masks all floating point exceptions sets rounding to nearest and sets the FPU precision to 64 bits 7 16 intel FLOATING POINT UNIT Infinity Control Rounding Control i Precision Control 151413121110 98 7 6 5 4 3 2 0 P U O Z M M M M D X RC PC M M Excepti
70. floating point error conditions This pin is provided to inhibit floating point exceptions from being generated while the floating point exception handler is servicing a previously signaled floating point exception Appendix D Guidelines for Writing FPU Exception Handlers describes the MS DOS compat ibility mode in much greater detail This mode is somewhat more complicated in the Intel486 and Pentium processor implementations as described in Appendix D 7 7 3 3 TYPICAL FLOATING POINT EXCEPTION HANDLER ACTIONS After the floating point exception handler is invoked the processor handles the exception in the same manner that it handles non FPU exceptions The floating point exception handler is normally part of the operating system or executive software A typical action of the exception handler is to store FPU state information in memory with the FSTENV FNSTENV or FSAVE FNSAVE instructions so that it can evaluate the exception and formulate an appropriate response see Section 7 3 9 Saving the FPU s State Other typical exception handler actions include Examining stored FPU state information control status and tag words and FPU instruction and operand pointers to determine the nature of the error Correcting the condition that caused the error Clearing the exception bits in the status word Returning to the interrupted program and resuming normal execution 7 46 Intel FLOATING POINT UNIT If the faulti
71. from interrupt and exception handler procedures that are located in a code segment other than the current code segment With these instructions however the switching of code segments is handled transparently from the application program 6 14 4 Load Far Pointer Instructions The load far pointer instructions LDS load far pointer using DS LES load far pointer using ES LFS load far pointer using FS LGS load far pointer using GS and LSS load far pointer using SS load a far pointer from memory into a segment register and a general purpose general register The segment selector part of the far pointer is loaded into the selected segment register and the offset is loaded into the selected general purpose register 6 15 MISCELLANEOUS INSTRUCTIONS The following instructions perform miscellaneous operations that are of interest to applications programmers 6 15 1 Address Computation Instruction The LEA load effective address instruction computes the effective address in memory offset within a segment of a source operand and places it in a general purpose register This instruc tion can interpret any of the Pentium Pro processor s addressing modes and can perform any indexing or scaling that may be needed It is especially useful for initializing the ESI or EDI 6 39 INSTRUCTION SET SUMMARY intel e registers before the execution of string instructions or for initializing the EBX register before an XLAT instruction 6 15
72. handler is invoked see Section 7 7 3 Software Exception Handling The top of stack pointer TOP and source and destination operands remain unchanged If the destination is the register stack the exponent of the rounded result is multiplied by 224576 and the product is stored along with the significand in the destination operand Condition code bit Cl in the FPU the status register acting here as a round up bit is set if the significand was rounded upward and cleared if the result was rounded toward 0 After the result is stored the UE flag is set and a software exception handler is invoked The scaling bias value 24 576 is the same as is used for the overflow exception and has the same effect which is to translate the result as nearly as possible to the middle of the extended real exponent range When using the FSCALE instruction massive underflow can occur where the result is too tiny to be represented even with a bias adjusted exponent Here if underflow occurs again after the result has been biased a properly signed 0 is stored in the destination operand 7 52 Intel FLOATING POINT UNIT 7 8 6 Inexact Result Precision Exception P The inexact result exception also called the precision exception occurs if the result of an oper ation is not exactly representable in the destination format For example the fraction 1 3 cannot be precisely represented in binary form This exception occurs frequently and indi
73. include segment limit checking read only and execute only segment options and up to four privilege levels to protect operating system code in several subdivisions if desired from application or user programs Furthermore hardware task switching and the Local Descriptor Tables allow the operating system to protect application or user programs from each other The Intel386 processor introduced 32 bit registers into the architecture for use both as operands for calculations and for addressing The lower half of each 32 bit register retained the properties of one of the 16 bit registers of the earlier two generations to provide complete upward compat 2 1 INTRODUCTION TO THE INTEL ARCHITECTURE intel e ibility A new virtual 8086 mode was provided to yield greater efficiency when executing programs created for the 8086 and 8088 processors on the new 32 bit machine The 32 bit addressing was supported with an external 32 bit address bus giving a 4 GByte address space and also allowed each segment to be as large as 4 GBytes The original instructions were enhanced with new 32 bit operand and addressing forms and completely new instructions were provided including those for bit manipulation The Intel386 processor also introduced paging into the Intel Architecture with the fixed 4 KByte page size providing a method for virtual memory management that was significantly superior compared to using segments for the purpose it was much more effici
74. is an implied value The exponent is a binary integer that represents the base 2 power that the significand is raised to Sign Exponent Significand Fraction Integer or J Bit A Figure 7 2 Binary Floating Point Format Table 7 1 shows how the real number 178 125 in ordinary decimal format is stored in floating point format The table lists a progression of real number notations that leads to the single real 32 bit floating point format which is one of the floating point formats that the FPU supports In this format the significand is normalized see Section 7 2 2 1 Normalized Numbers and the exponent is biased see Section 7 2 2 2 Biased Exponent For the single real format the biasing constant is 127 7 2 2 1 NORMALIZED NUMBERS In most cases the FPU represents real numbers in normalized form This means that except for Zero the significand is always made up of an integer of 1 and the following fraction 1 fff ff For values less than 1 leading zeros are eliminated For each leading zero eliminated the expo nent is decremented by one 7 4 Intel FLOATING POINT UNIT Table 7 1 Real Number Notation Notation Value Ordinary Decimal 178 125 Scientific Decimal 1 78125E p2 Scientific Binary 1 0110010001E 111 Scientific Binary 1 0110010001E 10000110 Biased Exponent Single Real Format Sign Biased Exponent Normaliz
75. not use the EMMS instruction when the interface to the MMX code has been defined to retain values in the MMX register If a high level language such as C is used the data types could be defined as a 64 bit structure with packed data types When implementing usage of MMX instructions in high level languages other approaches can be taken such as Intel PROGRAMMING WITH THE INTEL MMX TECHNOLOGY Passing parameters to an MMX routine by passing a pointer to a structure via the integer stack Returning a value from a function by returning the pointer to a structure 8 5 4 Writing Code with MMX and Floating Point Instructions The MMX technology aliases the MMX registers on the floating point registers The main reason for this is to enable MMX technology to be fully compatible and transparent to existing software environments operating systems and applications This way operating systems will be able to include new applications and drivers that use the MMX technology An application can contain both floating point and MMX code However the user is discour aged from causing frequent transitions between MMX and floating point instructions by mixing MMX code and floating point code 8 5 4 1 RECOMMENDATIONS AND GUIDELINES Do not mix MMX code and floating point code at the instruction level for the following reasons The TOS top of stack value of the floating point status word is set to O after each MMX instruction
76. occurring while the kernel is executing cannot happen in native mode The problem as actually found in practical code in a MS DOS compatibility system happens when the DNA handler uses FNSAVE to switch FPU contexts If an FPU exception is active then FNSAVE triggers FERR briefly which usually will cause the FPU exception handler to be invoked inside the DNA handler In native mode neither FNSAVE nor any other no wait instructions can trigger interrupt 16 As discussed above FERR gets asserted independent of the value of the NE bit but when NE 1 the O S should not enable its path through the PIC Another possible very rare way a floating point exception interrupt could occur while the kernel is executing is by an FPU immediate exception case having its interrupt delayed by the external hardware until execution has switched to the kernel This also cannot happen in native mode because there is no delay through external hardware Thus the native mode FPU exception handler can omit the test to see if the kernel is the FPU owner and the DNA handler for a native mode system can omit the step of setting the kernel as the FPU owner at the handler s beginning Since however these simplifications are minor and save little code it would be a reasonable and conservative habit as long as the MS DOS compatibility mode is widely used to include these steps in all systems Note that the special DP Dual Processing mode for Pentium Processors and also
77. of the operating system or executive When multitasking is imple mented individual tasks can use different memory models Real address mode When in real address mode the processor only supports the real address mode memory model System management mode When in SMM the processor switches to a separate address space called the system management RAM SMRAM The memory model used to address bytes in this address space is similar to the real address mode model See Chapter 11 in the Intel Architecture Software Developer s Manual Volume 3 for more information on the memory model used in SMM 3 4 intel e BASIC EXECUTION ENVIRONMENT 3 5 32 BIT VS 16 BIT ADDRESS AND OPERAND SIZES The processor can be configured for 32 bit or 16 bit address and operand sizes With 32 bit address and operand sizes the maximum linear address or segment offset is FFFFFFFFH 232 and operand sizes are typically 8 bits or 32 bits With 16 bit address and operand sizes the maximum linear address or segment offset is FFFFH 2 5 and operand sizes are typically 8 bits or 16 bits When using 32 bit addressing a logical address or far pointer consists of a 16 bit segment selector and a 32 bit offset when using 16 bit addressing it consists of a 16 bit segment selector and a 16 bit offset Instruction prefixes allow temporary overrides of the default address and or operand sizes from within a program When operating in protected mode the
78. operand addressing 5 8 5 9 DIN instruction 6 23 Division by zero exception 4 7 49 Double extended precision IEEE floating point formats cssc ee eta 7 25 Double precision IEEE floating point formiab o mte n he Gee 7 25 Double real floating point format 7 25 Do blewotd 5 1 DS register 3 7 3 9 DX register 3 7 Dynamic data flow analysis 2 7 Dynamic execution 2 7 E EAXrGglslor x ee iere cte 3 5 3 5 4 4 4 6 EBX 3 5 ECX register e aem 3 5 EDI tegister er yam m ems 3 5 EDX urere enn 3 5 Effective address 5 8 EFLAGS Condition Codes B 1 EFLAGS register 3 10 restoring from procedure stack 4 7 saving on a procedure call 4 7 status flags 7 15 7 16 7 35 EIP register mre 3 8 3 14 EMMS instruction 8 9 8 11 ENTER instruction 4 16 6 36 ES register lu unusual s ima n eges 3 7 3 9 ES exception summary flag FPU status word 7 55 ESC instructions FPU 7 30 ESI register iu l n eee A mes 3 5 ESP register 3 5 4 1 4 3 4 4 Exception flags FPU status word
79. operation similar to the WAIT FWAIT instructions to check for and handle pending unmasked FPU exceptions before they perform their primary operation such as adding two real numbers These instructions are called waiting instructions Some of the FPU control instructions such as FSTSW ENSTSW have both a waiting and a non waiting versions The waiting version with the F prefix executes a wait operation before it performs its primary operation whereas the non waiting version with the FN prefix ignores pending unmasked exceptions Non waiting instructions allow software to save the current FPU state without first handling pending excep tions or to reset or reinitialize the FPU without regard for pending exceptions NOTE When operating a Pentium or Intel486 processor in MS DOS compatibility mode it is possible under unusual circumstances for a non waiting instruction to be interrupted prior to being executed to handle a pending FPU exception The circumstances where this can happen and the resulting action of the processor are described in Section D 2 1 3 No Wait FPU Instructions Can Get FPU Interrupt in Window When operating a Pentium Pro processor in MS DOS compatibility mode non waiting instructions can not be interrupted in this way see Section D 2 2 MS DOS Compatibility Mode in the Pentium Pro Processor 7 40 Intel FLOATING POINT UNIT 7 5 13 Unsupported FPU Instructions The Intel 8087 instruc
80. operations EDX I O pointer ESI Pointer to data in the segment pointed to by the DS register source pointer for string operations EDI Pointer to data or destination in the segment pointed to by the ES register destination pointer for string operations ESP Stack pointer in the SS segment EBP Pointer to data on the stack in the SS segment As shown in Figure 3 4 the lower 16 bits of the general purpose registers map directly to the register set found in the 8086 and Intel 286 processors and can be referenced with the names AX BX CX DX BP SP SI and DI Each of the lower two bytes of the EAX EBX ECX and EDX registers can be referenced by the names AH BH CH and DH high bytes and AL BL CL and DL low bytes General Purpose Registers 31 1615 87 0 16 bit 32 bit AH AL AX EAX BH BL BX EBX CH CL CX ECX DH DL DX EDX BP EBP SI ESI DI EDI SP ESP Figure 3 4 Alternate General Purpose Register Names 3 6 2 Segment Registers The segment registers CS DS SS ES FS and GS hold 16 bit segment selectors A segment selector is a special pointer that identifies a segment in memory To access a particular segment in memory the segment selector for that segment must be present in the appropriate segment register When writing application code you generally create segment selectors with assembler direc tives and symbols The assembler
81. order parts of each 32 bit result The PMULHW packed multiply high and PMULLW packed multiply low instructions multiply the signed words of the source and destination operands and write the high order or low order 16 bits of each of the results to the destination operand 8 3 2 3 PACKED MULTIPLY ADD The PMADDWD packed multiply and add instruction calculates the products of the signed words of the source and destination operands The four intermediate 32 bit doubleword products are summed in pairs to produce two 32 bit doubleword results 8 3 8 Comparison Instructions The PCMPEQB PCMPEQW and PCMPEQD packed compare for equal and PCMPGTB PCMPGTW and PCMPGTD packed compare for greater than instructions compare the corre sponding data elements in the source and destination operands for equality or value greater than respectively These instructions generate a mask of ones or zeros which are written to the desti nation operand Logical operations can use the mask to select elements This can be used to 8 8 Intel PROGRAMMING WITH THE INTEL MMX TECHNOLOGY implement a packed conditional move operation without a branch or a set of branch instructions No flags are set These instructions support packed byte packed word and packed doubleword data types 8 3 4 Conversion Instructions The conversion instructions convert the data elements within a packed data type The PACKSSWB and PACKSSDW packed with signed satura
82. pass a large number of parameters to the called procedure the parameters can be placed on the stack in the stack frame for the calling procedure Here it is useful to use the stack frame base pointer in the EBP register to make a frame boundary for easy access to the parameters The stack can also be used to pass parameters back from the called procedure to the calling procedure 4 6 Intel PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS 4 3 3 3 PASSING PARAMETERS IN AN ARGUMENT LIST An alternate method of passing a larger number of parameters or a data structure to the called procedure is to place the parameters in an argument list in one of the data segments in memory A pointer to the argument list can then be passed to the called procedure through a general purpose register or the stack Parameters can also be passed back to the calling procedure in this same manner 4 3 4 Saving Procedure State Information The processor does not save the contents of the general purpose registers segment registers or the EFLAGS register on a procedure call A calling procedure should explicitly save the values in any of the general purpose registers that it will need when it resumes execution after a return These values can be saved on the stack or in memory in one of the data segments The PUSHA and POPA instruction facilitates saving and restoring the contents of the general purpose registers PUSHA pushes the values in all the general pur
83. positions on the upper end of the operand This instruc tion supports packed word and packed doubleword data types 8 3 7 EMMS Empty MMX State Instruction The EMMS instruction empties the MMX state This instruction must be used to clear the MMX state empty the floating point tag word at the end of an MMX routine before calling other routines that can execute floating point instructions 8 9 PROGRAMMING WITH THE INTEL MMX TECHNOLOGY intel 8 4 COMPATIBILITY WITH FPU ARCHITECTURE The MMX state is aliased upon the Intel Architecture floating point state No new state or mode is added to support the MMX technology The same floating point instructions that save and restore the floating point state also handle the MMX state for example during context switching MMX technology uses the same interface techniques between the floating point architecture and the operating system primarily for task switching purposes For more details see Chapter 10 MMX Technology System Programming Model in the Intel Architecture Software Devel oper s Manual Volume 3 8 4 1 MMX Instructions and the Floating Point Tag Word After each MMX instruction the entire floating point tag word is set to Valid 00s The Empty MMX state EMMS instruction sets the entire floating point tag word to Empty 11s Chapter 10 MMX Technology System Programming Model in the Intel Architecture Software Developer s Manual Volume 3 describes
84. processor also supports segmented addressing This is a form of addressing where a program may have many independent address spaces called segments For example a program can keep its code instructions and stack in separate segments Code addresses would always refer to the code space and stack addresses would always refer to the stack space The following notation is used to specify a byte address within a segment Segment register Byte address For example the following segment address identifies the byte at address FF79H in the segment pointed by the DS register DS FF79H The following segment address identifies an instruction address in the code segment The CS register points to the code segment and the EIP register contains the address of the instruction CS EIP 1 4 6 Exceptions An exception is an event that typically occurs when an instruction causes an error For example an attempt to divide by zero generates an exception However some exceptions such as break points occur under other conditions Some types of exceptions may provide error codes An error code reports additional information about the error An example of the notation used to show an exception and error code is shown below PF fault code This example refers to a page fault exception under conditions where an error code naming a type of fault is reported Under some conditions exceptions which produce error codes may not be able to report an accura
85. program must first load the segment selector for the segment to be accessed into a segment register Linear Address Space for Program Segment Registers Overlapping Segments of up to BS z 4G Bytes Beginning at Address 0 ES FS GS A The segment selector in each segment register points to an overlapping segment in the linear address space Figure 3 5 Use of Segment Registers for Flat Memory Model 3 8 intel e BASIC EXECUTION ENVIRONMENT Code Segment Segment Registers gt Data CS Segment DS Stack SS Segment ES 34 All segments FS gt are mapped GS gt to the same linear address space Data Segment Data Segment gt Data Segment gt Figure 3 6 Use of Segment Registers in Segmented Memory Model Each of the segment registers is associated with one of three types of storage code data or stack For example the CS register contains the segment selector for the code segment where the instructions being executed are stored The processor fetches instructions from the code segment using a logical address that consists of the segment selector in the CS register and the contents of the EIP register The EIP register contains the linear address within the code seement of the next instr
86. software performance by allowing the same operation to be carried out on multiple data elements in parallel The MMX technology supports parallel operations on byte word and doubleword data elements when contained in MMX registers 8 3 PROGRAMMING WITH THE INTEL MMX TECHNOLOGY intel The SIMD execution model supported in the MMX technology directly addresses the needs of modern media communications and graphics applications which often use sophisticated algo rithms that perform the same operations on a large number of small data types bytes words and doublewords For example most audio data is represented in 16 bit word quantities The MMX instructions can operate on 4 of these words simultaneously with one instruction Video and graphics information is commonly represented as palletized 8 bit byte quantities Here one MMX instruction can operate on 8 of these bytes simultaneously 8 1 4 Memory Data Formats When stored in memory the bytes words and doublewords in the packed data types are stored in consecutive addresses with the least significant byte word or doubleword being stored in the at the lowest address and the more significant bytes words or doubleword being stored at consecutively higher addresses see Figure 8 3 The ordering bytes words or doublewords in memory is always little endian That is the bytes with the lower addresses are less significant than the bytes with the higher addresses 56 55
87. stops further execution of the numeric instruction and causes a branch to a software exception handler When an FPU exception handler will be invoked synchronization must always be considered to assure reliable performance The following examples illustrate the need to always consider exception synchronization when writing numeric code even when the code is initially intended for execution with exceptions masked D 3 3 2 EXCEPTION SYNCHRONIZATION EXAMPLES In the following examples three instructions are shown to load an integer calculate its square root then increment the integer The synchronous execution of the FPU will allow both of these programs to execute correctly with INC COUNT being executed in parallel in the processor as long as no exceptions occur on the FILD instruction However if the code is later moved to an environment where exceptions are unmasked the code in the first example will not work cor rectly Incorrect Error Synchronization FILD COUNT FPU instruction INC COUNT integer instruction alters operand FSORT subsequent FPU instruction error from previous FPU instruction detected here Proper Error Synchronization FILD COUNT FPU instruction FSORT subsequent FPU instruction error from previous FPU instruction detected here INC COUNT integer instruction alters operand GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS intel In some operating systems supporting the FPU the numer
88. subsequently transferred back into memory from any of the FPU registers the results can be left in the extended real format or converted back into one of the other FPU formats real integer or packed BCD inte gers shown in Figure 7 17 The FPU instructions treat the eight FPU data registers as a register stack see Figure 7 6 All addressing of the data registers is relative to the register on the top of the stack The register number of the current top of stack register is stored in the TOP stack TOP field in the FPU status word Load operations decrement TOP by one and load a value into the new top of stack register and store operations store the value from the current TOP register in memory and then increment TOP by one For the FPU a load operation is equivalent to a push and a store oper ation is equivalent to a pop 7 9 FLOATING POINT UNIT FPU Data Registers i 7978 64 63 0 Sign R7 Exponent Significand R6 R5 R4 R3 R2 R1 RO 15 47 0 PRADA FPU Instruction Pointer Status FPU Operand Data Pointer Register p Register Opcode Figure 7 5 FPU Execution Environment If a load operation is performed when TOP is at 0 register wraparound occurs and the new value of TOP is set to 7 The floating point stack overflow exception indicates when wraparound might cause an unsaved value to be overwritten see Section 7 8 1
89. that they set the status flags ZF PF and CF in the EFLAGS register to indicate the results of the comparison see Table 7 16 instead of the FPU condition code flags The FCOMI and FCOMIP instructions allow condition branch instructions Jcc to be executed directly from the results of their comparison Table 7 16 Setting of EFLAGS Status Flags for Real Number Comparisons Comparison Results ZF PF CF STO gt ST i 0 0 0 STO ST i 0 0 1 STO ST i 1 0 0 Unordered 1 1 1 The FUCOMI and FUCOMIP instructions operate the same as the FCOMI and FCOMIP instructions except that they do not generate a floating point invalid operation exception if the 7 35 FLOATING POINT UNIT intel unordered condition is the result of one or both of the operands being a QNaN The FCOMIP and FUCOMIP instructions pop the FPU register stack following the comparison operation The FXAM instruction determines the classification of the real value in the ST 0 register that is whether the value is zero a denormal number a normal finite number a NaN or unsup ported format or that the register is empty It sets the FPU condition code flags to indicate the classification see FXAM Examine in Chapter 3 Instruction Set Reference of the Intel Architecture Software Developer s Manual Volume 2 It also sets the C1 flag to indicate the sign of the value 7 5 6 1 BRANCHING ON THE FPU CONDITION CODES The pr
90. that would have been obtained by subtracting the destination operand from the value in the EAX register The CMPXCRG instruction is commonly used for testing and modifying semaphores It checks to see if a semaphore is free If the semaphore is free it is marked allocated otherwise it gets the ID of the current owner This is all done in one uninterruptible operation In a single processor system the CMPXCHG instruction eliminates the need to switch to protection level 0 to disable interrupts before executing multiple instructions to test and modify a semaphore For multiple processor systems CMPXCHG can be combined with the LOCK prefix to perform the compare and exchange operation atomically See Locked Atomic Operations in Chapter 7 of the Intel Architecture Software Developer s Manual Volume 3 for more information on atomic opera tions The CMPXCHGSB instruction also requires three operands a 64 bit value in EDX EAX a 64 bit value in ECX EBX and a destination operand in memory The instruction compares the 64 bit value in the EDX EAX registers with the destination operand If they are equal the 64 bit value in the ECX EBX register is stored in the destination operand If the EDX EAX register and the destination are not equal the destination is loaded in the EDX EAX register The CMPXCHG8B instruction can be combined with LOCK prefix to perform the operation atomically 6 3 2 Stack Manipulation Instructions The PUSH POP
91. the processor when it recognizes the exception is often not in the context in which it occurred The processor may have changed many of its internal registers and be executing a totally different program by the time the exception occurs If the exception handler cannot recapture the original context it cannot reliably determine the cause of the exception or to recover successfully from the exception To handle this situation the FPU has special registers updated at the start of each numeric instruction to describe the state of the numeric program when the failed instruction was attempted This provides tools to help the exception handler recapture the original context but the application code must also be written with synchronization in mind Overall exception synchronization must ensure that the FPU and other relevant parts of the context are in a well defined state when the handler is invoked after an unmasked numeric exception occurs When the FPU signals an unmasked exception condition it is requesting help The fact that the exception was unmasked indicates that further numeric program execution under the arithmetic and programming rules of the FPU will probably yield invalid results Thus the exception must be handled and with proper synchronization or the program will not operate reliably For programmers in higher level languages all required synchronization is automatically provided by the appropriate compiler However for assembly langu
92. the full protection of the 80286 memory management and protection features The Intel 387 DX and SX math coprocessors are Intel s third generation numeric processors They implement the final IEEE Std 754 adding new trigonometric instructions and using a new design and CHMOS III process to allow higher clock rates and require fewer clocks per instruc tion Together the Intel 387 math coprocessor with additional instructions and the improved standard brought even more convenience and reliability to numeric programming and made this convenience and reliability available to applications that need the high speed and large memory capacity of the 32 bit environment of the Intel386 microprocessor The Intel486 processor FPU is an on chip equivalent of the Intel 387 DX math coprocessor conforming to both IEEE Std 754 and the more recent generalized IEEE Std 854 Having the FPU on chip results in a considerable performance improvement in numeric intensive computa tion The Pentium processor FPU has been completely redesigned over the Intel486 processor FPU while maintaining conformance to both the IEEE Std 754 and 854 Faster algorithms provide at least three times the performance over the Intel486 processor FPU for common operations including ADD MUL and LOAD Many applications can achieve five times the performance of the Intel486 processor FPU or more with instruction scheduling and pipelined execution 2 4 INTRODUCTION TO THE PENTIUM PRO PROC
93. to a register that already contains a value The term stack underflow refers to the opposite condition from stack overflow Here a program has popped eight values from the FPU register stack and the next value popped from the stack causes stack wraparound to an empty register 7 8 1 2 INVALID ARITHMETIC OPERAND EXCEPTION IA The FPU is able to detect a variety of invalid arithmetic operations that can be coded in a program These operations generally indicate a programming error such as dividing by eo Table 7 20 lists the invalid arithmetic operations that the FPU detects This group includes the invalid operations defined in IEEE Std 854 When the FPU detects an invalid arithmetic operand it sets the IE flag bit 0 in the FPU status word to 1 If the invalid operation exception is masked the FPU then returns an indefinite value to the destination operand or sets the floating point condition codes as shown in Table 7 20 If the invalid operation exception is not masked a software exception handler is invoked see Section 7 7 3 Software Exception Handling and the top of stack pointer TOP and source operands remain unchanged 7 48 Intel FLOATING POINT UNIT Table 7 20 Invalid Arithmetic Operations and the Masked Responses to Them Condition Masked Response Any arithmetic operation on an operand that is in an unsupported format Return the real indefinite value to the destination operand Any ar
94. to access it a word that starts on an odd address but does not cross a word boundary is considered aligned and can still be accessed in one bus cycle EH 7AH pH A Word at Address BH FEH CH Doubleword at Address AH Contains FEO6H 06H BH Contains 7AFE0636H Y 36H AH Byte at Address 9H 1FH 9H Wa Contains 1FH Quadword at Address 6H A A4H 8H Contains 7AFE06361FA4230BH Word at Address 6H 23H 7H Contains 230BH OBH 6H y 5H 4H Word at Address 2H Contains 74CBH 74 3H Word at Address 1H CBH 2H Contains CB31H 31H 1H 0H Figure 5 2 Bytes Words Doublewords and Quadwords in Memory 5 2 NUMERIC POINTER BIT FIELD AND STRING DATA TYPES Although bytes words and doublewords are the fundamental data types of the Intel Architec ture some instructions support additional interpretations of these data types to allow operations to be performed on numeric data types signed and unsigned integers and BCD integers See Figure 5 3 Also some instructions recognize and operate on additional pointer bit field and string data types The following sections describe these additional data types 5 2 1 Integers Integers are signed binary numbers held in a byte word or doubleword All operations assume a two s complement representation The sign bit is located in bit 7 in a byte integer bit 15 ina word integer and bit 31 in a doubleword integer The sign bit is set for nega
95. to the ST 0 register the register at the top of the register stack rather than by a physical register number Often the ST 0 register is an implied operand Operands in memory can be referenced using the same operand addressing methods available for the integer and system instructions 7 5 3 Data Transfer Instructions The data transfer instructions see Table 7 13 perform the following operations Load real integer or packed BCD operands from memory into the ST 0 register Store the value in the ST 0 register in memory in real integer or packed BCD format Move values between registers in the FPU register stack Table 7 13 Data Transfer Instructions Real Integer Packed Decimal FLD Load Real FILD Load Integer FBLD Load Packed Decimal FST Store Real FIST Store Integer FSTP Store Real and FISTP Store Integer FBSTP Store Packed Pop and Pop Decimal and Pop FXCH Exchange Register Contents FCMOVcc Conditional Move Operands are normally stored in the FPU data registers in extended real format see Section 7 3 4 2 Precision Control Field The FLD load real instruction pushes a real operand from memory onto the top of the FPU data register stack If the operand is in single or double real format it is automatically converted to extended real format This instruction can also be used to push the value in a selected FPU data register onto the top of the register stack The FILD loa
96. units two floating point units and one memory interface unit allowing up to five micro ops can be scheduled per clock The two integer units can handle two integer micro ops in parallel One of the integer units is designed to handle branch micro ops This unit has the ability to detect branch mispredictions and signal the branch target buffer to restart the pipeline This operation is handled as follows The instruction decoder tags each branch micro op with both branch destination addresses the predicted destination and the fall through destination When the integer unit executes the branch micro op it is able to determine whether the predicted or the fall through destination was taken If the predicted branch is taken then speculatively executed micro ops are marked usable and execution continues along the predicted instruction path If the predicted branch was not taken a jump execution unit in the integer unit changes the status of all of the micro ops following the branch to remove them from the instruction pool It then provides the proper branch destination to the branch target buffer which in turn restarts the pipeline from the new target address The memory interface unit handles load and store micro ops A load access only needs to specify the memory address so it can be encoded in one micro op A store access needs to specify both an address and the data to be written so it is encoded in two micro ops The part of the memory interf
97. with an FIST FISTP instruction and the invalid operation exception is masked the FPU stores the integer indefinite encoding in the destination operand as a masked response to the exception In situations where the origin of a value with this encoding may be ambiguous the invalid opera tion exception flag can be examined to see if the value was produced as a response to an exception If the integer indefinite is stored in memory and is later loaded back into an FPU data register it is interpreted as the largest negative number supported by the format 7 4 8 Decimal Integers Decimal integers are stored in a 10 byte packed BCD format Table 7 8 gives the precision and range of this data type and Figure 7 17 shows the format In this format the first 9 bytes hold 18 BCD digits 2 digits per byte see Section 5 2 3 BCD Integers The least significant digit is contained in the lower half byte of byte 0 and the most significant digit is contained in the upper half byte of byte 9 The most significant bit of byte 10 contains the sign bit 0 positive and 1 negative Bits O through 6 of byte 10 are don t care bits Negative decimal integers are not stored in two s complement form they are distinguished from positive decimal integers only by the sign bit Table 7 11 gives the possible encodings of value in the decimal integer data type The decimal integer format exists in memory only When a decimal integer is loaded in a data regis
98. 001111 After 1 bit SHL SAL Instruction 1 4 1000100010001000100010001000117 110 0 After 10 bit SHL SAL Instruction 0 MH 00100010001000100011110000000000 0 Figure 6 6 SHL SAL Instruction Operation The SHR instruction shifts the source operand right by from 1 to 31 bit positions see Figure 6 7 As with the SHL SAL instruction the empty bit positions are cleared and the CF flag is loaded with the last bit shifted out of the operand 6 25 INSTRUCTION SET SUMMARY intel e Initial State Operand CF 10001000100010001000100010001111 X After 1 bit SHR Instruction 0 1000100010001000100010001000 111 4 1 After 10 bit SHR Instruction 0 00000000001000100010001000100010m210 Figure 6 7 SHR Instruction Operation The SAR instruction shifts the source operand right by from 1 to 31 bit positions see Figure 6 8 This instruction differs from the SHR instruction in that it preserves the sign of the source operand by clearing empty bit positions if the operand is positive or setting the empty bits if the operand is negative Again the CF flag is loaded with the last bit shifted out of the operand The SAR and SHR instructions can also be used to perform division by powers of 2 see SAL SAR SHL SHR Shift Instructions in Chapter 3 of the Intel Architecture Software Developer s Manual Volume 2 Initial State
99. 1 Stack Overflow or Under flow Exception IS FPU Data Register Stack Growth Stack ST 2 ST 1 Top ST 0 2 011B N wo QO O 7 10 Figure 7 6 FPU Data Register Stack ntel e FLOATING POINT UNIT Many floating point instructions have several addressing modes that permit the programmer to implicitly operate on the top of the stack or to explicitly operate on specific registers relative to the TOP Assemblers supports these register addressing modes using the expression ST 0 or simply ST to represent the current stack top and ST 1 to specify the ith register from TOP in the stack 0 i 7 For example if TOP contains 011B register 3 is the top of the stack the following instruction would add the contents of two registers in the stack registers 3 and 5 FADD ST ST 2 Figure 7 7 shows an example of how the stack structure of the FPU registers and instructions are typically used to perform a series of computations Here a two dimensional dot product is computed as follows 1 The first instruction FLD value1 decrements the stack register pointer TOP and loads the value 5 6 from memory into ST 0 The result of this operation is shown in snap shot a 2 The second instruction multiplies the value in ST 0 by the value 2 4 from memory and stores the result in ST 0 shown in snap shot b 3 The third instruction
100. 2 4 or 8 bytes The displacement component encodes the static offset to the beginning of the array The base register holds the results of a calculation to determine the offset to a specific element within the array To access a field of a record The base register holds the address of the beginning of the record while the displacement is an static offset to the field An important special case of this combination is access to parameters in a procedure activation record A procedure activation record is the stack frame created when a procedure is entered Here the EBP register is the best choice for the base register because it automatically selects the stack segment This is a compact encoding for this common function Index Scale Displacement This address mode offers an efficient way to index into a static array when the element size is 2 4 or 8 bytes The displacement locates the beginning of the array the index register holds the subscript of the desired array element and the processor automatically converts the subscript into an index by applying the scaling factor Base Index Displacement Using two registers together supports either a two dimensional array the displacement holds the address of the beginning of the array or one of several instances of an array of records the displacement is an offset to a field within the record Base Index Scale Displacement Using all the addressing components toget
101. 2 Table Lookup Instructions The XLAT and XLATB table lookup instructions replace the contents of the AL register with a byte read from a translation table in memory The initial value in the AL register is interpreted as an unsigned index into the translation table This index is added to the contents of the EBX register which contains the base address of the table to calculate the address of the table entry These instructions are used for applications such as converting character codes from one alphabet into another for example an ASCII code could be used to look up its EBCDIC equiv alent in a table 6 15 3 Processor Identification Instruction The CPUID processor identification instruction provides information about the processor on which the instruction is executed To obtain processor information a value of from 0 to 2 is loaded in the EAX register and then the CPUID instruction is executed The resulting processor information is placed in the EAX EBX ECX and EDX registers Table 6 5 shows the informa tion that is provided depending on the value initially entered in the EAX register See Section 10 1 Processor Identification for detailed information on the output of the CPUID instruc tion Table 6 5 Information Provided by the CPUID Instruction Initial EAX Value Information Provided about the Processor 0 Maximum CPUID input value Vendor identification string Genuinelntel 1 Version information
102. 22 6 23 Integer FPU data type description of 7 27 7 27 Inter privilege level call description of 4 7 etes 4 9 Inter privilege level return description of 4 7 A cetero e rem els 4 9 Interrupt 4 11 Interrupt handler 4 10 Interrupt vector 4 11 Interrupts description of 4 10 implicit call to an interrupt handler procedure 4 11 implicit call to an interrupt handler task 4 14 in real address mode 4 15 maskable 4 11 summaryof 4 12 user defined 4 11 VOCIO s e S Mes a Sey ma TG aa a 4 11 INTn instruction 6 34 INTO instruction 4 15 6 34 6 39 Invalid arithmetic operand exception A FPU description of 7 48 masked responseto 7 49 Invalid operation exception 7 47 INVD instruction 6 2 INVLPG instruction 6 2 IOPL I O privilege level field EFLAGS register 3 13 9 4 IRET instruction 3 14 4 14 4 15 6 31 6 39 9 5 I O address space 9 2 I O instructions overview Of
103. 4 20 intel PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS Old EBP Main s EBP Main s EBP Main s EBP Procedure A s EBP Procedure A s EBP Main s EBP Procedure A s EBP Procedure B s EBP Procedure B s EBP lt 1 Main s EBP Display Procedure A s EBP Procedure C s EBP Dynamic Storage ESP Figure 4 10 Stack Frame after Entering Procedure C 4 5 2 LEAVE Instruction The LEAVE instruction which does not have any operands reverses the action of the previous ENTER instruction The LEAVE instruction copies the contents of the EBP register into the ESP register to release all stack space allocated to the procedure Then it restores the old value of the EBP register from the stack This simultaneously restores the ESP register to its original value A subsequent RET instruction then can remove any arguments and the return address pushed on the stack by the calling program for use by the procedure 4 21 Intel Data Types and Addressing Modes intel CHAPTER 5 DATA TYPES AND ADDRESSING MODES This chapter describes data types and addressing modes available to programmers of the Intel Architecture processors 5 1 FUNDAMENTAL DATA TYPES The fundamental data types of the Intel Architecture are bytes words doublewords and quad words see Figure 5 1 A byte is eight bits a word i
104. 48 47 40 39 32 31 24 23 16 15 Byte 7 Byte 6 Byte 5 Byte 4 Byte 3 Byte 2 Byte 1 Byte 0 Memory Address 1008h Memory Address 1000h 3006045 Figure 8 3 Eight Packed Bytes in Memory at address 1000H 8 1 5 Data Formats for MMX Registers Values in MMX registers have the same format as a 64 bit quantity in memory MMX registers have two data access modes 64 bit access mode and 32 bit access mode The 64 bit access mode is used for 64 bit memory access 64 bit transfer between MMX regis ters all pack logical and arithmetic instructions and some unpack instructions The 32 bit access mode is used for 32 bit memory access 32 bit transfer between integer regis ters and MMX registers and some unpack instructions 8 2 MMX INSTRUCTION SET The MMX instruction set consists of 57 instructions grouped into the following categories Data transfer instructions Arithmetic instructions 8 4 Intel PROGRAMMING WITH THE INTEL MMX TECHNOLOGY Comparison instructions Conversion instructions Logical instructions Shift instructions Empty MMX state instruction EMMS When operating on packed data within an MMX register the data is cast by the type specified by the instruction For example the PADDB add packed bytes instruction treats the packed data in an MMX register as 8 packed bytes whereas the PADDW add packed words instruc tion treats the packed data as 4 packed words 8 2 1
105. 56 55 48 47 40 39 32 31 24 23 16 15 87 0 Packed word 4x16 bits 63 48 47 32 31 16 15 0 Packed doublewords 2x32 bits 63 32 31 0 Quadword 64 bits 63 0 e E 3006002 Figure 8 2 MMX Data Types The MMX instructions move the packed data types packed bytes packed words or packed doublewords and the quadword data type to and from memory or to and from the Intel Archi tecture general purpose registers in 64 bit blocks However when performing arithmetic or logical operations on the packed data types the MMX instructions operate in parallel on the individual bytes words or doublewords contained in a 64 bit MMX register as described in the following section Section 8 1 3 Single Instruction Multiple Data SIMD Execution Model When operating on the bytes words and doublewords within packed data types the MMX instructions recognize and operate on both signed and unsigned byte integers word integers and doubleword integers 8 1 3 Single Instruction Multiple Data SIMD Execution Model The MMX technology uses the single instruction multiple data SIMD technique for performing arithmetic and logical operations on the bytes words or doublewords packed into 64 bit MMX registers For example the PADDSB instruction adds 8 signed bytes from the source operand to 8 signed bytes in the destination operand and stores 8 byte results in the desti nation operand This SIMD technique speeds up
106. 6 4 shows the mnemonics for the CMOV cc instructions and the conditions being tested for each instruction The condition code mnemonics are appended to the letters CMOV to form the mnemonics for the CMOVcc instructions The instructions listed in Table 6 4 as pairs for example CMOVA CMOVNBE are alternate names for the same instruction The assembler provides these alternate names to make it easier to read program listings The CMOVcc instructions are useful for optimizing small IF constructions They also help elim inate branching overhead for IF statements and the possibility of branch mispredictions by the processor These instructions may not be supported on some processors in the Pentium Pro processor family Software can check if the CMOVcc instructions are supported by checking the processor s feature information with the CPUID instruction see CPUID CPU Identification in Chapter 3 of the Intel Architecture Software Developer s Manual Volume 2 6 3 1 3 EXCHANGE INSTRUCTIONS The exchange instructions swap the contents of one or more operands and in some cases performs additional operations such as asserting the LOCK signal or modifying flags in the EFLAGS register The XCHG exchange instruction swaps the contents of two operands This instruction takes the place of three MOV instructions and does not require a temporary location to save the contents of one operand location while the other is being loaded When a memory ope
107. 8 3 2 1 Packed Addition And 5 8 8 8 3 2 2 Packed Multiplication 8 8 8 3 2 3 Packed Multiply 8 8 8 3 3 Comparison Instructions eee 8 8 8 3 4 Conversion Instructions mh 8 9 8 3 5 logical Instr ctl rnis s rhe tene RR ee RD S oa Sty a Rd 8 9 8 3 6 shift Instr ctlorns eo co menle p RUE 8 9 8 3 7 EMMS Empty MMX State Instruction 8 9 8 4 COMPATIBILITY WITH FPU ARCHITECTURE 8 10 8 4 1 MMX Instructions and the Floating Point Tag Word 8 10 8 4 2 Effect of Instruction Prefixes on MMX Instructions 8 10 8 5 WRITING APPLICATIONS WITH MMX CODE 8 10 8 5 1 Detecting Support for MMX Technology Using the CPUID Instruction 8 11 8 5 2 Using the EMMS Instruction 8 11 8 5 3 Interfacing with MMX 8 12 8 5 4 Writing Code with MMX and Floating Point Instructions 8 13 8 5 4 1 RECOMMENDATIONS AND GUIDELINES 8 13 8 5 5 Using MMX Code in a Multitasking Operating System Environment 8 14 8 5 5 1 COOPERATIVE MULTITA
108. AVE ENS AVE or by overwriting the flags with an FRSTOR or FLDENV instruction This allows a programmer to mask all exceptions run a calculation and then inspect the status word to see if any exceptions were detected at any point in the calculation Intel GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS D 3 2 2 SOFTWARE EXCEPTION HANDLING If the FPU in or with an Intel Architecture processor Intel 286 and onwards encounters an unmasked exception condition with the system operated in the MS DOS compatibility mode and with IGNNE not asserted a software exception handler is invoked through a PIC and the processor s INTR pin The FERR or ERROR output from the FPU that begins the process of invoking the exception handler may occur when the error condition is first detected or when the processor encounters the next WAIT or FPU instruction Which of these two cases occurs depends on the processor generation and also on which exception and which FPU instruction triggered it as discussed earlier in Section D 1 Origin of the MS DOS Compatibility Mode for Handling FPU Exceptions and Section D 2 Implementation of the MS DOS Compati bility Mode In the Intel486 Pentium and Pentium Pro Processors The elapsed time between the initial error signal and the invocation of the FPU exception handler depends of course on the external hardware interface and also on whether the external interrupt for FPU errors is enabled But the architecture
109. Although all of these registers are available for general storage of operands results and pointers caution should be used when referencing the ESP register The ESP register holds the stack pointer and as a general rule should not be used for any other purpose Many instructions assign specific registers to hold operands For example string instructions use the contents of the ECX ESI and EDI registers as operands When using a segmented memory model some instructions assume that pointers in certain registers are relative to specific segments For instance some instructions assume that a pointer in the EBX register points to a memory location in the DS segment The special uses of general purpose registers by instructions are described in Chapter 6 Instruc tion Set Summary in this volume and Chapter 3 Instruction Set Reference in the Intel Architec ture Software Developer s Manual Volume 2 The following is a summary of these special uses EAX Accumulator for operands and results data EBX Pointer to data in the DS segment General Purpose Registers 31 0 EAX EBX ECX EDX ESI EDI EBP ESP Segment Registers 15 0 CS DS ss ES FS GS 31 Status and Control Registers 0 EFLAGS 31 0 EIP Figure 3 3 Application Programming Registers 3 6 intel e BASIC EXECUTION ENVIRONMENT ECX Counter for string and loop
110. CLR Pentium or Pentium Pro hi processor O PR 5V IGNNE lt INTR lt Interrupt Controller Legend FF n Flip Flop n CLR Clear or Reset Figure D 1 Recommended Circuit for MS DOS Compatibility FPU Exception Handling D 6 Intel GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS In the circuit in Figure D 1 when the FPU exception handler accesses I O port OFOH it clears the IRQ13 interrupt request output from Flip Flop 1 and also clocks out the IGNNE signal active from Flip Flop 2 So the handler can activate IGNNE if needed by doing this access before clearing the FPU exception condition which de asserts FERR However the circuit does not depend on the order of actions by the FPU exception handler to guarantee the correct hardware state upon exit from the handler Flip Flop 2 which drives IGNNE to the processor has its CLEAR input attached to the inverted FERR This ensures that IGNNE can never be active when FERR is inactive So if the handler clears the FPU exception condition before the OFOH access IGNNE does not get activated and left on after exit from the handler FERR FP_IRQ IGNNE A B C Figure D 2 Behavior of Signals During FPU Exception Handling D 2 1 3 NO WAIT FPU INSTRUCTIONS CAN GET FPU INTERRUPT IN WINDOW The Pentium and Intel486 processors implement the no wait floating point instructions FNINIT FNCLEX
111. Contd Mnemonic Instruction 5 AD Z 0 U P FIADD Integer add Y Y Y Y Y Y FICOM P Integer compare Y Y Y FIDIV Integer divide Y Ye Y Y Y Y FIDIVR Integer divide reversed Y Y Y XII Y NY Y FILD Integer load Y FIMUL Integer multiply Y Y Y Y Y Y FINCSTP Increment stack pointer FINIT Initialize processor FIST P Integer store Y Y Y FISUB R Integer subtract Y Y Y Y Y Y FLD extended or stack Load real Y FLD single or double Load real Y Y Y FLD1 Load 1 0 Y FLDCW Load Control word Y Y Y Y Y Y Y FLDENV Load environment Y Y Y Y Y Y Y FLDL2E Load Y FLDL2T Load log210 Y FLDLG2 Load 10912 Y FLDLN2 Load log 2 Y FLDPI Load x Y FLDZ Load 0 0 Y FMUL P Multiply real Y Y Y Y Y Y FNOP No operation FPATAN Partial arctangent Y Y Y Y Y FPREM Partial remainder Y Y Y Y FPREM1 IEEE partial remainder Y Y Y Y FPTAN Partial tangent Y Y Y Y Y FRNDINT Round to integer Y Y Y Y FRSTOR Restore state Y Y Y Y Y Y Y FSAVE Save state FSCALE Scale Y Y Y Y Y Y FSIN Sine Y Y FSINCOS Sine and cosine Y Y Y Y Y C 2 Intel FLOATING POINT EXCEPTIONS SUMMARY Table C 1 Floating Point Exceptions Summary Contd Mnemonic Instruction HIS IA D Z 0 U P FSQRT Square root Y Y Y Y FST P stack or extended Store real Y FST P single or double Store real Y Y Y Y Y Y FSTCW Store control word FSTE
112. ENTAL DATA TYPES 5 1 5 1 1 Alignment of Words Doublewords and 5 5 1 5 2 NUMERIC POINTER BIT FIELD AND STRING DATA TYPES 5 2 5 2 1 5 2 5 2 2 Unsigned Integers eco lem m eee xr Ee 5 4 5 2 3 BGbD Integers es es oO i o dm 5 4 5 2 4 Poilntets xcix mes A ee ee eed ola Bees 5 4 5 2 5 Bit Fields u ico ECRIRE I PD OI P en rl eer 5 4 5 2 6 SUINGS sio i reap ne Re d a eM PE Neu e dunt net PEERS Ever d 5 4 5 2 7 Floating Point Data lt 5 5 5 2 8 MMX Technology Data lt 5 5 5 3 OPERAND ADDRESSING m n 5 5 5 3 1 Immediate lt 5 5 5 3 2 Register Operands lee eran ER uqa bet su uqa ec RR RR a 5 5 5 3 3 Memory lt 5 6 5 3 3 1 Specifying a Segment 5 6 5 3 3 2 Specifying an Offset u maura aspas eee 5 7 5 3 3 3 Assembler and Compiler Addressing Modes 5 9 5 3 4 I O Port Addressing i oriente tT RE RR eee 5 10 vi intel e TABLE OF CONTENTS PAGE CHAPTER 6 INSTRUCTION SET SUMMARY 6 1 NEW INTEL ARCHITECTURE INSTRUCTIONS
113. ERMINATION intel Machine check architecture Conditional move instruction CMOVcc MMX technology Cache and TLB information To use this instruction a source operand value of 0 1 or 2 is placed in the EAX register Processor identification and feature information is then returned in the EAX EBX ECX and EDX registers See CPUID CPU Identification in Chapter 3 of the Intel Architecture Soft ware Developer s Manual Volume 2 for more detailed information about the instruction AP 485 Intel Processor Identification and the CPUID Instruction Order Number 241618 provides additional information and example source code for use in identifying Intel Architec ture processors It also contains guidelines for using the CPUID instruction to help maintain the widest range of software compatibility The following guidelines are among the most important and should always be followed when using the CPUID instruction to determine available features Always begin by testing for the GenuineIntel message in the EBX EDX and ECX registers when the CPUID instruction is executed with EAX equal to 0 If the processor is not genuine Intel the feature identification flags may have different meanings than are described in CPUID CPU Identification in Chapter 3 of the Intel Architecture Software Developer s Manual Volume 2 Do not assume a value of 1 in a feature identification flag indicates that a given feature is
114. ESSOR S ADVANCED MICROARCHITECTURE The Pentium Pro processor introduced by Intel in 1995 represents the most recent implemen tation of the Intel Architecture Like its predecessor the Pentium processor introduced by Intel in 1993 the Pentium Pro processor with its advanced superscalar microarchitecture sets an impressive performance standard In designing the Pentium Pro processor one of the primary 2 5 INTRODUCTION TO THE INTEL ARCHITECTURE intel e goals of the Intel chip architects was to exceed the performance of the Pentium processor signif icantly while still using the same 0 6 micrometer four layer metal BICMOS manufacturing process Using the same manufacturing process as the Pentium processor meant that perfor mance gains could only be achieved through substantial advances in the microarchitecture The resulting Pentium Pro processor microarchitecture is a three way superscalar pipelined architecture The term three way superscalar means that using parallel processing techniques the processor is able on average to decode dispatch and complete execution of retire three instructions per clock cycle To handle this level of instruction throughput the Pentium Pro processor uses a decoupled 12 stage superpipeline that supports out of order instruction execu tion Figure 2 1 shows a conceptual view of this pipeline with the pipeline divided into four processing units the fetch decode unit the dispatch execute u
115. Execution Environment CHAPTER 3 BASIC EXECUTION ENVIRONMENT This chapter describes the basic execution environment of an Intel Architecture processor as seen by assembly language programmers It describes how the processor executes instructions and how it stores and manipulates data The parts of the execution environment described here include memory the address space the general purpose data registers the segment registers the EFLAGS register and the instruction pointer register The execution environment for the floating point unit FPU is described in Chapter 7 Floating Point Unit 3 1 MODES OF OPERATION The Intel Architecture supports three operating modes protected mode real address mode and system management mode The operating mode determines which instructions and architectural features are accessible Protected mode The native state of the processor In this mode all instructions and archi tectural features are available providing the highest performance and capability This is the recommended mode for all new applications and operating systems Among the capabilities of protected mode is the ability to directly execute real address mode 8086 software in a protected multi tasking environment This feature is called virtual 8086 mode although it is not actually a processor mode Virtual 8086 mode is actually a protected mode attribute that can be enabled for any task Real address mode Provides the pr
116. FPREM1 instruction 7 13 7 33 7 37 FPTAN instruction 7 13 FPU architecture 7 8 compatibility with Intel Architecture FPUs and math coprocessors 7 1 floating point format 7 2 7 3 IEEE 7 1 presence 10 1 transcendental instruction accuracy 7 39 FPU control word description of c ize Le DIEA 7 16 exception flag masks 7 17 PC field e reu o ab hard US E eos es 7 17 RG field spewed ied ee he tens 7 18 FPU data pointer 7 21 INDEX FPU data registers 7 9 FPU instruction pointer 7 21 FPU instructions arithmetic vs non arithmetic instructions 7 43 instruction 7 29 Operahds Rede rete eth eis 7 31 OVENVIOW ii eed be oe ea ons 7 29 unsupported 7 41 FPU integer description of 7 27 encodings 7 27 FPUlastopcode 7 21 FPU register stack description of 7 9 parameter passing 7 11 FPU state Im genes ee 7 22 7 23 SAVING ales cores I end 7 21 FPU status word condition code 7 12 DEJAD sua 0 rd 7 50 description of 7 12 exception flags
117. Handling If an inexact result occurs in conjunction with numeric overflow or underflow one of the following operations is carried out inexact result occurs along with masked overflow or underflow the OE or UE flag and the PE flag are set and the result is stored as described for the overflow or underflow exceptions see Section 7 8 4 Numeric Overflow Exception O or Section 7 8 5 Numeric Underflow Exception U If the inexact result exception is unmasked the FPU also invokes the software exception handler Ifan inexact result occurs along with unmasked overflow or underflow and the destination operand is a register the OE or UE flag and the PE flag are set the result is stored as described for the overflow or underflow exceptions and the software exception handler is invoked Ifan inexact result occurs along with unmasked overflow or underflow and the destination operand is a memory location the inexact result condition is ignored 7 8 7 Exception Priority The processor handles exceptions according to a predetermined precedence When an instruc tion generates two or more exception conditions the exception precedence sometimes results in the higher priority exception being handled and the lower priority exceptions being ignored For 7 53 FLOATING POINT UNIT intel example dividing an SNaN by zero can potentially signal an invalid arithmetic operand excep tion due to the SNaN operan
118. ING PROCEDURES USING CALL AND RET 4 4 4 3 1 Near CALL and RET 4 5 4 3 2 Far CALL and RET 4 6 4 3 3 Parameter Passing a tenute n sees qt E eee Ook Pewee ee dates Rus 4 6 4 3 3 1 Passing Parameters Through the General Purpose Registers 4 6 4 3 3 2 Passing Parameters on the Stack 4 6 4 3 3 3 Passing Parameters in an Argument List 4 7 4 3 4 Saving Procedure State 1 4 7 4 3 5 Calls to Other Privilege Levels 4 7 4 3 6 CALL and RET Operation Between Privilege Levels 4 9 4 4 INTERRUPTS AND EXCEPTIONS 4 10 4 4 1 Call and Return Operation for Interrupt or Exception Handling Procedures 4 11 4 4 2 Calls to Interrupt or Exception Handler Tasks 4 14 4 4 8 Interrupt and Exception Handling in Real Address Mode 4 15 4 4 4 INT n INTO INT and BOUND Instructions 4 15 4 5 PROCEDURE CALLS FOR BLOCK STRUCTURED LANGUAGES 4 16 4 5 1 ENTER Instructions umero Te Eana deed Soon tees 4 16 4 5 2 EEAVE Iristr ctlori uda coe tite tina Ex DIA Pdl 4 21 CHAPTER 5 DATA TYPES AND ADDRESSING MODES 5 1 FUNDAM
119. INT EXCEPTION CONDITIONS The following sections describe the various conditions that cause a floating point exception to be generated and the masked response of the FPU when these conditions are detected Chapter 3 Instruction Set Reference in the Intel Architecture Software Developer s Manual Volume 2 lists the floating point exceptions that can be signaled for each floating point instruction 7 8 4 Invalid Operation Exception The floating point invalid operation exception occurs in response to two general types of oper ations Stack overflow or underflow IS Invalid arithmetic operand The flag for this exception IE is bit 0 of the FPU status word and the mask bit IM is bit 0 of the FPU control word The stack fault flag SF of the FPU status word indicates the type of operation caused the exception When the SF flag is set to 1 a stack operation has resulted in stack overflow or underflow when the flag is cleared to 0 an arithmetic instruction has encoun tered an invalid operand Note that the FPU explicitly sets the SF flag when it detects a stack overflow or underflow condition but it does not explicitly clear the flag when it detects an invalid arithmetic operand condition As a result the state of the SF flag can be 1 following an invalid arithmetic operation exception if it was not cleared from the last time a stack overflow or underflow condition occurred See Section 7 3 2 4 Stack Fault Flag for mo
120. Intel Architecture Software Developer s Manual Volume 2 Table 7 12 Unsupported Extended Real Encodings Class Sign Biased Exponent Significand Integer Fraction Positive 0 11 11 0 11 11 Pseudo NaNs Quiet 0 11 11 10 00 0 11 11 0 01 11 Signaling 0 11 11 00 01 Positive Reals Pseudo infinity 0 11 11 0 00 00 0 11 10 0 11 11 Unnormals 0 00 01 00 00 Pseudo denormals 0 00 00 1 11 11 0 00 00 00 00 Negative Reals Pseudo denormals 1 00 00 1 11 11 1 00 00 00 00 1 11 10 0 11 01 Unnormals z 1 00 01 00 00 Pseudo infinity 1 11 11 0 00 00 Negative 1 11 11 0 01 11 Pseudo NaNs Signaling 1 11 11 00 01 1 11 11 0 11 11 Quiet 1 11 11 10 00 15 bits lt 63 bits 7 5 1 Escape ESC Instructions All of the instructions in the FPU instruction set fall into a class of instructions known as escape ESC instructions All of these instructions have a common opcode format which is slightly different from the format used by the integer and operating system instructions 7 30 Intel FLOATING POINT UNIT 7 5 2 FPU Instruction Operands Most floating point instructions require one or two operands which are located on the FPU data register stack or in memory None of the floating point instructions accept immediate operands When an operand is located in a data register it is referenced relative
121. Intel Architecture Software Developer s Manual Volume 1 Basic Architecture NOTE The ntel Architecture Software Developer s Manual consists of three books Basic Architecture Order Number 243190 Instruction Set Reference Manual Order Number 243191 and the System Programming Guide Order Number 243192 Please refer to all three volumes when evaluating your design needs 1997 Information in this document is provided in connection with Intel products No license express or implied by estoppel or otherwise to any intellectual property rights is granted by this document Except as provided in Intel s Terms and Conditions of Sale for such products Intel assumes no liability whatsoever and Intel disclaims any express or implied warranty relating to sale and or use of Intel products including liability or warranties relating to fitness for a particular purpose merchantability or infringement of any patent copyright or other intellectual property right Intel products are not intended for use in medical life saving or life sustaining applications Intel may make changes to specifications and product descriptions at any time without notice Designers must not rely on the absence or characteristics of any features or instructions marked reserved or undefined Intel reserves these for future definition and shall have no responsibility whatsoever for conflicts or incompatibilities arising from future changes to them Intel s Inte
122. KByte L1 data cache is dual ported and two way set associative supporting one load and one store operation per cycle Coherency between the caches and system memory are maintained using the MESI modified exclusive shared invalid cache protocol This protocol fosters cache coherency in single and multiple processor systems It is also able to detect coherency problems created by self modi fying code Memory requests from the processor s execution units go through the memory interface unit and the memory order buffer These units have been designed to support a smooth flow of memory access requests through the cache and system memory hierarchy to prevent memory access blocking The L1 data cache automatically forwards a cache miss on to the L2 cache and then if necessary the bus interface unit forwards an L2 cache miss to system memory 2 8 intel e INTRODUCTION TO THE INTEL ARCHITECTURE System Bus External L2 Cache Cache Bus Bus Interface Unit p Y Y Next IP Instruction Fetch Unit Instruction Cache L1 lt a Y Branch Memory Instruction Decoder Ll gt Target Reorder U Buffer Buffer Simple Simple Complex Instruction Instruction Instruction Decoder Decoder Decoder i M
123. MPATIBILITY MODE FOR HANDLING FPU EXCEPTIONS o oocccccccccc eee IMPLEMENTATION OF THE MS DOS COMPATIBILITY MODE IN THE INTEL486 PENTIUMO AND PENTIUM PRO PROCESSORS MS DOS Compatibility Mode in the Intel486 and Pentium Processors Basic Rules When FERR Is Recommended External Hardware to Support the MS DOS Compatibility No Wait FPU Instructions Can Get FPU Interrupt in Window MS DOS Compatibility Mode in the PentiumO Pro Processor RECOMMENDED PROTOCOL FOR MS DOS COMPATIBILITY HANDLERS Floating Point Exceptions and Their Defaults Two Options for Handling Numeric Exceptions Automatic Exception Handling Using Masked Exceptions Software Exception Handling Synchronization Required for Use of FPU Exception Handlers Exception Synchronization What Why and When Exception Synchronization Examples Proper Exception Synchronization in General FPU Exception Handling Need for Storing State of IGNNE Circuit If Using FPU and SMM Considerations When FPU Shared Between Tasks Speculatively Deferring FPU Saves General Overview
124. ND OR and TEST instructions may be used to manipulate bits in the control and status registers of a memory mapped peripheral devices When using memory mapped I O caching of the address space mapped for I O operations must be prevented With the Pentium Pro processors caching of I O accesses can be prevented by using memory type range registers MTRRs to map the address space used for the memory mapped I O as uncacheable UC See Chapter 9 Memory Cache Control in the Intel Architec ture Software Developer s Manual Volume 3 for a complete discussion of the MTRRs The Pentium and Intel486 processors do not support MTRRs Instead they provide the KEN pin which when held inactive high prevents caching of all addresses sent out on the system bus To use this pin external address decoding logic is required to block caching in specific address spaces 9 2 intel INPUT OUTPUT Physical Memory FFFF FFFFH EPROM 1 0 Port 1 0 Port I O Port RAM 0 Figure 9 1 Memory Mapped l O All the Intel Architecture processors that have on chip caches also provide the PCD page level cache disable flag in page table and page directory entries This flag allows caching to be disabled on a page by page basis See Page Directory and Page Table Entries in Chapter 3 of in the Intel Architecture Software Developer s Manual Volume 3 9 4 lO INSTRUCTIONS The processor s I O instructions prov
125. NT memory location might be overwritten before the exception handler is called Rearranging the instructions as follows so that the FSQRT instruction follows the FILD instruction synchronizes the exception handling and eliminates the possibility of the exception being handled incorrectly FILD COUNT Floating point instruction FSORT Subsequent floating point instruction synchronizes any exceptions generated by the FILD instruction INC COUNT Integer instruction The FSQRT instruction does not require any synchronization because the results of this instruc tion are stored in the FPU data registers and will remain there undisturbed until the next floating point or WAIT FWAIT instruction is executed To absolutely insure that any exceptions emanating from the FSQRT instruction are handled for example prior to a procedure call a WAIT instruction can be placed directly after the FSQRT instruction Note that some floating point instructions non waiting instructions do not check for pending unmasked exceptions see Section 7 5 11 FPU Control Instructions They include the FNINIT FNSTENV ENSAVE ENSTSW FNSTCW and FNCLEX instructions When an FNINIT FNSTENV FNSAVE or FNCLEX instruction is executed all pending exceptions are essentially lost either the FPU status register is cleared or all exceptions are masked The FNSTSW and FNSTCW instructions do not check for pending interrupts but they do not modify the FPU status
126. NV Store environment FSTSW AX Store status word FSUB R P Subtract real Y Y Y Wo 7S ey FTST Test Y Y Y FUCOM P P Unordered compare real Y Y Y FWAIT CPU Wait FXAM Examine FXCH Exchange registers Y FXTRACT Extract Y Y Y Y FYL2X Y logoX Y Y Y Y Y Y Y FYL2XP1 Y loge X 1 Y Y Y Y Y C 3 Intel D Guidelines for Writing FPU Exception Handlers APPENDIX D GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS As described in Chapter 7 Floating Point Unit the Intel Architecture supports two mechanisms for accessing exception handlers to handle unmasked FPU exceptions native mode and MS DOS compatibility mode The primary purpose of this appendix is to provide detailed informa tion to help software engineers design and write FPU exception handling facilities to run on PC systems that use the MS DOS compatibility mode for handling FPU exceptions Some of the information in this appendix will also be of interest to engineers who are writing native mode FPU exception handlers The information provided is as follows Discussion of the origin of the MS DOS FPU exception handling mechanism and its relationship to the FPU s native exception handling mechanism Description of the Intel Architecture flags and processor pins that control the MS DOS FPU exception handling mechanism Description of the external hardware typically required to support MS DOS exception handling mechanism Description of the FPU s except
127. P 63 0 3006044 Figure 8 1 MMX Register Set Although the MMX registers are defined in the Intel Architecture as separate registers they are aliased to the registers in the FPU data register stack RO through R7 See Chapter 10 MMXTM Technology System Programming Model in the Intel Architecture Software Developer s Manual Volume 3 for more a detailed discussion of the aliasing of MMX registers 8 1 2 MMX Data Types The MMX technology defines the following new 64 bit data types see Figure 8 2 Packed bytes Eight bytes packed into one 64 bit quantity Packed words Four 16 bit words packed into one 64 bit quantity Packed doublewords Two 32 bit doublewords packed into one 64 bit quantity Quadword One 64 bit quantity The bytes in the packed bytes data type are numbered 0 through 7 with byte 0 being contained in the least significant bits of the data type bits O through 7 and byte 7 being contained in the most significant bits bits 56 through 63 The words in the packed words data type are numbered 0 through 4 with word 0 being contained in the bits 0 through 15 of the data type and word 4 being contained in bits 48 through 63 The doublewords in a packed doublewords data type are numbered 0 and 1 with doubleword 0 being contained in bits 0 through 31 and doubleword 1 being contained in bits 32 through 63 8 2 Intel PROGRAMMING WITH THE INTEL MMX TECHNOLOGY Packed bytes 8x8 bits 63
128. P instruction 6 1 7 16 7 34 FCOMP instruction 7 15 7 34 FCOMPP instruction 7 15 7 34 FCOS instruction 7 18 7 37 FDIV instruction 7 33 FDIVP instruction 7 33 FDIVRiinstruction 7 33 FDIVRP instruction 7 33 Feature determination of processor 10 1 Fetch decode unit 2 10 FIADD instruction 7 33 FICOM instruction 7 15 7 34 FICOMP instruction 7 15 7 34 FIDIV instruction 7 33 FIDIVR instruction 7 33 FILD instruction 7 31 FIMUL instruction 7 33 FINIT FNINIT instructions 7 15 7 16 7 20 7 40 FIST instruction 7 31 FISTP instruction 7 32 FISUB 7 33 FISUBR instruction 7 33 Flat memory model 3 3 3 8 FLD instruction 7 31 INDEX FLD1 instruction 7 33 FLDOW instruction 7 16 7 40 FLDENV instruction 7 15 7 20 7 24 7 40 FLDL2E instruction 7 33 FLDL2T instruction 7 33 FLDLG2 instruction 7 33 FLDLN2 instruction 7 33 FLDPI instru
129. PU exception the processor in most cases uses a deferred method of reporting the error This means that the processor does not respond immediately but rather freezes just before executing the next WAIT or FPU instruction except for no wait instructions which the FPU executes regardless of an error condition 2 When the processor freezes it also asserts the FERR output 3 The frozen processor waits for an external interrupt which must be supplied by external hardware in response to the FERR assertion 4 In MS DOS compatibility systems FERR is fed to the IRQ13 input in the cascaded PIC The PIC generates interrupt 75H which then branches to interrupt 2 as described earlier in this appendix for systems using the Intel 286 and Intel 287 or Intel386 and Intel 387 processors The deferred method of error reporting is used for all exceptions caused by the basic arithmetic instructions including FADD FSUB FMUL FDIV FSQRT FCOM and FUCOM for preci sion exceptions caused by all types of FPU instructions and for numeric underflow and over flow exceptions caused by all types of FPU instructions except stores to memory Some FPU instructions with some FPU exceptions use an immediate method of reporting errors Here the FERR is asserted immediately at the time that the exception occurs The immediate method of error reporting is used for FPU stack fault invalid operation and denormal exceptions caused by all transcende
130. PU instruction is actually encoun tered in another task Since kernel tasks rarely use floating point and some applications do not use floating point or use it infrequently the amount of time saved by avoiding unnecessary stores of the floating point state is significant Speculative deferral of FPU saves does however place an extra burden on the kernel in three key ways 1 The kernel must keep track of which thread owns the FPU which may be different from the currently executing thread 2 The kernel must associate any floating point exceptions with the generating task This requires special handling since floating point exceptions are delivered asynchronous with other system activity 3 There are conditions under which spurious floating point exception interrupts are generated which the kernel must recognize and discard D 21 GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS intel D 3 6 1 SPECULATIVELY DEFERRING FPU SAVES GENERAL OVERVIEW In order to support multi tasking each thread in the system needs a save area for the general purpose registers and each task that is allowed to use floating point needs an FPU save area large enough to hold the entire FPU stack and associated FPU state such as the control word and status word See Section 7 3 9 Saving the FPU s State for a complete description of the FPU save image On a task switch the general purpose registers are swapped out to their save area for the suspendi
131. Positive Operand Operand CF 01000100010001000100010001000111 X After 1 bit SAR Instruction r 3 00100010001000100010001000100011 gt 1 Ld Initial State Negative Operand CF 11000100010001000100010001000111p gt X After 1 bit SAR Instruction 111000100010001000100010001000 11 1 Figure 6 8 SAR Instruction Operation 6 26 intel e INSTRUCTION SET SUMMARY 6 7 2 Double Shift Instructions The SHLD shift left double and SHRD shift right double instructions shift a specified number of bits from one operand to another see Figure 6 9 They are provided to facilitate operations on unaligned bit strings They can also be used to implement a variety of bit string move operations SHLD Instruction CF lt Destination Memory or Register lt 31 0 Source Register SHRD Instruction 31 0 Source Register Destination Memory or Register gt CF Figure 6 9 SHLD and SHRD Instruction Operations The SHLD instruction shifts the bits in the destination operand to the left and fills the empty bit positions in the destination operand with bits shifted out of the source operand The destination and source operands must be the same length either words or doublewords The shift count can range from 0 to 31 bits The result of this shift operation is stored in the
132. SKING OPERATING SYSTEM 8 14 8 5 5 2 PREEMPTIVE MULTITASKING OPERATING SYSTEM 8 14 8 5 6 Exception Handling in MMX Code 8 15 8 5 7 Register Mapping rrr nore Rem eR Ee tee ae 8 15 CHAPTER 9 INPUT OUTPUT 9 1 VO PORT ADDRESSING i cede dia Bale Ese Meee Bee emake ads 9 1 intel 9 2 VO PORT HARDWARE mr 9 3 FO ADDRESS SPAGE uuu is aguas ds Pre Rest le ed 9 3 1 Memory Mapped 9 4 l OINSTRUGTIONS zit eno oce esas 9 5 PROTECTED MODE l O ike Rer eme re Be 9 5 1 O Privilege Level u 522 ic 4 c OS RR 9 5 2 l O Permission Bit Map u serio un p assaka eee 9 6 ORDERING I Q eiu pamapa oe erg ERE ES CHAPTER 10 PROCESSOR IDENTIFICATION AND FEATURE DETERMINATION 10 1 PROCESSOR IDENTIFICATION 10 2 IDENTIFICATION OF EARLIER INTEL ARCHITECTURE PROCESSORS APPENDIX A EFLAGS CROSS REFERENCE APPENDIX B EFLAGS CONDITION CODES APPENDIX C FLOATING POINT EXCEPTIONS SUMMARY APPENDIX D GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS D 1 D 2 D 2 1 D 2 1 1 D 2 1 2 D 2 1 3 D 2 2 D 3 D 3 1 D 3 2 D 3 2 1 D 3 2 2 D 3 3 D 3 3 1 D 3 3 2 D 3 3 3 D 3 4 D 3 5 D 3 6 D 3 6 1 D 3 6 2 D 3 6 3 D 3 6 4 ORIGIN OF THE MS DOS CO
133. Saturation Arithmetic and Wraparound Mode The MMX technology supports a new arithmetic capability known as saturating arithmetic Saturation is best defined by contrasting it with wraparound mode In wraparound mode results that overflow or underflow are truncated and only the lower least significant bits of the result are returned that is the carry is ignored In saturation mode results of an operation that overflow or underflow are clipped saturated to a data range limit for the data type see Table 8 1 The result of an operation that exceeds the range of a data type saturates to the maximum value of the range A result that is less than the range of a data type saturates to the minimum value of the range This method of handling over flow and underflow is useful in many applications such as color calculations Table 8 1 Data Range Limits for Saturation Data Type Lower Limit Upper Limit Hexadecimal Decimal Hexadecimal Decimal Signed Byte 80H 128 7FH 127 Signed Word 8000H 32 768 7FFFH 32 767 Unsigned Byte 00H 0 FFH 255 Unsigned Word 0000H 0 FFFFH 65 535 For example when the result exceeds the data range limit for signed bytes it is saturated to 7FH FFH for unsigned bytes If a value is less than the data range limit it is saturated to 80H for signed bytes 00H for unsigned bytes Saturation provides a useful feature of avoiding wraparound artifacts In the example of color calculations
134. TRUCTIONS MOVD Move doubleword MOVQ Move quadword 6 2 2 2 MMX CONVERSION INSTRUCTIONS PACKSSWB Pack words into bytes with signed saturation PACKSSDW Pack doublewords into words with signed saturation PACKUSWB Pack words into bytes with unsigned saturation PUNPCKHBW Unpack high order bytes from words PUNPCKHWD Unpack high order words from doublewords PUNPCKHDQ Unpack high order doublewords from quadword PUNPCKLBW Unpack low order bytes from words PUNPCKLWD Unpack low order words from doublewords PUNPCKLDQ Unpack low order doublewords from quadword 6 9 INSTRUCTION SET SUMMARY intel e 6 2 2 3 PADDB PADDW PADDD PADDSB PADDSW PADDUSB PADDUSW PSUBB PSUBW PSUBD PSUBSB PSUBSW PSUBUSB PSUBUSW PMULHW PMULLW PMADDWD 6 2 2 4 PCMPEQB PCMPEQW PCMPEQD PCMPGTB PCMPGTW PCMPGTD 6 2 2 5 PAND PANDN POR PXOR 6 10 MMX PACKED ARITHMETIC INSTRUCTIONS Add packed bytes Add packed words Add packed doublewords Add packed bytes with saturation Add packed words with saturation Add packed unsigned bytes with saturation Add packed unsigned words with saturation Subtract packed bytes Subtract packed words Subtract packed doublewords Subtract packed bytes with saturation Subtract packed words with saturation Subtract packed unsigned bytes with saturation Subtract packed unsigned words with saturation Multiply packed words and store high result Multiply packed words and store low result Mult
135. TSW FNSTSW instruction in a general purpose register The FSTENV FNSTENV and FSAVE FNSAVE instructions save the FPU environment and state respectively in memory The FPU environment includes all the FPU s control and status registers the FPU state includes the FPU environment and the data registers in the FPU register stack The FSAVE FNSAVE instruction also initializes the FPU to default values like the FINIT ENINIT instruction after it saves the original state of the FPU The FLDENV and FRSTOR instructions load the FPU environment and state respectively from memory into the FPU These instructions are commonly used when switching tasks or contexts The WAIT FWAIT instructions are synchronization instructions They are actually mnemonics for the same opcode These instructions check the FPU status word for pending unmasked FPU exceptions If any pending unmasked FPU exceptions are found they are handled before the processor resumes execution of the instructions integer floating point or system instruction in the instruction stream The WAIT FWAIT instructions are provided to allow synchronization of instruction execution between the FPU and the processor s integer unit See Section 7 9 Floating Point Exception Synchronization for more information on the use of the WAIT FWAIT instructions 7 5 12 Waiting Vs Non waiting Instructions All of the floating point instructions except a few special control instructions perform a wait
136. U makes an implicit call traps to the floating point software exception handler The exception handler can then execute recovery procedures for selected or all floating point exceptions Synchronization problems occur in the time frame between when the exception is signaled and when it is actually handled Because of concurrent execution integer or system instructions can be executed during this time frame It is thus possible for the source or destination operands for a floating point instruction that faulted to be overwritten in memory making it impossible for the exception handler to analyze or recover from the exception To solve this problem an exception synchronizing instruction either a floating point instruction or a WAIT FWAIT instruction can be placed immediately after any floating point instruction that might present a situation where state information pertaining to a floating point exception might be lost or corrupted Floating point instructions that store data in memory are prime candi dates for synchronization For example the following three lines of code have the potential for exception synchronization problems FILD COUNT Floating point instruction INC COUNT Integer instruction FSORT Subsequent floating point instruction In this example the INC instruction modifies the result of a floating point instruction FILD If an exception is signaled during the execution of the FILD instruction the result stored in the COU
137. U operate independently and in parallel The actual microarchitecture of an Intel Architecture processor varies among the various families of processors For example the Pentium Pro processor has two integer units and two FPUs whereas the Pentium processor has two integer units and one FPU and the Intel486 processor has one integer unit and one FPU 7 8 intel FLOATING POINT UNIT Instruction Decoder and Sequencer Y Y Integer Unit FRU Data Bus Figure 7 4 Relationship Between the Integer Unit and the FPU The instruction execution environment of the FPU see Figure 7 5 consists of 8 data registers called the FPU data registers and the following special purpose registers The status register The control register The tag word register Instruction pointer register Last operand data pointer register Opcode register These registers are described in the following sections 7 3 1 FPU Data Registers The FPU data registers shown in Figure 7 5 consist of eight 80 bit registers Values are stored in these registers in the extended real format shown in Figure 7 17 When real integer or packed BCD integer values in any of the formats shown in Figure 7 17 are loaded from memory into any of the FPU data registers the values are automatically converted into extended real format Gf they are not already in that format When computation results are
138. V FPTAN FSTSW FNSTSW FRNDINT WAIT FWAIT FSCALE FXAM FSIN FXCH FSINCOS FSQRT FST FSTP conversion FSUB FSUBP FSUBR FSUBRP FTST FUCOM FUCOMP FUCOMPP FXTRACT FYL2X FYL2XP1 7 44 Intel FLOATING POINT UNIT Note that when exceptions are masked the FPU may detect multiple exceptions in a single instruction because it continues executing the instruction after performing its masked response For example the FPU can detect a denormalized operand perform its masked response to this exception and then detect numeric underflow 7 7 3 Software Exception Handling The FPU in the Pentium Pro Pentium and Intel486 processors provides two different modes of operation for invoking a software exception handler for floating point exceptions native mode and MS DOS compatibility mode The mode of operation is selected with the NE flag of control register CRO See Chapter 2 System Architecture Overview in the Intel Architecture Software Developer s Manual Volume 3 for more information about the NE flag 7 7 3 1 NATIVE MODE The native mode for handling floating point exceptions is selected by setting the NE flag in control register CRO to 1 In this mode if the FPU detects an exception condition while executing a floating point instruction and the exception is unmasked the mask bit for the excep tion is cleared the FPU sets the flag for the exception and the ES flag in the FPU status word It then invokes the software exception handl
139. Vcc instructions are represented by the status flags in the EFLAGS register The condi tion code mnemonics are appended to the letters FCMOV to form the mnemonic for a FCMOVcc instruction Table 7 14 Floating Point Conditional Move Instructions Instruction Mnemonic Status Flag States Condition Description FCMOVB CF 1 Below FCMOVNB CF 0 Not below FCMOVE ZF 1 Equal FCMOVNE ZF 0 Not equal FCMOVBE CF or ZF 1 Below or equal FCMOVNBE CF or ZF 0 Not below nor equal FCMOVU PF 1 Unordered FCMOVNU PF 0 Not unordered Like the CMOVcc instructions FCMOVcc instructions are useful for optimizing small IF constructions They also help eliminate branching overhead for IF operations and the possibility of branch mispredictions by the processor NOTE The FCMOVcc instructions may not be supported on some processors in the Pentium Pro processor family Software can check if the FCMOVcc instruc tions are supported by checking the processor s feature information with the CPUID instruction see CPUID CPU Identification in Chapter 3 of the Intel Architecture Software Developer s Manual Volume 2 7 32 Intel FLOATING POINT UNIT 7 5 4 Load Constant Instructions The following instructions push commonly used constants onto the top ST 0 of the FPU register stack FLDZ Load 40 0 FLDI Load 1 0 FLDPI Load x FLDL2T Load log 10 FLDL2E Load log e FLDLG2 Load log 92 FLDLN2 Load log 2 The constant
140. X register The CDQ instruction copies the sign bit 31 of the doubleword in the EAX register into every bit position in the EDX register The CWD instruction can be used to produce a doubleword dividend from a word before a word division and the CDQ instruction can be used to produce a quadword dividend from a doubleword before doubleword division 6 3 2 3 MOVE AND CONVERT The MOVSX move with sign extension and MOVZX move with zero extension instructions move the source operand into a register then perform the sign extension The MOVSX instruction extends an 8 bit value to a 16 bit value or an 8 or 16 bit value to 32 bit value by sign extending the source operand as shown in Figure 6 5 The MOVZX instruction extends an 8 bit value to a 16 bit value or an 8 or 16 bit value to 32 bit value by zero extending the source operand 6 4 BINARY ARITHMETIC INSTRUCTIONS The binary arithmetic instructions operate on 8 16 and 32 bit numeric data encoded as signed or unsigned binary integers Operations include the add subtract multiply and divide as well as increment decrement compare and change sign negate The binary arithmetic instructions may also be used in algorithms that operate on decimal BCD values 6 4 1 Addition and Subtraction Instructions The ADD add integers ADC add integers with carry SUB subtract integers and SBB subtract integers with borrow instructions perform addition and subtraction operations on s
141. Y 5 1 8 2 Quiet NaN see QNaN R RC rounding control field FPU control word 7 18 RCL instruction 6 28 RCR instruction 6 28 RDMSR instructi0N 6 2 10 1 RDPMC instruction 6 1 RDTSC instruction 6 2 10 1 Real numbers encoding nueces ee pu 7 5 7 6 7 26 floating point format 7 25 indefinite 7 26 notation z pus ner uay uuu q aus u 7 5 SS Mia S enun nu ER ESP 7 2 Real address mode 3 4 handling exceptions in 4 15 handling interrupts 4 15 Register operands 5 5 Register stack FPU 7 9 Registers EFLAGS register 3 10 ElP registers cir pc ad pa 3 14 general purpose registers 3 5 segment registerS 3 5 3 7 Related literature 1 7 REP REPE REPZ REPNE REPNZ prefixes 6 35 9 4 Reserved bits 1 5 RESET DIN ee deca ai Ee 3 10 RET instruction 3 14 4 4 6 31 6 39 Retirementunit 2 12 Return instruction pointer 4 4 INDEX 7 INDEX Returns from procedure calls exception handler return from 4 11 far retur orn om act 4 6 interrupt handler return from 4 11 Returns from procedure
142. YL2XPI Compute log epsilon y log x 1 F2XMI Compute exponential 2 1 FSCALE Scale The FYL2X and FYL2XPI instructions perform two different base 2 logarithmic operations The FYL2X instruction computes the log of y log x This operation permits the calculation of the log of any base using the following equation log x 1 log b log x The FYL2XPI instruction computes the log epsilon of y log x 1 This operation provides optimum accuracy for values of epsilon that are close to 0 7 38 Intel FLOATING POINT UNIT The F2XMI instruction computes the exponential 2 1 This instruction only operates on source values in the range 1 0 to 1 0 The FSCALE instruction multiplies the source operand by a power of 2 7 5 10 Transcendental Instruction Accuracy The algorithms that the Pentium and Pentium Pro processors use for the transcendental instruc tions FSIN FCOS FSINCOS FPTAN FPATAN 2 FYL2X and FYL2XP1 allow a higher level of accuracy than was possible in earlier Intel Architecture math coprocessors and FPUs The accuracy of these instructions is measured in terms of units in the last place ulp For a given argument x let f x and F x be the correct and computed approximate function values respectively The error in ulps is defined to be error Pe 7 x 2 k 63 k where k is an integer such that 1 lt 2 f x 2 With the Pentium and Pentium Pro processors the w
143. ace unit that handles stores has two ports allowing it to process the address and the data micro op in parallel The memory interface unit can thus execute both a load and a store in parallel in one clock cycle The floating point execution units are similar to those found in the Pentium processor Several new floating point instructions have been added to the Pentium Pro processor to streamline conditional branches and moves INTRODUCTION TO THE INTEL ARCHITECTURE intel e 2 5 5 Retirement Unit The retirement unit commits the results of speculatively executed micro ops to permanent machine state and removes the micro ops from the reorder buffer Like the reservation station the retirement unit continuously checks the status of micro ops in the reorder buffer looking for ones that have been executed and no longer have any dependencies with other micro ops in the instruction pool It then retires completed micro ops in their original program order taking into accounts interrupts exceptions breakpoints and branch mispredictions The retirement unit can retire three micro ops per clock In retiring a micro op it writes the results to the retirement register file and or memory The retirement register file contains the Intel Architecture registers eight general purpose registers and eight floating point data regis ters After the results have been committed to machine state the micro op is removed from the reorder buffer Intel Basic
144. ad BCD Store BCD and pop Exchange registers Floating point conditional move if equal Floating point conditional move if not equal Floating point conditional move if below 6 11 INSTRUCTION SET SUMMARY intel e FCMOVBE FCMOVNB FCMOVNBE FCMOVU FCMOVNU 6 2 3 2 FADD FADDP FIADD FSUB FSUBP FISUB FSUBR FSUBRP FISUBR FMUL FMULP FIMUL FDIV FDIVP FIDIV FDIVR FDIVRP FIDIVR FPREM FPREMI FABS FCHS FRNDINT FSCALE 6 12 Floating point conditional move if below or equal Floating point conditional move if not below Floating point conditional move if not below or equal Floating point conditional move if unordered Floating point conditional move if not unordered BASIC ARITHMETIC Add real Add real and pop Add integer Subtract real Subtract real and pop Subtract integer Subtract real reverse Subtract real reverse and pop Subtract integer reverse Multiply real Multiply real and pop Multiply integer Divide real Divide real and pop Divide integer Divide real reverse Divide real reverse and pop Divide integer reverse Partial remainder IEEE Partial remainder Absolute value Change sign Round to integer Scale by power of two intel e INSTRUCTION SET SUMMARY FSQRT Square root FXTRACT Extract exponent and significand 6 2 3 3 COMPARISON FCOM Compare real FCOMP Compare real and pop FCOMPP Compare real and pop twice FUCOM Unordered compare real FUCOMP Unordered compare real and pop
145. adjust the results of arithmetic operations performed in unpacked BCD values see Section 5 2 3 BCD Integers All these instructions assume that the value to be adjusted is stored in the AL register or in one instance the AL and AH registers The AAA instruction adjusts the contents of the AL register following the addition of two unpacked BCD values It converts the binary value in the AL register into a decimal value and stores the result in the AL register in unpacked BCD format the decimal number is stored in the lower 4 bits of the register and the upper 4 bits are cleared If a decimal carry occurred as a result of the addition the CF flag is set and the contents of the AH register are incremented by 1 The AAS instruction adjusts the contents of the AL register following the subtraction of two unpacked BCD values Here again a binary value is converted into an unpacked BCD value If a borrow was required to complete the decimal subtract the CF flag is set and the contents of the AH register are decremented by 1 The AAM instruction adjusts the contents of the AL register following a multiplication of two unpacked BCD values It converts the binary value in the AL register into a decimal value and stores the least significant digit of the result in the AL register in unpacked BCD format and the most significant digit if there is one in the AH register also in unpacked BCD format The AAD instruction adjusts a two digit BCD
146. age programmers exception synchronization remains the responsibility of the programmer It is not uncommon for a programmer to expect that their numeric program will not cause numeric exceptions after it has Intel GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS been tested and debugged but in a different system or numeric environment exceptions may occur regularly nonetheless An obvious example would be use of the program with some numbers beyond the range for which it was designed and tested The example in Section D 3 3 2 Exception Synchronization Examples shows a more subtle way in which unex pected exceptions can occur As described in Section D 3 1 Floating Point Exceptions and Their Defaults depending on options determined by the software system designer the processor can perform one of two possible courses of action when a numeric exception occurs The FPU can provide a default fix up for selected numeric exceptions If the FPU performs its default action for all exceptions then the need for exception synchronization is not manifest However code is often ported to contexts and operating systems for which it was not originally designed The example below illustrates that it is safest to always consider exception synchronization when designing code that uses the FPU Alternatively a software exception handler can be invoked to handle the exception When a numeric exception is unmasked and the exception occurs the FPU
147. ain bytes words or doublewords and can range from zero to 2 1 bytes 4 gigabytes 5 4 intel e DATA TYPES AND ADDRESSING MODES 5 2 7 Floating Point Data Types The processor s floating point instructions recognize a set of real integer and BCD integer data types See Section 7 4 Floating Point Data Types and Formats for a description of FPU data types 5 2 8 MMX Technology Data Types Intel Architecture processors that implement the Intel MMX technology recognize a set of packed 64 bit data types See Section 8 1 2 MMX Data Types for a description of the MMX data types 5 3 OPERAND ADDRESSING An Intel Architecture machine instruction acts on zero or more operands Some operands are specified explicitly in an instruction and others are implicit to an instruction An operand can be located in any of the following places The instruction itself an immediate operand register A memory location TO port 5 3 1 Immediate Operands Some instructions use data encoded in the instruction itself as a source operand These operands are called immediate operands or simply immediates For example the following ADD instruction adds an immediate value of 14 to the contents of the EAX register ADD EAX 14 All the arithmetic instructions except the DIV and IDIV instructions allow the source operand to be an immediate value The maximum value allowed for an immediate operand va
148. al level should not give access to its variables In this case the ENTER instruction copies only that part of the display from the calling procedure which refers to previously nested procedures operating at higher lexical levels The new stack frame does not include the pointer for addressing the calling procedure s stack frame The ENTER instruction treats a re entrant procedure as a call to a procedure at the same lexical level In this case each succeeding iteration of the re entrant procedure can address only its own variables and the variables of the procedures within which it is nested A re entrant procedure always can address its own variables it does not require pointers to the stack frames of previous iterations PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS intel By copying only the stack frame pointers of procedures at higher lexical levels the ENTER instruction makes certain that procedures access only those variables of higher lexical levels not those at parallel lexical levels see Figure 4 6 Main Lexical Level 1 Procedure A Lexical Level 2 Procedure B Lexical Level 3 Procedure C Lexical Level 3 Procedure D Lexical Level 4 Figure 4 6 Nested Procedures Block structured languages can use the lexical levels defined by ENTER to control access to the variables of nested procedures In Figure 4 6 for example if procedure A calls procedure B which in
149. alidate cache instruction INVLPG invalidate TLB entry instruction 6 2 INSTRUCTION SET LIST This section lists all the Intel Architecture instructions divided into three major groups integer MMX technology floating point and system instructions For each instruction the mnemonic and descriptive names are given When two or more mnemonics are given for example CMOVA CMOVNBE they represent different mnemonics for the same instruction opcode Assemblers support redundant mnemonics for some instructions to make it easier to read code listings For instance CMOVA Conditional move if above and CMOVNBE Conditional move is not below or equal represent the same condition 6 2 INSTRUCTION SET SUMMARY intel 6 2 1 Integer instructions perform the integer arithmetic logic and program flow control operations that programmers commonly use to write application and system software to run on an Intel Architecture processor In the following sections the integer instructions are divided into several Integer Instructions instruction subgroups 6 2 1 1 DATA TRANSFER INSTRUCTIONS MOV Move CMOVE CMOVZ Conditional move if equal Conditional move if zero CMOVNE CMOVNZ Conditional move if not equal Conditional move if not zero CMOVA CMOVNBE Conditional move if above Conditional move if not below or equal CMOVAE CMOVNB Conditional move if above or equal Conditional move if not below CMOVB CMOVNAE Conditional move if
150. an optional n argument Increments the stack pointer by the number of bytes specified with the n operand to release parameters from the stack If the call gate descriptor specifies that one or more parameters be copied from one stack to the other a RET n instruction must be used to release the parameters from both stacks Here the n operand specifies the number of bytes occupied on each stack by the parameters On a return the processor increments ESP by n for each stack to step over effectively remove these parameters from the stacks 4 Restores the SS and ESP registers to their values prior to the call which causes a switch back to the stack of the calling procedure 5 Ifthe RET instruction has an optional n argument Increments the stack pointer by the number of bytes specified with the n operand to release parameters from the stack see explanation in step 3 6 Resumes execution of the calling procedure See Chapter 4 Protection in the Intel Architecture Software Developer s Manual Volume 3 for detailed information on calls to privileged levels and the call gate descriptor 4 4 INTERRUPTS AND EXCEPTIONS The processor provides two mechanisms for interrupting program execution interrupts and exceptions An interrupt is an asynchronous event that is typically triggered by an I O device An exception is a synchronous event that is generated when the processor detects or more predefined conditions while execut
151. an the infinitely precise result toward 00 Round toward 11B Rounded result is close to but no greater in absolute value than the zero Truncate infinitely precise result The round up and round down modes are termed directed rounding and can be used to imple ment interval arithmetic Interval arithmetic is used to determine upper and lower bounds for the true result of a multistep computation when the intermediate results of the computation are subject to rounding The round toward zero mode sometimes called the chop mode is commonly used when performing integer arithmetic with the FPU Whenever possible the FPU produces an infinitely precise result in the destination format single double or extended real However it is often the case that the infinitely precise result of an arithmetic or store operation cannot be encoded exactly in the format of the destination operand 7 18 Intel FLOATING POINT UNIT For example the following value a has a 24 bit fraction The least significant bit of this frac tion the underlined bit cannot be encoded exactly in the single real format which has only a 23 bit fraction a 1 0001 0000 1000 0011 1001 01116 101 To round this result a the FPU first selects two representable fractions b and c that most closely bracket a in value b lt a lt c 5 1 0001 0000 1000 0011 1001 O11E 101 c 1 0001 0000 1000 0011 1001 100E 101 The FPU then sets the result to b
152. and other tools then create the actual segment selector values associated with these directives and symbols If you are writing system code you may need to create segment selectors directly A detailed description of the segment selector data structure is given in Chapter 3 Protected Mode Memory Management of the Intel Architecture Software Developer s Manual Volume 3 3 7 BASIC EXECUTION ENVIRONMENT intel e How segment registers are used depends on the type of memory management model that the operating system or executive is using When using the flat unsegmented memory model the segment registers are loaded with segment selectors that point to overlapping segments each of which begins at address 0 of the linear address space as shown in Figure 3 5 These overlap ping segments then comprise the linear address space for the program Typically two overlap ping segments are defined one for code and another for data and stacks The CS segment register points to the code segment and all the other segment registers point to the data and stack segment When using the segmented memory model each segment register is ordinarily loaded with a different segment selector so that each segment register points to a different segment within the linear address space as shown in Figure 3 6 At any time a program can thus access up to six segments in the linear address space To access a segment not pointed to by one of the segment registers a
153. ansfer program control to a destination instruction if the conditions specified with the condition code cc associated with the instruction are satisfied see Table 6 4 If the condition is not satisfied execution continues with the instruction following the Jcc instruction As with the JMP instruction the transfer is one way that is a return address is not saved Table 6 4 Conditional Jump Instructions Instruction Mnemonic Condition Flag States Description Unsigned Conditional Jumps JA JNBE CF or ZF 0 Above not below or equal JAE JNB CF 0 Above or equal not below JB JNAE CF 1 Below not above or equal JBE JNA CF or ZF 1 Below or equal not above JC CF 1 Carry JE JZ ZF 1 Equal zero JNC CF 0 Not carry JNE JNZ ZF 0 Not equal not zero JNP JPO PF 0 Not parity parity odd JP JPE PF 1 Parity parity even JCXZ CX 0 Register CX is zero JECXZ ECX 0 Register ECX is zero Signed Conditional Jumps JG JNLE SF xor OF or ZF 0 Greater not less or equal JGE JNL SF xor OF 0 Greater or equal not less JL JNGE SF xor OF 1 Less not greater or equal JLE JNG SF xor OF or ZF 1 Less or equal not greater JNO OF 0 Not overflow JNS SF 0 Not sign non negative JO OF 1 Overflow JS SF 1 Sign negative The destination operand specifies a relative address a signed offset with respect to the address in the EIP register that points to an instruction in the current code segment The Jcc instructions do not
154. as continued to be true for over three decades the computing power and the complexity or roughly the number of transistors per CPU chip of the silicon integrated circuit micropro cessor doubles every one to two years and the cost per CPU chip is cut in half This law is the main explanation for the computer revolution in which the Intel Architecture plays such a significant role The table below shows the dramatic increases in performance and transistor count of the Intel Architecture processors over their history as predicted by Moore s Law and also summarizes the evolution of other key features of the architecture Table 2 1 Processor Performance Over Time and Other Key Features of the Intel Architecture No of Date of Perfor Max CPU Transis Main Extern Max Caches Product mance Frequency tors CPU Data Extern in CPU Intel Intro in at Intro onthe Register Bus Addr Pack Processor duction MIPs duction Die Size Size Space age 8086 1978 0 8 8 MHz 29K 16 16 1 MB None Intel 286 1982 2 7 12 5 MHz 134 K 16 16 16 MB Note 3 Intel3861M 1985 6 0 20 MHz 275 K 32 32 4 GB Note 3 DX Intel486 1989 20 25 MHz 1 2M 32 32 4GB 8KB L1 DX Pentium 1993 100 60 MHz 3 1M 32 64 4GB 16KB L1 Pentium Pro 1995 440 200 MHz 5 5M 32 64 64 GB 16KBL1 256KB or 512KB L2 NOTES 1 Performance here is indicated by Dhrystone MIPs Millions of Instructions per Second because even
155. as results Denormalized numbers represent an underflow condition A denormalized number is computed through a technique called gradual underflow Table 7 2 gives an example of gradual underflow in the denormalization process Here the single real format is being used so the minimum exponent unbiased is 126 The true result in this example requires an exponent of 129 in order to have a normalized number Since 129 is beyond the allowable exponent range the result is denormalized by inserting leading zeros until the minimum exponent of 126 is reached Table 7 2 Denormalization Process Operation Sign Exponent Significand True Result 0 129 1 01011100000 00 Denormalize 0 128 0 10101110000 00 Denormalize 0 127 0 01010111000 00 Denormalize 0 126 0 00101011100 00 Denormal Result 0 126 0 00101011100 00 NOTE Expressed as an unbiased decimal number In the extreme case all the significant bits are shifted out to the right by leading zeros creating a zero result The FPU deals with denormal values in the following ways Jt avoids creating denormals by normalizing numbers whenever possible It provides the floating point underflow exception to permit programmers to detect cases when denormals are created e It provides the floating point denormal operand exception to permit procedures programs to detect when denormals are being used as source operands for
156. ating point exception it sets the appropriate flags in the FPU status word then takes one of two possible courses of action Handles the exception automatically producing a predefined and often times usable result while allowing program execution to continue undisturbed Invokes a software exception handler to handle the exception The following sections describe how the FPU handles exceptions either automatically or by calling a software exception handler how the FPU detects the various floating point excep tions and the automatic masked response to the floating point exceptions 7 7 1 Arithmetic vs Non arithmetic Instructions When dealing with floating point exceptions it is useful to distinguish between arithmetic instructions and non arithmetic instructions Non arithmetic instructions have no operands or do not make substantial changes to their operands Arithmetic instructions do make signifi cant changes to their operands in particular they make changes that could result in a floating point exception being signaled Table 7 19 lists the non arithmetic and arithmetic instructions It should be noted that some non arithmetic instructions can signal a floating point stack fault exception but this exception is not the result of an operation on an operand 7 7 2 Automatic Exception Handling If the FPU detects an exception condition for a masked exception an exception with its mask bit set it sets the exception f
157. be located in a memory location or in a 16 bit register For example the following MOV instruction moves a segment selector located in register BX into segment register DS MOV DS BX Segment selectors can also be specified explicitly as part of a 48 bit far pointer in memory Here the first doubleword in memory contains the offset and the next word contains the segment selector 5 3 3 2 SPECIFYING AN OFFSET The offset part of a memory address can be specified either directly as an static value called a displacement or through an address computation made up of one or more of the following components 5 7 DATA TYPES AND ADDRESSING MODES intel e Displacement An 8 16 or 32 bit value Base The value in a general purpose register ndex The value in a general purpose register Scale factor A value of 2 4 or 8 that is multiplied by the index value The offset which results from adding these components is called an effective address Each of these components can have either a positive or negative 2s complement value with the excep tion of the scaling factor Figure 5 5 shows all the possible ways that these components can be combined to create an effective address in the selected segment Base Index Scale Displacement EAX EBX EAX 1 None EBX ECX EDX ECX 2 8 bit ESP EDX EBP 3 16 bit EBP ESI EDI Offset Base Index Scale Displacement
158. be regis tered during the state swap process itself and the kernel and floating point exception interrupt handler must be prepared for this case A simple way to handle the case of exceptions arriving during FPU state swaps is to allow the kernel to be one of the FPU owning threads A reserved thread identifier is used to indicate ker nel ownership of the FPU During an floating point state swap the FPU owner variable should be set to indicate the kernel as the current owner At the completion of the state swap the vari able should be set to indicate the new owning thread The numeric exception handler needs to check the FPU owner and discard any numeric exceptions that occur while the kernel is the FPU owner A more general flow for a DNA exception handler that handles this case is shown in Fig ure D 5 Numeric exceptions received while the kernel owns the FPU for a state swap must be discarded in the kernel without being dispatched to a handler A flow for a numeric exception dispatch routine is shown in Figure D 6 It may at first glance seem that there is a possibility of floating point exceptions being lost because of exceptions that are discarded during state swaps This is not the case as the exception will be re issued when the floating point state is reloaded Walking through state swaps both with and without pending numeric exceptions will clarify the operation of these two handlers Case 1 FPU State Swap Without Numeric Exc
159. bit wide stack the value is automatically padded with zeros out to 32 bits The processor does not check stack pointer alignment It is the responsibility of the programs tasks and system procedures running on the processor to maintain proper alignment of stack pointers Misaligning a stack pointer can cause serious performance degradation and in some instances program failures 4 2 3 Address Size Attributes for Stack Accesses Instructions that use the stack implicitly such as the PUSH and POP instructions have two address size attributes each of either 16 or 32 bits This is because they always have the implicit address of the top of the stack and they may also have an explicit memory address for example PUSH Array1 EBX The attribute of the explicit address is determined by the D flag of the current code segment and the presence or absence of the 67H address size prefix as usual The address size attribute of the top of the stack determines whether SP or ESP is used for the stack access Stack operations with an address size attribute of 16 use the 16 bit SP stack pointer register and can use a maximum stack address of FFFFH stack operations with a address size attribute of 32 bits use the 32 bit ESP register and can use a maximum address of FFFFFFFFH The default address size attribute for data segments used as stacks is controlled by the B flag of the segment s descriptor When this flag is clear the default address size attribute is
160. built into the chip design with the addition of the NE bit in control register CRO and the addition of the FERR Floating point ERRor and IGNNE IGNore Numeric Error pins The NE bit selects the native FPU exception handling mode NE 1 or the MS DOS compat ibility mode NE 0 When native mode is selected all signaling of floating point exceptions is handled internally in the Intel486 chip resulting in the generation of an interrupt 16 When MS DOS compatibility mode is selected the FERRR and IGNNE pins are used to signal floating point exceptions The FERR output pin which replaces the ERROR pin from the previous generations of Intel Architecture numeric coprocessors is connected to a PIC A new input signal IGNNE is provided to allow the FPU exception handler to execute FPU instructions if desired without first clearing the error condition and without triggering the inter rupt a second time This IGNNE feature is needed to replicate the capability that was provided on MS DOS compatibility Intel 286 and Intel 287 and Intel386 and Intel 387 systems by turning off the BUSY signal when inside the FPU exception handler before clearing the error condi tion Note that Intel in order to provide Intel486 processors for market segments which had no need for an FPU created the SX versions These Intel486 SX processors did not contain the floating point unit Intel also produced Intel 487 SX processors for end users who later
161. calls the breakpoint exception BP handler The BOUND instruction explicitly calls the BOUND range exceeded exception BR handler if an operand is found to be not within predefined boundaries in memory This instruction is provided for checking references to arrays and other data structures Like the overflow exception the BOUND range exceeded exception can only be raised explicitly with the BOUND instruction or the INT n instruction with an argument of 5 the vector number of the bounds check exception The processor does not implicitly perform bounds checks and raise the BOUND range exceeded exception 4 15 PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS intel 4 5 PROCEDURE CALLS FOR BLOCK STRUCTURED LANGUAGES The Intel Architecture supports an alternate method of performing procedure calls with the ENTER enter procedure and LEAVE leave procedure instructions These instructions auto matically create and release respectively stack frames for called procedures The stack frames have predefined spaces for local variables and the necessary pointers to allow coherent returns from called procedures They also allow scope rules to be implemented so that procedures can access their own local variables and some number of other variables located in other stack frames The ENTER and LEAVE instructions offer two benefits They provide machine language support for implementing block structured languages such as C and Pascal They
162. cates that some normally acceptable accuracy has been lost The exception is supported for applications that need to perform exact arithmetic only Because the rounded result is generally satisfactory for most applications this exception is commonly masked Note that the transcendental instruc tions FSIN FCOS FSINCOS FPTAN FPATAN F2XM1 FYL2X and FYL2XP1 by nature produce inexact results The inexact result exception flag PE is bit 5 of the FPU status word and the mask bit PM is bit 5 of the FPU control word If the inexact result exception is masked when an inexact result condition occurs and a numeric overflow or underflow condition has not occurred the FPU sets the PE flag and stores the rounded result in the destination operand The current rounding mode determines the method used to round the result see Section 7 3 4 3 Rounding Control Field The C1 round up bit in the FPU status word indicates whether the inexact result was rounded up C1 is set or not rounded up C1 is cleared In the not rounded up case the least significant bits of the inexact result are truncated so that the result fits in the destination format If the inexact result exception is not masked when an inexact result occurs and numeric overflow or underflow has not occurred the FPU performs the same operation described in the previous paragraph and in addition invokes a software exception handler see Section 7 7 3 Software Exception
163. cc Condition Tested For Subcode Status Flags Setting Overflow 0000 OF 1 No overflow 0001 OF 0 B Below 0010 CF 1 NAE Neither above nor equal NB Not below 0011 CF 0 AE Above or equal E Equal 0100 ZF 1 Z Zero NE Not equal 0101 ZF 0 NZ Not zero BE Below or equal 0110 CF OR ZF 1 NA Not above NBE Neither below nor equal 0111 CF OR ZF 0 A Above S Sign 1000 SF 1 NS No sign 1001 SF 0 P Parity 1010 1 Parity even NP No parity 1011 0 PO Parity odd Instruction Mnemonic Meaning Subcode Condition Tested L Less 1100 SF xOR OF 1 NGE Neither greater nor equal NL Not less 1101 SF xOR OF 0 GE Greater or equal B 1 EFLAGS CONDITION CODES intel Table B 1 EFLAGS Condition Codes Contd Mnemonic cc Condition Tested For Instruction Subcode Status Flags Setting LE Less or equal 1110 SF XOR OF OR ZF 1 NG Not greater NLE Neither less nor equal 1111 SF XOR OF OR ZF 0 G Greater Many of the test conditions are described in two different ways For example LE less or equal and NG not greater describe the same test condition Alternate mnemonics are provided to make code more intelligible The terms above and below are associated with the CF flag and refer to the relation between two unsigned integer values The terms greater and less are associated with the SF and OF flags and refer to the relation between two signe
164. cessors and brief history of the Intel Architecture Chapter 3 Basic Execution Environment Introduces the models of memory organization and describes the register set used by applications Chapter 4 Procedure Calls Interrupts and Exceptions Describes the procedure stack and the mechanisms provided for making procedure calls and for servicing interrupts and exceptions 1 1 ABOUT THIS MANUAL intel Chapter 5 Data Types and Addressing Modes Describes the data types and addressing modes recognized by the processor Chapter 6 Instruction Set Summary Gives an overview of all the Intel Architecture instructions except those executed by the processor s floating point unit The instructions are presented in functionally related groups Chapter 7 Floating Point Unit Describes the Intel Architecture floating point unit including the floating point registers and data types gives an overview of the floating point instruction set and describes the processor s floating point exception conditions Chapter 8 Programming with Intel MMX Technology Describes the Intel MMX technology including MMX registers and data types and gives an overview of the MMX instruction set Chapter 9 Input Output Describes the processor s I O architecture including I O port addressing the I O instructions and the I O protection mechanism Chapter 10 Processor Identification and Feature Determination Describes how to
165. chanisms provided in the Intel Architecture to support oper ating systems and executives including the system oriented registers and data structures and the system oriented instructions The steps necessary for switching between real address and protected modes are also identified Chapter 3 Protected Mode Memory Management Describes the data structures registers and instructions that support segmentation and paging and explains how they can be used to implement a flat unsegmented memory model or a segmented memory model Chapter 4 Protection Describes the support for page and segment protection provided in the Intel Architecture This chapter also explains the implementation of privilege rules stack switching pointer validation user and supervisor modes Chapter 5 Interrupt and Exception Handling Describes the basic interrupt mechanisms defined in the Intel Architecture shows how interrupts and exceptions relate to protection and describes how the architecture handles each exception type Reference information for each Intel Architecture exception is given at the end of this chapter Chapter 6 Task Management Describes the mechanisms the Intel Architecture provides to support multitasking and inter task protection Chapter 7 Multiple Processor Management Describes the instructions and flags that support multiple processors with shared memory memory ordering and the advanced program mable interrupt contr
166. ck TOP pointer stored in the FPU status word can be used to associate tags with registers relative to ST 0 The FPU uses the tag values to detect stack overflow and underflow conditions Stack overflow occurs when the TOP pointer is decremented due to a register load or push operation to point to a non empty register Stack underflow occurs when the TOP pointer is incremented due to a save or pop operation to point to an empty register or when an empty register is also referenced as a source operand A non empty register is defined as a register containing a zero 01 a valid value 00 or an special 10 value Application programs and exception handlers can use this tag information to check the contents of an FPU data register without performing complex decoding of the actual data in the register To read the tag register it must be stored in memory using either the FSTENV FNSTENV or FSAVE ENS AVE instructions The location of the tag word in memory after being saved with one of these instructions is shown in Figures 7 13 through 7 16 Software cannot directly load or modify the tags in the tag register The FLDENV and FRSTOR instructions load an image of the tag register into the FPU however the FPU uses those tag 7 20 Intel FLOATING POINT UNIT values only to determine if the data registers are empty 11B or non empty 00B 01B or 10B If the tag register image indicates that a data register is empty the tag in the tag regis
167. complete reduction or partial remainder When a successful reduction has been completed the CO C3 and C1 condition code flags are set to the three least significant bits of the quotient Q2 Q1 and QO respectively See FPREM1 Partial Remainder in Chapter 3 Instruction Set Reference of the Intel Architecture Software Developer s Manual Volume 2 for more infor mation on how these instructions use the condition code flags The FPTAN FSIN FCOS and FSINCOS instructions set the C2 flag to 1 to indicate that the source operand is beyond the allowable range of 26 Where the state of the condition code flags are listed as undefined in Table 7 3 do not rely on any specific value in these flags 7 13 FLOATING POINT UNIT Table 7 3 FPU Condition Code Interpretation Instruction co C3 C2 C1 FCOM FCOMP FCOMPP FICOM FICOMP FTST FUCOM FUCOMP FUCOMPP Result of Comparison Operands are not Comparable 0 or HIS FCOMI FCOMIP FUCOMI FUCOMIP Undefined These instructions set the status flags in the EFLAGS register 5 FXAM Operand class Sign FPREM FPREM1 Q2 Q1 O reduction complete 1 reduction incomplete QO or IS F2XM1 FADD FADDP FBSTP FCMOVcc FIADD FDIV FDIVP FDIVR FDIVRP FIDIV FIDIVR FIMUL FIST FISTP FISUB FISUBR FMUL FMULP FPATAN FRNDINT FSCALE FST FSTP FSUB FSUBP FSUBR FSUBRPFSQRT FYL2X FYL2XP1 Und
168. computations When a denormal number in single or double real format is used as a source operand and the denormal exception is masked the FPU automatically normalizes the number when it is converted to extended real format 7 7 FLOATING POINT UNIT intel 7 2 3 3 SIGNED INFINITIES The two infinities and ee represent the maximum positive and negative real numbers respectively that can be represented in the floating point format Infinity is always represented by a zero significand fraction and integer bit and the maximum biased exponent allowed in the specified format for example 255 for the single real format The signs of infinities are observed and comparisons are possible Infinities are always inter preted in the affine sense that is ee is less than any finite number and is greater than any finite number Arithmetic on infinities is always exact Exceptions are generated only when the use of an infinity as a source operand constitutes an invalid operation Whereas denormalized numbers represent an underflow condition the two infinity numbers represent the result of an overflow condition Here the normalized result of a computation has a biased exponent greater than the largest allowable exponent for the selected result format 7 2 3 4 NANS Since NaNs are non numbers they are not part of the real number line In Figure 7 3 the encoding space for NaNs in the FPU floating point formats is shown ab
169. critical code modules in the system usually the kernel of an operating system The outer rings with progressively lower privileges are used for segments that contain code modules for less critical software Code modules in lower privilege segments can only access modules operating at higher privilege segments by means of a tightly controlled and protected interface called a gate Attempts to access higher privilege segments without going through a protection gate and without having sufficient access rights causes a general protection exception GP to be generated 4 7 PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS intel Protection Rings Operating System Kernel Operating System Services Device N Drivers Etc N Level 1 Applications Highest Lowest 0 1 2 3 L Privilege Levels Figure 4 3 Protection Rings If an operating system or executive uses this multilevel protection mechanism a call to a proce dure that is in a more privileged protection level than the calling procedure is handled in a similar manner as a far call see Section 4 3 2 Far CALL and RET Operation The differences are as follows The segment selector provided in the CALL instruction references a special data structure called a call gate descriptor Among other things the call gate descriptor provides the following Access rights information The segment selector for the code segment of the called proced
170. ction 7 33 FLDSW instruction 7 40 FLDZ instruction 7 33 Floating point data types 7 24 Floating point exceptions automatic handling 7 43 denormal operand exception 7 50 division by zero 7 49 exception conditions 7 47 exception priorily 7 53 guidelines for writing exception handlers D 1 inexact result precision 7 53 invalid arithmetic operand 7 47 7 48 MS DOS compatibility mode D 1 numeric overfloW 7 50 numeric underfloW 7 52 software handling 7 45 stack overflow 7 13 7 47 stack underflow 7 13 7 47 7 48 7 42 synchronization 7 54 Floating point format biasedexponent 7 5 description 7 24 exponeht eer egli 7 3 fracti n boli ee Ee che as 7 3 real number system 7 2 realnumbers 7 25 S ON ze a am eee De a oe ae 7 3 significand 7 3 FMUL instruction 7 33 FMULP instruction 7 33 FNOP instruction 7 39 FPATAN instruction 7 37 FPREM instruction 7 13 7 33 7 37
171. ctions This will not happen in the Pentium Pro processor because this sampling window has been removed from the no wait group of FPU instructions D 3 RECOMMENDED PROTOCOL FOR MS DOS COMPATIBILITY HANDLERS The activities of numeric programs can be split into two major areas program control and arith metic The program control part performs activities such as deciding what functions to perform calculating addresses of numeric operands and loop control The arithmetic part simply adds subtracts multiplies and performs other operations on the numeric operands The processor is designed to handle these two parts separately and efficiently An FPU exception handler if a system chooses to implement one is often one of the most complicated parts of the program control code D 3 1 Floating Point Exceptions and Their Defaults The FPU can recognize six classes of floating point exception conditions while executing floating point instructions 1 I Invalid operation IS Stack fault IEEE standard invalid operation Intel GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS 2 HZ Divide by zero 3 D Denormalized operand 4 O Numeric overflow 5 0 Numeric underflow 6 P Inexact result precision For complete details on these exceptions and their defaults see Section 7 7 Floating Point Exception Handling and Section 7 8 Floating Point Exception Conditions D 3 2 Two Options for Handl
172. ctions are given in Chapter 3 Instruction Set Reference in the Intel Architecture Software Developer s Manual Volume 2 7 1 COMPATIBILITY AND EASE OF USE OF THE INTEL ARCHITECTURE FPU The architecture of the Intel Architecture FPU has evolved in parallel with the architecture of early Intel Architecture processors The first Intel Math Coprocessors the Intel 8087 Intel 287 and Intel 387 were companion processors to the Intel 8086 8088 Intel 286 and Intel386 processors respectively and were designed to improve and extend the numeric processing capa bility of the Intel Architecture The Intel486 DX processor for the first time integrated the CPU and the FPU architectures on one chip The Pentium processor s FPU offered the same architec ture as the Intel486 DX processor s FPU but with improved performance The Pentium Pro processor s FPU further extended the floating point processing capability of Intel Architecture family of processors and added several new instructions to improve processing throughput Throughout this evolution compatibility among the various generations of FPUs and math coprocessors has been maintained For example the Pentium Pro processor s FPU is fully compatible with the Pentium and Intel486 DX processors s FPUs Each generation of the Intel Architecture FPUs have been explicitly designed to deliver stable accurate results when programmed using straightforward pencil and paper algorithms bringing the functi
173. d and a divide by zero exception Here if both exceptions are masked the FPU handles the higher priority exception only the invalid arithmetic operand exception returning a real indefinite to the destination Alternately a denormal operand or inexact result exception can accompany a numeric underflow or overflow exception with both exceptions being handled The precedence for floating point exceptions is as follows 1 Invalid operation exception subdivided as follows a Stack underflow b Stack overflow c Operand of unsupported format SNaN operand 2 QNaN operand Though this is not an exception the handling of a QNaN operand has precedence over lower priority exceptions For example a QNaN divided by zero results in a QNaN not a zero divide exception 3 Any other invalid operation exception not mentioned above or a divide by zero exception 4 Denormal operand exception If masked then instruction execution continues and a lower priority exception can occur as well 5 Numeric overflow and underflow exceptions in conjunction with the inexact result exception 6 Inexact result exception Invalid operation zero divide and denormal operand exceptions are detected before a floating point operation begins whereas overflow underflow and precision errors are not detected until a true result has been computed When a pre operation exception is detected the FPU register stack and memory have not yet been updat
174. d integer instruction converts an integer operand in memory into extended real format and pushes the value onto the top of the register stack The FBLD load packed decimal instruction performs the same load operation for a packed BCD operand in memory The FST store real and FIST store integer instructions store the value in register ST 0 in memory in the destination format real or integer respectively Again the format conversion is carried out automatically 7 31 FLOATING POINT UNIT intel The FSTP store real and pop FISTP store integer and pop and FBSTP store packed decimal and pop instructions store the value in the ST 0 registers into memory in the destination format real integer or packed BCD then performs a pop operation on the register stack A pop oper ation causes the ST 0 register to be marked empty and the stack pointer TOP in the FPU control work to be incremented by 1 The FSTP instruction can also be used to copy the value in the ST 0 register to another FPU register ST 1 The FXCH exchange register contents instruction exchanges the value in a selected register in the stack ST 1 with the value in ST 0 The FCMOVcc conditional move instructions move the value in a selected register in the stack ST 1 to register ST 0 These instructions move the value only if the conditions specified with a condition code cc are satisfied see Table 7 14 The conditions being tested with the FCMO
175. d integer values B 2 Intel Floating Point Exceptions Summary Intel APPENDIX C FLOATING POINT EXCEPTIONS SUMMARY Table C 1 lists the floating point instruction mnemonics in alphabetical order For each mnemonic it summarizes the exceptions that the instruction may cause See Section 7 8 Floating Point Exception Conditions for a detailed discussion of the floating point excep tions The following codes indicate the floating point exceptions 15 Invalid operation exception for stack underflow or stack overflow HIA Invalid operation exception for invalid arithmetic operands and unsupported formats 0 Denormal operand exception 7 Divide by zero exception 0 Numeric overflow exception U Numeric underflow exception P Inexact result precision exception Table C 1 Floating Point Exceptions Summary Mnemonic Instruction 5 AD Z 0 U P F2XM1 2X1 Y Y Y Y Y FABS Absolute value Y FADD P Add real Y Y Y Y Y Y FBLD BCD load Y FBSTP BCD store and pop Y Y Y FCHS Change sign Y FCLEX Clear exceptions FCMOVcc Floating point conditional move Y FCOM FCOMP FCOMPP Compare real Y Y FCOMI FCOMIP FUCOMI Compare real and set EFLAGS Y Y FUCOMIP FCOS Cosine Y Y Y Y Y FDECSTP Decrement stack pointer FDIV R P Divide real Y Y Y Y Y Y Y FFREE Free register C 1 FLOATING POINT EXCEPTIONS SUMMARY Table C 1 Floating Point Exceptions Summary
176. ddressed by the ESI register to the location addressed by the EDI register The assembler recognizes three short forms of this instruction which specify the size of the string to be moved MOVSB move byte string MOVSW move word string and MOVSD move doubleword string The CMPS instruction subtracts the destination string element from the source string element and updates the status flags CF ZF OF SF PF and AF in the EFLAGS register according to the results Neither string element is written back to memory The assembler recognizes three short forms of the CMPS instruction CMPSB compare byte strings CMPSW compare word strings and CMPSD compare doubleword strings The SCAS instruction subtracts the destination string element from the contents of the EAX AX or AL register depending on operand length and updates the status flags according to the results The string element and register contents are not modified The following short forms of the SCAS instruction specifies the operand length SCASB scan byte string SCASW scan word string and SCASD scan doubleword string The LODS instruction loads the source string element identified by the ESI register into the EAX register for a doubleword string the AX register for a word string or the AL register for a byte string The short forms for this instruction are LODSB load byte string LODSW load word string and LODSD load doubleword string This inst
177. decided to upgrade to a system with an FPU These Intel 487 SX processors are similar to standard Intel486 processors with a working FPU on board Thus the external circuitry necessary to support the MS DOS compatibility mode for Intel 487 SX processors is the same as for standard Intel486 DX processors The Pentium and Pentium Pro processors offer the same mechanism the NE bit and the FERR and IGNNE pins as the Intel486 processors for generating FPU exceptions in MS DOS compatibility mode The actions of these mechanisms are slightly different and more straight D 3 GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS intel forward for the Pentium Pro processors as described in Section D 2 2 MS DOS Compati bility Mode in the Pentium Pro Processor For Pentium and Pentium Pro processors it is important to note that the special DP Dual Processing mode for Pentium Processors and also the more general Intel MultiProcessor Spec ification for systems with multiple Pentium or Pentium Pro processors support FPU exception handling only in the native mode Intel does not recommend using the MS DOS compatibility FPU mode for systems using more than one processor D 2 1 1 BASIC RULES WHEN FERR IS GENERATED When MS DOS compatibility mode is enabled for the Intel486 or Pentium processors NE bit is set to 0 and the IGNNE input pin is de asserted the FERR signal is generated as follows 1 When an FPU instruction causes an unmasked F
178. decrements TOP and loads the value 3 8 in ST 0 4 The fourth instruction multiplies the value in ST 0 by the value 10 3 from memory and stores the result in ST 0 shown in snap shot c 5 The fifth instruction adds the value and the value in ST 1 and stores the result in ST 0 shown in snap shot d The style of programming demonstrated in this example is supported by the floating point instruction set In cases where the stack structure causes computation bottlenecks the FXCH exchange FPU register contents instruction can be used to streamline a computation 7 3 1 1 PARAMETER PASSING WITH THE FPU REGISTER STACK Like the general purpose registers in the processor s integer unit the contents of the FPU data registers are unaffected by procedure calls or in other words the values are maintained across procedure boundaries A calling procedure can thus use the FPU data registers as well as the procedure stack for passing parameter between procedures The called procedure can reference parameters passed through the register stack using the current stack register pointer TOP and the ST 0 and ST 1 nomenclature It is also common practice for a called procedure to leave a return value or result in register ST 0 when returning execution to the calling procedure or program 7 11 FLOATING POINT UNIT intel Computation Dot Prod
179. derflow FPU stack 7 47 7 48 Underflow numeric 7 7 Un normal number 7 28 Unsigned integers 5 4 6 22 6 23 Unsupported floating point formats 7 28 Unsupported FPU instructions 7 41 V Vector see Interrupt vector VIF virtual interrupt flag EFLAGS register 3 13 VIP virtual interrupt pending flag EFLAGS register 3 13 Virtual 8086 mode description of 3 13 memory model 3 4 VM virtual 8086 mode flag EFLAGS register cha errada da ERR 3 13 Waiting instructions 7 40 WAIT FWAIT instructions 7 40 7 55 WBINVD 6 2 WOK E ere csi tees ue as eater a ea 5 1 Wraparound mode MMX instructions 8 5 WRMSR instruction 6 2 10 1 X XADD instruction 6 2 6 18 XCHG instruction 6 17 XLAT XLATB instruction 6 40 XOR instruction 6 25 2 division by zero exception flag FPU status word 7 14 Zero floating point format 7 6 ZF zero flag EFLAGS register 3 11 INDEX INDEX 9
180. describe the basic methods of addressing memory when memory manage ment is used When employing the processor s memory management facilities programs do not directly address physical memory Instead they access memory using any of three memory models flat segmented or real address mode 3 2 intel e BASIC EXECUTION ENVIRONMENT With the flat memory model see Figure 3 2 memory appears to a program as a single contin uous address space called a linear address space Code a program s instructions data and the procedure stack are all contained in this address space The linear address space is byte addressable with addresses running contiguously from 0 to 2 1 An address for any byte in the linear address space is called a linear address Flat Model Linear Address Linear Address Space Segmented Model A Segments m at Linear E Offset Address Logical spaco Address Segment Selector pro 2 o 0 Real Address Mode Model Linear Address Offset Space Divided 4 Into Equal Sized Segments _ _ 0069 Segment Selector gt p gt 4 The linear address space can be paged when using the flat or segmented model Figure 3 2 Three Memory Management Models With the segmented memory model memory appears to a program as a group of independent add
181. descriptions of the MMX instructions are provided in Chapter 3 Instruction Set Reference of the Intel Archi tecture Software Developer s Manual Volume 2 The manner in which the MMX technology is integrated into the Intel Architecture system programming model is described in Chapter 10 MMX Technology System Programming Model in the Intel Architecture Software Developer s Manual Volume 3 8 1 OVERVIEW OF THE MMX TECHNOLOGY PROGRAMMING ENVIRONMENT MMX technology provides the following new extensions to the Intel Architecture programming environment Eight MMX registers MMO through MM7 Four MMX data types packed bytes packed words packed doublewords and quadword The MMX instruction set The MMX registers and data types are described in the following sections See Section 8 3 Overview of the MMX Instruction Set for an overview of the MMX instructions 8 1 PROGRAMMING WITH THE INTEL MMX TECHNOLOGY intel 8 1 1 MMX Registers The MMX register set consists of eight 64 bit registers see Figure 8 1 The MMX instructions access the MMX registers directly using the register names MMO through MM7 These registers can only be used to perform calculations on MMX data types they cannot be used to address memory Addressing of MMX instruction operands in memory is handled by using the standard Intel Architecture addressing modes and general purpose registers EAX EBX ECX EDX EBP ESI EDI and ES
182. deter mine the CPU type and the features that are available in the processor Appendix A EFLAGS Cross Reference Summaries how the Intel Architecture instructions affect the flags in the EFLAGS register Appendix B EFLAGS Condition Codes Summarizes how the conditional jump move and byte set on condition code instructions use the condition code flags OF CE ZF SF and PF in the EFLAGS register Appendix C Floating Point Exceptions Summary Summarizes the exceptions that can be raised by floating point instructions Appendix D Guidelines for Writing FPU Exception Handlers Describes how to design and write MS DOS compatible exception handling facilities for FPU exceptions including both software and hardware requirements and assembly language code examples This appendix also describes general techniques for writing robust FPU exception handlers 1 2 OVERVIEW OF THE INTEL ARCHITECTURE SOFTWARE DEVELOPER S MANUAL VOLUME 2 INSTRUCTION SET REFERENCE The contents of the Intel Architecture Software Developer s Manual Volume 2 are as follows Chapter 1 About This Manual Gives an overview of all three volumes of the Intel Archi tecture Software Developer s Manual It also describes the notational conventions in these manuals and lists related Intel manuals and documentation of interest to programmers and hard ware designers Chapter 2 Instruction Format Describes the machine level instruction format used fo
183. determine if an FPU or NPX is present See Chapter 17 Intel Architecture Compatibility in the Intel Architecture Software Developer s Manual Volume 3 for a complete list of the features that are available for the different Intel Architecture processors 10 1 PROCESSOR IDENTIFICATION The CPUID instruction returns the processor type for the processor that executes the instruction It also indicates the features that are present in the processor including the existence of an on chip FPU The following information can be obtained with this instruction The highest operand value the instruction responds to 2 for the Pentium Pro processors and 1 for the Pentium processors and recent Intel486TM processors The processor s family identification ID number model ID and stepping ID The presence of an on chip FPU Support for or the presence of the following architectural extensions and enhancements Virtual 8086 mode enhancements Debugging extensions Page size extensions Read time stamp counter RDTSC instruction Read model specific registers RDMSR and write model specific registers WRMSR instructions Physical address extension Machine check exceptions Compare and exchange 8 bytes instruction CMPXCHGSB On chip advanced programmable interrupt controller APIC Memory type range registers MTRRs Page global flag 10 1 PROCESSOR IDENTIFICATION AND FEATURE DET
184. dling floating point exceptions by providing a dedicated input pin ERROR for receiving floating point exception signals and a dedicated interrupt number 16 Interrupt 16 was used to signal floating point errors also called math faults It was intended that the ERROR pin on the Intel 286 be connected to a corre sponding ERROR pin on the Intel 287 numeric coprocessor When the Intel 287 signals a floating point exception using this mechanism the Intel 286 generates an interrupt 16 to invoke the floating point exception handler To maintain compatibility existing PC software the native floating point exception handling mode of the Intel 286 and 287 was not used in the IBM PC AT system design Instead the ERROR pin on the Intel 286 was tied permanently high and the ERROR pin from the Intel 287 was routed to a second cascaded PIC The resulting output of this PIC was routed through an exception handler and eventually caused an interrupt 2 NMI interrupt Here the NMI inter rupt was shared with PC AT s new parity checking feature Interrupt 16 remained assigned to the BIOS video interrupt handler The external hardware for the MS DOS compatibility mode must prevent the Intel 286 processor from executing past the next FPU instruction when an unmasked exception has been generated To do this it asserts the BUSY signal into the Intel 286 when the ERROR signal is asserted by the Intel 287 The Intel386 processor and its companion Intel 387 n
185. e classes of real numbers that is zero denormalized finite normalized finite and and NaNs for each of the three real data types It also gives the format for the real indefinite value When storing real values in memory single real values are stored in 4 consecutive bytes in memory double real values are stored in 8 consecutive bytes and extended real values are stored in 10 consecutive bytes As a general rule values should be stored in memory in double real format This format provides sufficient range and precision to return correct results with a minimum of programmer attention 7 25 FLOATING POINT UNIT intel The single real format is appropriate for applications that are constrained by memory however it provides less precision and a greater chance of overflow The single real format is also useful for debugging algorithms because rounding problems will manifest themselves more quickly in this format The extended real format is normally reserved for holding intermediate results in the FPU registers and constants Its extra length is designed to shield final results from the effects of rounding and overflow underflow in intermediate calculations However when an application requires the maximum range and precision of the FPU for data storage computa tions and results values can be stored in memory in extended real format The real indefinite value is a QNaN encoding that is stored by several floating point instr
186. e first operand from the contents of the ESP register to allocate dynamic storage The non nested form differs from the nested form in that no stack frame pointers are copied The nested form of the ENTER instruction occurs when the second parameter lexical level is not zero The following pseudo code shows the formal definition of the ENTER instruction STORAGE is the number of bytes of dynamic storage to allocate for local variables and LEVEL is the lexical nesting level PUSH EBP FRAME PTR lt ESP IF LEVEL 0 THEN DO LEVEL 1 times EBP EBP 4 PUSH Pointer EBP doubleword pointed to by EBP OD PUSH FRAME PTR Fl EBP FRAME PTR ESP ESP STORAGE The main procedure in which all other procedures are nested operates at the highest lexical level level 1 The first procedure it calls operates at the next deeper lexical level level 2 A level 2 procedure can access the variables of the main program which are at fixed locations specified by the compiler In the case of level 1 the ENTER instruction allocates only the requested dynamic storage on the stack because there is no previous display to copy A procedure which calls another procedure at a lower lexical level gives the called procedure access to the variables of the caller The ENTER instruction provides this access by placing a pointer to the calling procedure s stack frame in the display A procedure which calls another procedure at the same lexic
187. e old mechanism is available in FPU s prior to the Pentium Pro processor and in the Pentium Pro processor This mechanism uses the floating point compare instructions FCOM FCOMP FCOMPP FTST FUCOMPP FICOM and FICOMP to compare two floating point values and set the condition code flags CO through C3 according to the results The contents of the condi tion code flags are then copied into the status flags of the EFLAGS register using a two step process see Figure 7 9 1 The FSTSW AX instruction moves the FPU status word into the AX register 2 The SAHF instruction copies the upper 8 bits of the AX register which includes the condition code flags into the lower 8 bits of the EFLAGS register When the condition code flags have been loaded into the EFLAGS register conditional jumps or conditional moves can be performed based on the new settings of the status flags in the EFLAGS register 7 15 FLOATING POINT UNIT intel 15 FPU Status Word 0 Condition Status C C CIC Code Flag 3 211 0 1 Y C2 PF FSTSW AX Instruction C3 ZF 15 AX Register 0 C c cC 3 21110 SAHF Instruction 31 EFLAGS Register 7 0 PC F F F Figure 7 9 Moving the FPU Condition Codes to the EFLAGS Register The new mechanism is available only in the Pentium Pro processor Using this mechanism the new
188. e the invalid opcode exception D 22 Intel GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS 2 Save the FPU contents to the old thread s save area typically using an FNSAVE instruction 3 Setthe FPU Owner variable to the identify the currently executing thread 4 Reload the FPU contents from the new thread s save area typically using an FRSTOR instruction 5 Clear TS using the CLTS instruction and exit the DNA exception handler While this flow covers the basic requirements for speculatively deferred FPU state swaps there are some additional subtleties that need to be handled in a robust implementation D 3 6 3 INTERACTION OF FPU STATE SAVES AND FLOATING POINT EXCEPTION ASSOCIATION Recall these key points from earlier in this document When considering floating point excep tions across all implementations of the Intel Architecture and across all floating point instruc tions an floating point exception can be initiated from any time during the excepting floating point instruction up to just before the next floating point instruction The next floating point instruction may be the FNSAVE used to save the FPU state for a task switch In the case of no wait instructions such as FNSAVE the interrupt from a previously excepting instruction NE 0 case may arrive just before the no wait instruction during or shortly thereafter with a system dependent delay Note that this implies that an floating point exception might
189. e top of stack value the return instruction pointer into the EIP register 2 If the RET instruction has an optional n argument Increments the stack pointer by the number of bytes specified with the n operand to release parameters from the stack 3 Resumes execution of the calling procedure Stack Frame Before Call Stack Frame After Y Call Stack During Stack During Stack During Near Return Far Call Near Call i Frame Param 1 CU Param 1 Param 2 Param 2 Param 3 ESP Before Call Param 3 Calling EIP amp ESP After Call T Calling CS Stack Calling EIP Famey After Call Stack During Far Return r ESP After Return Param 1 Param 2 Param 1 Param 3 Param 2 Calling EIP Param 3 lt ESP Before Return Calling CS Calling EIP Note On a near or far return parameters are released from the stack if the correct value is given for the n operand in the RET n instruction ESP Before Call amp ESP After Call lt ESP After Return ESP Before Return Figure 4 2 Stack on Near and Far Calls 4 5 PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS intel 4 3 2 Far CALL and RET Operation When executing a far call the processor performs these actions see Figure 4 4 1 Pushes current value of the CS register on the stack
190. eading to the Intel Architecture can be traced back through the 8085 and 8080 microprocessors to the 4004 microprocessor the first microprocessor designed by Intel in 1969 However the first actual processor in the Intel Architecture family is the 8086 quickly followed by a more cost effective version for smaller systems the 8088 The object code programs created for these processors starting in 1978 will still execute on the latest members of the Intel Architecture family The 8086 has 16 bit registers and a 16 bit external data bus with 20 bit addressing giving a 1 MByte address space The 8088 is identical except for a smaller external data bus of 8 bits These processors introduced Intel Architecture segmentation but only in Real Mode 16 bit registers can act as pointers to address into segments of up to 64 KBytes in size The four segment regis ters hold the effectively 20 bit base addresses of the currently active segments up to 256 KBytes can be addressed without switching between segments and a total address range of 1 MByte is available The Intel 80286 processor introduced the Protected Mode into the Intel Architecture This new mode uses the segment register contents as selectors or pointers into descriptor tables The descriptors provide 24 bit base addresses allowing a maximum physical memory size of up to 16 MBytes support for virtual memory management on a segment swapping basis and various protection mechanisms These
191. ear the loop is terminated by transferring program control to the instruction immediately following the LOOPE LOOPZ instruction The LOOPNE and LOOPNZ instructions mnemonics for the same instruction operate the same as the LOOPE LOOPPZ instructions except that they terminate the loop if the ZF flag is set 6 9 2 3 JUMP IF ZERO INSTRUCTIONS The JECXZ jump if ECX zero instruction jumps to the location specified in the destination operand if the ECX register contains the value zero This instruction can be used in combination with a loop instruction LOOP LOOPE LOOPZ LOOPNE or LOOPNZ to test the ECX register prior to beginning a loop As described in Section 6 9 2 2 Loop Instructions the loop 6 33 INSTRUCTION SET SUMMARY intel e instructions decrement the contents of the ECX register before testing for zero If the value in the ECX register is zero initially it will be decremented to FFFFFFFFH on the first loop instruc tion causing the loop to be executed 2 times To prevent this problem a JECXZ instruction can be inserted at the beginning of the code block for the loop causing a jump out the loop if the EAX register count is initially zero When used with repeated string scan and compare instruc tions the JECXZ instruction can determine whether the loop terminated because the count reached zero or because the scan or compare conditions were satisfied The JCXZ jump if CX is zero instruction operates the same as the
192. earches the instruction pool for completed instructions that no longer have data dependencies with other instructions or unresolved branch predictions When completed instructions are found the retirement unit commits the results of these instructions to memory and or the Intel Architecture registers the processor s eight general purpose registers and eight floating point unit data registers in the order they were originally issued and retires the instructions from the instruction pool Through deep branch prediction dynamic data flow analysis and speculative execution dynamic execution removes the constraint of linear instruction sequencing between the tradi tional fetch and execute phases of instruction execution It allows instructions to be decoded deep into multi level branches to keep the instruction pipeline full It promotes out of order instruction execution to keep the processor s six instruction execution units running at full capacity And finally it commits the results of executed instructions in original program order to maintain data integrity and program coherency The following section describes the Pentium Pro processor microarchitecture in greater detail 2 7 INTRODUCTION TO THE INTEL ARCHITECTURE intel e 2 5 DETAILED DESCRIPTION OF THE PENTIUM PRO PROCESSOR MICROARCHITECTURE Figure 2 2 shows a functional block diagram of the Pentium Pro processor microarchitecture In this diagram the following blocks make up
193. ecause FNSAVE clears the FPU errors and thus de asserts FERR When the processor returns from SMM it will restore the FPU state with FRSTOR which will re assert FERR but the IGNNE Flip Flop will not get set Then when the FPU error handler executes the FLDCW instruction the active error condition will cause the processor to re enter the FPU error handler from the beginning This may cause the handler to malfunction To avoid this problem Intel recommends two measures 1 Do not use FPU for calculations inside SMM code The normal power management and sometimes security functions provided by SMM have no need for FPU calculations if they are needed for some special case use scaling or emulation instead This eliminates the need to do FNSAVE FRSTOR inside SMM code except when going into a 0 V suspend state in which in order to save power the CPU is turned off completely requiring its complete state to be saved 2 The system should not call upon SMM code to put the processor into 0 V suspend while the processor is running FPU calculations or just after an interrupt has occurred Normal power management protocol avoids this by going into power down states only after timed intervals in which no system activity occurs D 3 6 Considerations When FPU Shared Between Tasks The Intel Architecture allows speculative deferral of floating point state swaps on task switches This feature allows postponing an FPU state swap until an F
194. ed When denormal operands have reduced significance due to loss of low order bits it may be advisable to not operate on them Precluding denormal operands from computations can be accomplished by an exception handler that responds to unmasked denormal operand exceptions 7 8 4 Numeric Overflow Exception O The FPU reports a floating point numeric overflow exception 0 whenever the rounded result of an arithmetic instruction exceeds the largest allowable finite value that will fit into the real format of the destination operand For example if the destination format is extended real 80 bits overflow occurs when the rounded result falls outside the unbiased range of 1 0 216384 to 1 0 216584 exclusive Numeric overflow can occur on arithmetic operations where the result is stored in an FPU data register It can also occur on store real operations with the FST and 7 50 Intel FLOATING POINT UNIT FSTP instructions where a within range value in a data register is stored in memory in a single or double real format The overflow threshold range for the single real format is 1 0 2128 to 1 0 2128 the range for the double real format is 1 0 21 to 1 0 21024 The numeric overflow exception cannot occur when overflow occurs when storing values in an integer or BCD integer format Instead the invalid arithmetic operand exception is signaled The flag OE for the numeric overflow exception is bit 3 of the FPU status
195. ed and appear as if the offending instructions has not been executed When a post operation exception is detected the register stack and memory may be updated with a result depending on the nature of the error 7 9 FLOATING POINT EXCEPTION SYNCHRONIZATION Because the integer unit and FPU are separate execution units it is possible for the processor to execute floating point integer and system instructions concurrently No special programming techniques are required to gain the advantages of concurrent execution Floating point instruc tions are placed in the instruction stream along with the integer and system instructions However concurrent execution can cause problems for floating point exception handlers This problem is related to the way the FPU signals the existence of unmasked floating point exceptions Special exception synchronization is not required for masked floating point excep tions because the FPU always returns a masked result to the destination operand 7 54 ntel e FLOATING POINT UNIT When a floating point exception is unmasked and the exception condition occurs the FPU stops further execution of the floating point instruction and signals the exception event On the next occurrence of a floating point instruction or a WAIT FWAIT instruction in the instruction stream the processor checks the ES flag in the FPU status word for pending floating point exceptions It floating point exceptions are pending the FP
196. ed Significand 0 10000110 01100100010000000000000 1 Implied Representing numbers in normalized form maximizes the number of significant digits that can be accommodated in a significand of a given width To summarize a normalized real number consists of a normalized significand that represents a real number between 1 and 2 and an expo nent that specifies the number s binary point 7 2 2 2 BIASED EXPONENT The FPU represents exponents in a biased form This means that a constant is added to the actual exponent so that the biased exponent is always a positive number The value of the biasing constant depends on the number of bits available for representing exponents in the floating point format being used The biasing constant is chosen so that the smallest normalized number can be reciprocated without overflow See Section 7 4 1 Real Numbers for a list of the biasing constants that the FPU uses for the various sizes of real data types 7 2 8 Real Number and Non number Encodings A variety of real numbers and special values can be encoded in the FPU s floating point format These numbers and values are generally divided into the following classes Signed zeros Denormalized finite numbers Normalized finite numbers Signed infinities NaNs Indefinite numbers The term NaN stands for Not a Number 7 5 intel Figure 7 3 shows how the encodings for these numbers and non numbe
197. efined Roundup or 15 FCOS FSIN FSINCOS FPTAN Undefined 1 source operand out of range Roundup or IS Undefined if C2 1 FABS FBLD FCHS FDECSTP FILD FINCSTP FLD Load Constants FSTP ext real FXCH FXTRACT Undefined 0 or HIS FLDENV FRSTOR Each bit loaded from memory FFREE FLDCW FCLEX FNCLEX FNOP FSTCW FNSTCW FSTENV FNSTENV FSTSW FNSTSW Undefined FINIT FNINIT FSAVE FNSAVE 7 3 2 3 EXCEPTION FLAGS The six exception flags bits 0 through 5 of the status word indicate that one or more floating point exceptions has been detected since the bits were last cleared The individual exception flags IE DE ZE OE UE and PE are described in detail in Section 7 7 Floating Point Exception Handling Each of the exception flags can be masked by an exception mask bit in the FPU control word see Section 7 3 4 FPU Control Word The exception summary status ES flag bit 7 is set when any of the unmasked exception flags are set When the ES flag is 7 14 Intel FLOATING POINT UNIT set the FPU exception handler is invoked using one of the techniques described in Section 7 7 3 Software Exception Handling Note that if an exception flag is masked the FPU will still set the flag if its associated exception occurs but it will not set the ES flag The exception flags are sticky bits meaning that once set they remain set until
198. egrating an MMX routine into an application running under an existing operating system programmers need to take special precautions similar to those when writing floating point code When an MMX instruction executes the floating point tag word is marked valid 00s Subse quent floating point instructions that will be executed may produce unexpected results because the floating point stack seems to contain valid data The EMMS instruction marks the floating PROGRAMMING WITH THE INTEL MMX TECHNOLOGY intel point tag word as empty Therefore it is imperative to use the EMMS instruction at the end of every MMX routine if the next routine may contain FPU code The EMMS instruction must be used in each of the following cases When an application using the floating point instructions calls an MMX technology library DLL Use the EMMS instruction at the end of the MMX code When an application using MMX instructions calls a floating point library DLL Use the EMMS instruction before calling the floating point code When a switch is made between MMX code in a task thread and other tasks threads in cooperative operating systems unless it is certain that more MMX instructions will be executed before any FPU code If the EMMS instruction is not used when trying to execute a floating point instruction the following may occur Depending on the exception mask bits of the floating point control word a floating point excepti
199. either immediately or else delayed deferred until just before the execution of the next waiting floating point instruction or MMX instruction Whether the FERR pin is asserted immediately or delayed depends on the type of processor the instruction and the type of exception 3 Ifa preceding floating point instruction has set the exception flag for an unmasked FPU exception the processor freezes just before executing the next WAIT instruction waiting floating point instruction or MMX instruction Whether the FERR pin was asserted at the preceding floating point instruction or is just now being asserted the freezing of the processor assures that the FPU exception handler will be invoked before the new floating point or MMX instruction gets executed 4 The FERR pin is connected through external hardware to IRQ13 of a cascaded program mable interrupt controller PIC When the FERR pin is asserted the PIC is programmed to generate an interrupt 75H 5 The PIC asserts the INTR pin on the processor to signal the interrupt 75H 6 The BIOS for the PC system handles the interrupt 75H by branching to the interrupt 2 NMI interrupt handler 7 The interrupt 2 handler determines if the interrupt is the result of an NMI interrupt or a floating point exception 8 If a floating point exception is detected the interrupt 2 handler branches to the floating point exception handler If the IGNNE pin is asserted the processor ignores
200. emory in a single or double real format The under flow threshold range for the single real format is 1 0 27126 to 1 0 2 26 the range for the double real format is 1 0 2 102 to 1 0 271022 The numeric underflow exception cannot occur when storing values in an integer or BCD integer format The flag UE for the numeric underflow exception is bit 4 of the FPU status word and the mask bit UM is bit 4 of the FPU control word When a numeric underflow exception occurs and the exception is masked the FPU denormal izes the result see Section 7 2 3 2 Normalized and Denormalized Finite Numbers If the denormalized result is exact the FPU stores the result in the destination operand without setting the UE flag If the denormal result is inexact the FPU sets the UE flag then goes on to handle the inexact result exception condition see Section 7 8 6 Inexact Result Precision Exception It is important to note that if numeric underflow is masked a numeric underflow excep tion is signaled only if the denormalized result is inexact If the denormalized result is exact no flags are set and no exceptions are signaled The action that the FPU takes when numeric underflow occurs and the numeric underflow exception is not masked depends on whether the instruction is supposed to store the result in memory or on the register stack If the destination is a memory location the UE flag is set and a software exception
201. en the processor and the coprocessor See Section D 1 Origin of the MS DOS Compatibility Mode for Handling FPU Exceptions in this appendix and Chapter 17 Intel Architecture Compatibility in the Intel Architecture Software Developer s Manual Volume 3 for differences in FPU exception handling The exception handling routine is normally a part of the systems software The routine must clear or disable the active exception flags in the FPU status word before executing any floating point instructions that cannot complete execution when there is a pending floating point excep tion Otherwise the floating point instruction will trigger the FPU interrupt again and the system will be caught in an endless loop of nested floating point exceptions and hang In any event the routine must clear or disable the active exception flags in the FPU status word after handling them and before IRET D Typical exception responses may include Incrementing an exception counter for later display or printing Printing or displaying diagnostic information e g the FPU environment and registers GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS intel Aborting further execution or using the exception pointers to build an instruction that will run without exception and executing it Applications programmers should consult their operating system s reference manuals for the appropriate system response to numerical exceptions For systems programmer
202. ensures that the handler will be invoked before execution of the next WAIT or floating point instruction since an unmasked floating point exception causes the processor to freeze just before executing such an instruction unless the IGNNE input is active or it is a no wait FPU instruction The frozen processor waits for an external interrupt which must be supplied by external hard ware in response to the FERR or ERROR output of the processor or coprocessor usually through IRQ13 on the slave PIC and then through INTR Then the external interrupt invokes the exception handling routine Note that if the external interrupt for FPU errors is disabled when the processor executes an FPU instruction the processor will freeze until some other enabled interrupt occurs if an unmasked FPU exception condition is in effect If NE 0 but the IGNNE input is active the processor disregards the exception and continues Error reporting via an external interrupt is supported for MS DOS compatibility Chapter 17 Intel Architecture Compatibility of the Intel Architecture Software Developer s Manual Volume 3 contains further discussion of compatibility issues The references above to the ERROR output from the FPU apply to the Intel 387 and Intel 287 math coprocessors NPX chips If one of these coprocessors encounters an unmasked exception condition it signals the exception to the Intel 286 or Intel386 processor using the ERROR status line betwe
203. ent for operating systems and completely transparent to the applications without significant sacrifice of execution speed Furthermore the ability to define segments as large as the 4 GBytes physical address space together with paging allowed the creation of protected flat model addressing systems in the architecture including complete implementations of the widely used main frame operating system UNIX The Intel Architecture has been and is committed to the task of maintaining backward compat ibility at the object code level to preserve our customers very large investment in software but at the same time in each generation of the architecture the latest most effective microprocessor architecture and silicon fabrication technologies have been used to produce the fastest most powerful processors possible Intel has worked over the generations to adapt and incorporate increasingly sophisticated techniques from main frame architecture into microprocessor archi tecture Various forms of parallel processing have been the most performance enhancing of these techniques and the Intel386 processor was the first Intel Architecture processor to include a number of parallel stages six These are the Bus Interface Unit accesses memory and I O for the other units the Code Prefetch Unit receives object code from the Bus Unit and puts it into a 16 byte queue the Instruction Decode Unit decodes object code from the Prefetch unit into microcode the E
204. ental Instruction 7 39 7 5 11 FPU Control 5 7 39 7 5 12 Waiting Vs Non waiting Instructions 7 40 7 5 13 Unsupported FPU Instructions 7 41 7 6 OPERATING NANS oococccccccc m 7 41 7 6 1 Uses for Signaling 7 42 7 6 2 Uses for Quiet lt 7 42 7 7 FLOATING POINT EXCEPTION HANDLING 7 42 7 7 1 Arithmetic vs Non arithmetic Instructions 7 43 7 7 2 Automatic Exception Handling 7 43 7 7 3 Software Exception Handling 7 45 7 7 3 1 Native Mode L uuu Ta Usnu ad RO ALD 7 45 7 7 3 2 MS DOS Compatibility 7 45 7 7 3 3 Typical Floating Point Exception Handler Actions 7 46 7 8 FLOATING POINT EXCEPTION CONDITIONS 7 47 TABLE OF CONTENTS intel e PAGE 7 8 1 Invalid Operation 7 47 7 8 1 1 Stack Overflow or Underflow Exception IS 7 48 7 8 1 2 Invalid Arithmetic Operand Exception 7 48 7 8 2 Divide By Zero Exception HZ
205. epresent all the I O addresses I O addresses not spanned by the map are treated as if they had set bits in the map For example if the TSS segment limit is 10 bytes past the bit map base address the map has 11 bytes and the first 80 I O ports are mapped Higher addresses in the I O address space generate exceptions If the I O bit map base address is greater than or equal to the TSS segment limit there is no I O permission map and all I O instructions generate exceptions when the CPL is greater than the current IOPL The I O bit map base address must be less than or equal to DFFFH 9 6 ORDERING I O When controlling I O devices it is often important that memory and I O operations be carried out in precisely the order programmed For example a program may write a command to an I O port then read the status of the I O device from another I O port It is important that the status returned be the status of the device after it receives the command not before When using memory mapped I O caution should be taken to avoid situations in which the programmed order is not preserved by the processor To optimize performance the processor allows cacheable memory reads to be reordered ahead of buffered writes in most situations Internally processor reads cache hits can be reordered around buffered writes When using memory mapped I O therefore is possible that an I O read might be performed before the memory write of a previous instruction The rec
206. eption Assume two threads A and B both using the floating point unit Let A be the thread to have most recently executed a floating point instruction with no pending numeric exceptions Let B be the currently executing thread CRO TS was set when thread A was suspended When B starts to D 23 GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS intel execute a floating point instruction the instruction will fault with the DNA exception because TS is set At this point the handler is entered and eventually it finds that the current FPU Owner is not the currently executing thread To guard the FPU state swap from extraneous numeric exceptions the FPU Owner is set to be the kernel The old owner s FPU state is saved with FNSAVE and the current thread s FPU state is restored with FRSTOR Before exiting the FPU owner is set to thread B and the TS bit is cleared On exit thread B resumes execution of the faulting floating point instruction and continues Case 2 FPU State Swap with Discarded Numeric Exception Again assume two threads A and B both using the floating point unit Let A be the thread to have most recently executed a floating point instruction but this time let there be a pending numeric exception Let B be the currently executing thread When B starts to execute a floating point instruction the instruction will fault with the DNA exception and enter the DNA handler If both numeric and DNA exceptions are pending the DNA excepti
207. eption and asserts its ERROR pin The processor recognizes this active condition of the ERROR status line immediately before execution of the next WAIT or FPU instruction except for the no wait type in its instruction stream and branches to the routine at interrupt vector 16 Thus an FPU exception will be handled before any other FPU instruction after the one causing the error is executed except for no wait instructions which will be executed without triggering the FPU exception interrupt but it will remain pending Using the dedicated interrupt 16 for FPU exception handling is referred to as the native mode It is the simplest approach and the one recommended most highly by Intel D 4 2 Changes with Intel486 Pentium and Pentium Pro Processors with CRO NE 1 With these latest three generations of the Intel Architecture more enhancements and speedup features have been added to the corresponding FPUs Also the FPU is now built into the same chip as the processor which allows further increases in the speed at which the FPU can operate as part of the integrated system This also means that the native mode of FPU exception handling selected by setting bit NE of register CRO to 1 is now entirely internal If an unmasked exception occurs during an FPU instruction the FPU records the exception internally and triggers the exception handler through interrupt 16 immediately before execution of the next WAIT or FPU instruction except fo
208. equested immediately The following code examples show the ASM386 486 coding of three skeleton exception handlers with the save spaces given as correct for 32 bit protected mode They show how prologues and epilogues can be written for various situations but the application dependent exception handling body is just indicated by comments showing where it should be placed The first two are very similar their only substantial difference is their choice of instructions to save and restore the FPU The trade off here is between the increased diagnostic information provided by FNSAVE and the faster execution of FNSTENV Also after saving the original contents FNSAVE re initializes the FPU while FNSTENV only masks all FPU exceptions For applications that are sensitive to interrupt latency or that do not need to examine register contents FNSTENV reduces the duration of the critical region during which the processor does not recognize another interrupt request See the Section 7 3 9 Saving the FPU s State for a complete description of the FPU save image After the exception handler body the epilogues prepare the processor to resume execution from the point of interruption 1 the instruction following the one that generated the unmasked exception Notice that the exception flags in the memory image that is loaded into the FPU are cleared to zero prior to reloading in fact in these examples the entire status word image is cleared
209. er through the floating point error exception MF vector 16 immediately before execution of any of the following instructions in the processor s instruction stream The next floating point instruction unless it is one of the non waiting instructions FNINIT FNCLEX FNSTSW FNSTCW ENSTENV and FNSAVE The next WAIT FWAIT instruction The next MMX instruction If the next floating point instruction in the instruction stream is a non waiting instruction the FPU executes the instruction without invoking the software exception handler 7 7 3 2 MS DOS COMPATIBILITY MODE If the NE flag in control register CRO is set to 0 the MS DOS compatibility mode for handling floating point exceptions is selected In this mode the software exception handler for floating point exceptions is invoked externally using the processor s FERR INTR and IGNNE pins This method of reporting floating point errors and invoking an exception handler is provided to support the floating point exception handling mechanism used in PC systems that are running the MS DOS or Windows 95 operating system The MS DOS compatibility mode is typically used as follows to invoke the floating point excep tion handler 1 If the FPU detects an unmasked floating point exception it sets the flag for the exception and the ES flag in the FPU status word 7 45 FLOATING POINT UNIT intel 2 Ifthe IGNNE pin is deasserted the FPU then asserts the FERR pin
210. eration of the PUSHA Instruction The POP instruction copies the word or doubleword at the current top of stack indicated by the ESP register to the location specified with the destination operand and then increments the ESP register to point to the new top of stack see Figure 6 3 The destination operand may specify a general purpose register a segment register or a memory location Stack Before Popping Doubleword After Popping Doubleword GR ui 0 31 0 n n 4 lt ESP n 8 Doubleword Value lt ESP Figure 6 3 Operation of the POP Instruction The POPA instruction reverses the effect of the PUSHA instruction It pops the top eight words or doublewords from the top of the stack into the general purpose registers except for the ESP register see Figure 6 4 If the operand size attribute is 32 the doublewords on the stack are transferred to the registers in the following order EDI ESI EBP ignore doubleword EBX EDX ECX and EAX The ESP register is restored by the action of popping the stack If the operand size attribute is 16 the words on the stack are transferred to the registers in the following order DI SI BP ignore word BX DX CX and AX 6 20 intel e INSTRUCTION SET SUMMARY Stack Before Popping Registers After Popping Registers Stack 0 31 0 31 Growth n n 4 lt ESP n 8 EAX n 12 ECX n 16 EDX n 20 EBX n 24 Ignored n
211. espectively The contents of the remaining bits in the register bits 5 3 and 1 are undefined and the contents of the EFLAGS register remain unchanged The SAHF instruction copies bits 7 6 4 2 and 0 from the AH register into the SF ZF AF PF and CF flags respectively in the EFLAGS register The PUSHF push flags PUSHFD push flags double POPF pop flags and POPFD pop flags double instructions copy the flags in the EFLAGS register to and from the stack The PUSHF instruction pushes the lower word of the EFLAGS register onto the stack see Figure 6 11 The PUSHFD instruction pushes the entire EFLAGS register onto the stack with the RF and VM flags read as clear 6 37 INSTRUCTION SET SUMMARY intel e PUSHFD POPFD PUSHF POPF lt gt 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 131211109 87 6 5 4 3 2 1 0 in o lolbliz s A Pl lc P L Figure 6 11 Flags Affected by the PUSHF POPF PUSHFD and POPFD instructions The POPF instruction pops a word from the stack into the EFLAGS register Only bits 11 10 8 7 6 4 2 and 0 of the EFLAGS register are affected with all uses of this instruction If the current privilege level CPL of the current code segment is 0 most privileged the IOPL bits bits 13 and 12 also are affected If
212. ess space Like words in memory 16 bit ports should be aligned to even addresses 0 2 4 so that all 16 bits can be transferred in a single bus cycle Likewise 32 bit ports should be aligned to addresses that are multiples of four 0 4 8 The processor supports data transfers to unaligned ports but there is a performance penalty because one or more extra bus cycle must be used The exact order of bus cycles used to access unaligned ports is undefined and is not guaranteed to remain the same in future Intel Architecture processors If hardware or software requires that I O ports be written to in a particular order that order must be specified explicitly For example to load a word length I O port at address 2H and then another word port at 4H two word length writes must be used rather than a single doubleword write at 2H Note that the processor does not mask parity errors for bus cycles to the I O address space Accessing I O ports through the I O address space is thus a possible source of parity errors 9 3 1 Memory Mapped I O I O devices that respond like memory components can be accessed through the processor s phys ical memory address space see Figure 9 1 When using memory mapped I O any of the processor s instructions that reference memory can be used to access an I O port located at a physical memory address For example the MOV instruction can transfer data between any register and a memory mapped I O port The A
213. et of general data registers a set of segment registers and a set of status and control registers When a program calls a procedure a procedure stack is added to the execution environment Procedure calls and the procedure stack implementation are described in Chapter 4 Procedure Calls Interrupts and Exceptions 23 1 Eight 32 bit General Purpose Registers Registers Six 16 bit Segment Registers Address Registers Space 32 bits EFLAGS Register 32 bits EIP Instruction Pointer Register The address space can be flat or segmented 0 Figure 3 1 Pentium Pro Processor Basic Execution Environment 3 3 MEMORY ORGANIZATION The memory that the processor addresses on its bus is called physical memory Physical memory is organized as a sequence of 8 bit bytes Each byte is assigned a unique address called a physical address The physical address space ranges from zero to a maximum of 222 1 4 gigabytes Virtually any operating system or executive designed to work with an Intel Architecture processor will use the processor s memory management facilities to access memory These facil ities provide features such as segmentation and paging which allow memory to be managed effi ciently and reliably Memory management is described in detail in Chapter 3 Protected Mode Memory Management of the Intel Architecture Software Developer s Manual Volume 3 The following paragraphs
214. ey are executed and termi nate a loop when zero is reached The LOOPE LOOPZ LOOPNE and LOOPNZ instructions also accept the ZF flag as a condition for terminating the loop before the count reaches zero The LOOP instruction decrements the contents of the ECX register or the CX register if the address size attribute is 16 then tests the register for the loop termination condition If the count in the ECX register is non zero program control is transferred to the instruction address specified by the destination operand The destination operand is a relative address that is an offset relative to the contents of the EIP register and it generally points to the first instruction in the block of code that is to be executed in the loop When the count in the ECX register reaches zero program control is transferred to the instruction immediately following the LOOP instruction which terminates the loop If the count in the ECX register is zero when the LOOP instruction is first executed the register is pre decremented to FFFFFFFFH causing the loop to be executed 2 times The LOOPE and LOOPZ instructions perform the same operation they are mnemonics for the same instruction These instructions operate the same as the LOOP instruction except that they also test the ZF flag If the count in the ECX register is not zero and the ZF flag is set program control is transferred to the destination operand When the count reaches zero or the ZF flag is cl
215. ey are used correctly when encountered as operands The exponent is treated as if it were 00 01B and the mantissa is unchanged The denormal exception is generated Table 7 11 Packed Decimal Integer Encodings Magnitude Class Sign digit digit digit digit T digit Positive Largest 0 0000000 1001 1001 1001 1001 ds 1001 0 0000000 0000 0000 0000 0000 0001 Smallest Zero 0 0000000 0000 0000 0000 0000 in 0000 Negative Zero 1 0000000 0000 0000 0000 0000 x 0000 1 0000000 0000 0000 0000 0000 des 0001 Smallest Largest 1 0000000 1001 1001 1001 1001 it 1001 Decimal 1 1111111 1111 1111 UUUU UUUU UUUU Integer Indefinite lt 1 byte gt lt 9 bytes gt NOTE UUUU means bit values are undefined and may contain any value 7 5 FPU INSTRUCTION SET The floating point instructions that the Intel Architecture FPU supports can be grouped into six functional categories e Data transfer instructions Basic arithmetic instructions Comparison instructions Transcendental instructions Load constant instructions FPU control instructions 7 29 FLOATING POINT UNIT intel See Section 6 2 3 Floating Point Instructions for a list of the floating point instructions by category The following section briefly describes the instructions in each category Detailed descriptions of the floating point instructions are given in Chapter 3 Instruction Set Reference in the
216. f Register Segment Reference Used Used Default Selection Rule Instructions CS Code Segment All instruction fetches Stack SS Stack Segment All stack pushes and pops Any memory reference which uses the ESP or EBP register as a base register Local Data DS Data Segment All data references except when relative to stack or string destination Destination ES Data Segment Destination of string instructions Strings pointed to with the ES register When storing data in or loading data from memory the DS segment default can be overridden to allow other segments to be accessed Within an assembler the segment override is generally handled with a colon operator For example the following MOV instruction moves a value from register EAX into the segment pointed to by the ES register The offset into the segment is contained in the EBX register MOV ES EBX EAX At the machine level a segment override is specified with a segment override prefix which is a byte placed at the beginning of an instruction The following default segment selections cannot be overridden Instruction fetches must be made from the code segment Destination strings in string instructions must be stored in the data segment pointed to by the ES register Push and pop operations must always reference the SS segment Some instructions require a segment selector to be specified explicitly In these cases the 16 bit segment selector can
217. f system registers that are used to control memory manage ment interrupt and exception handling task management processor management and debug ging activities Some of these system registers are accessible by an application program the operating system or the executive through a set of system instructions When accessing a system register with a system instruction the register is generally an implied operand of the instruction 5 3 3 Memory Operands Source and destination operands in memory are referenced by means of a segment selector and an offset see Figure 5 4 The segment selector specifies the segment containing the operand and the offset the number of bytes from the beginning of the segment to the first byte of the operand specifies the linear or effective address of the operand 15 0 31 0 Segment Offset or Linear Address Selector Figure 5 4 Memory Operand Address 5 3 3 1 SPECIFYING A SEGMENT SELECTOR The segment selector can be specified either implicitly or explicitly The most common method of specifying a segment selector is to load it in a segment register and then allow the processor to select the register implicitly depending on the type of operation being performed The processor automatically chooses a segment according to the rules given in Table 5 1 5 6 intel e DATA TYPES AND ADDRESSING MODES Table 5 1 Default Segment Selection Rules Type o
218. f the old floating point context of thread A and the FRSTOR of the floating point context for thread B Thread A eventually gets an opportunity to handle the exception that was discarded during the task switch After some time thread B is suspended and thread A resumes execution When thread A starts to execute an floating point instruction once again the DNA exception handler is entered B s FPU state is Finessed and A s FPU state is Frustrate Note that in restoring the FPU state from A s save area the pending numeric exception flags are reloaded in to the floating point status word Now when the DNA exception handler returns thread A resumes execution of the faulting floating point instruction just long enough to immediately generate a numeric exception which now gets handled in the normal way The net result is that the task switch and resulting FPU state swap via the DNA exception handler causes an extra numeric exception which can be safely discarded D 3 6 4 INTERRUPT ROUTING FROM THE KERNEL In MS DOS an application that wishes to handle numeric exceptions hooks interrupt 16 by placing its handler address in the interrupt vector table and exiting via a jump to the previous interrupt 16 handler Protected mode systems that run MS DOS programs under a subsystem can emulate this exception delivery mechanism For example assume a protected mode O S that runs with CR NE 1 and that runs MS DOS programs in a virtual machine subsystem The
219. f these numeric coprocessors was the Intel 8087 which was followed by the Intel 287 and Intel 387 numeric coprocessors To allow the 8087 to signal floating point exceptions to its companion 8086 or 8088 the 8087 has an output pin INT which it asserts when an unmasked floating point exception occurs The designers of the 8087 recommended that the output from this pin be routed through a program mable interrupt controller PIC such as the Intel 8259A to the INTR pin of the 8086 or 8088 The accompanying interrupt vector number could then be used to access the floating point exception handler However the original IBM PC design and MS DOS operating system used a different mecha nism for handling the INT output from the 8087 It connected the INT pin directly to the NMI input pin of the 8086 or 8088 The NMI interrupt handler then had to determine if the interrupt was caused by a floating point exception or another NMI event This mechanism is the origin of want is now called the MS DOS compatibility mode The decision to use this latter float ing point exception handling mechanism came about because when the IBM PC was first de signed the 8087 was not available When the 8087 did become available other functions had already been assigned to the eight inputs to the PIC One of these functions was a BIOS video interrupt which was assigned to interrupt number 16 for the 8086 and 8088 The Intel 286 processor created the native mode for han
220. family ID model ID and stepping ID Feature information identifies the feature set for the processor model 2 Cache information about the processor s internal cache memory 6 15 4 No Operation and Undefined Instructions The NOP no operation instruction increments the EIP register to point at the next instruction but affects nothing else The UD2 undefined instruction generates an invalid opcode exception Intel reserves the opcode for this instruction for this function The instruction is provided to allow software to test an invalid opcode exception handler 6 40 Intel 7 Floating Point Unit CHAPTER 7 FLOATING POINT UNIT The Intel Architecture Floating Point Unit FPU provides high performance floating point processing capabilities It supports the real integer and BCD integer data types and the floating point processing algorithms and exception handling architecture defined in the IEEE 754 and 854 Standards for Floating Point Arithmetic The FPU executes instructions from the processor s normal instruction stream and greatly improves the efficiency of Intel Architecture processors in handling the types of high precision floating point processing operations commonly found in scientific engineering and business applications This chapter describes the data types that the FPU operates on the FPU s execution environ ment and the FPU specific instruction set Detailed descriptions of the FPU instru
221. for data items Operand identifiers are either reserved names of registers or are assumed to be assigned to data items declared in another part of the program which may not be shown in the example When two operands are present in an arithmetic or logical instruction the right operand is the source and the left operand is the destination For example LOADREG MOV EAX SUBTOTAL In this example LOADREG is a label MOV is the mnemonic identifier of an opcode EAX is the destination operand and SUBTOTAL is the source operand Some assembly languages put the source and destination in reverse order 1 4 4 Hexadecimal and Binary Numbers Base 16 hexadecimal numbers are represented by a string of hexadecimal digits followed by the character H for example F82EH A hexadecimal digit is a character from the following set 0 1 2 3 4 5 6 7 8 9 A B C D E and F Base 2 binary numbers are represented by a string of 1s and Os sometimes followed by the character B for example 1010B The B designation is only used in situations where confu sion as to the type of number might arise intel ABOUT THIS MANUAL 1 4 5 Segmented Addressing The processor uses byte addressing This means memory is organized and accessed as a sequence of bytes Whether one or more bytes are being accessed a byte address is used to locate the byte or bytes memory The range of memory that can be addressed is called an address space The
222. fter the result is stored the OE flag is set and a software exception handler is invoked The scaling bias value 24 576 is equal to 3 213 Biasing the exponent by 24 576 normally trans lates the number as nearly as possible to the middle of the extended real exponent range so that if desired it can be used in subsequent scaled operations with less risk of causing further exceptions When using the FSCALE instruction massive overflow can occur where the result is too large to be represented even with a bias adjusted exponent Here if overflow occurs again after the result has been biased a properly signed is stored in the destination operand 7 51 FLOATING POINT UNIT intel 7 8 5 Numeric Underflow Exception 0 The FPU reports a floating point numeric underflow exception U whenever the rounded result of an arithmetic instruction is tiny that is less than the smallest possible normalized finite value that will fit into the real format of the destination operand For example if the desti nation format is extended real 80 bits underflow occurs when the rounded result falls in the unbiased range of 1 0 2716382 to 1 0 2716382 exclusive Like numeric overflow numeric underflow can occur on arithmetic operations where the result is stored in an FPU data register It can also occur on store real operations with the FST and FSTP instructions where a within range value in a data register is stored in m
223. g SS lt ESP After Return Calling ESP Param 1 Param 1 Param 2 Param 2 Param 3 Param 3 Calling CS ESP Before Return 3 Calling EIP Note On a return parameters are released on both stacks if the correct value is given for the n operand in the RET n instruction Figure 4 4 Stack Switch on a Call to a Different Privilege Level 3 Loads the segment selector and stack pointer for the new stack that is the stack for the privilege level being called from the TSS into the SS and ESP registers and switches to the new stack 4 Pushes the temporarily saved SS and ESP values for the calling procedure s stack onto the new stack 5 Copies the parameters from the calling procedure s stack to the new stack A value in the call gate descriptor determines how many parameters to copy to the new stack 6 Pushes the temporarily saved CS and EIP values for the calling procedure to the new stack 4 9 PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS intel 7 Loads the segment selector for the new code segment and the new instruction pointer from the call gate into the CS and EIP registers respectively 8 Begins execution of the called procedure at the new privilege level When executing a return from the privileged procedure the processor performs these actions Performs a privilege check 2 Restores the CS and EIP registers to their values prior to the call 3 If the RET instruction has
224. gest 1 00 00 Integer Indefinite 1 00 00 Word Integer lt 15 bits gt Short Integer lt 31 Bits gt Long Integer lt 63 Bits gt The most significant bit of each format is the sign bit 0 for positive and 1 for negative Negative values are represented in standard two s complement notation The quantity zero is represented with all bits including the sign bit set to zero Note that the FPU s word integer data type is identical to the word integer data type used by the processor s integer unit and the short integer format is identical to the integer unit s doubleword integer data type Word integer values are stored in 2 consecutive bytes in memory short integer values are stored in 4 consecutive bytes and long integer values are stored in 8 consecutive bytes When loaded into the FPU s data registers all the binary integers are exactly representable in the extended real format The binary integer encoding 100 00B represents either of two things depending on the circum stances of its use The largest negative number supported by the format 2 5 23 or 26 The integer indefinite value 7 27 FLOATING POINT UNIT intel If this encoding is used as a source operand as in an integer load or integer arithmetic instruc tion the FPU interprets it as the largest negative number representable in the format being used If the FPU detects an invalid operation when storing an integer value in memory
225. gs 3 13 BASIC EXECUTION ENVIRONMENT intel e 3 7 INSTRUCTION POINTER The instruction pointer EIP register contains the offset in the current code segment for the next instruction to be executed It is advanced from one instruction boundary to the next in straight line code or it is moved ahead or backwards by a number of instructions when executing JMP Jcc CALL RET and IRET instructions The EIP register cannot be accessed directly by software it is controlled implicitly by control transfer instructions such as JMP Jcc CALL and RET interrupts and exceptions The only way to read the EIP register is to execute a CALL instruction and then read the value of the return instruction pointer from the procedure stack The EIP register can be loaded indirectly by modi fying the value of a return instruction pointer on the procedure stack and executing a return instruction RET or IRET See Section 4 2 4 2 Return Instruction Pointer All Intel Architecture processors prefetch instructions Because of instruction prefetching an instruction address read from the bus during an instruction load does not match the value in the EIP register Even though different processor generations use different prefetching mechanisms the function of EIP register to direct program flow remains fully compatible with all software written to run on Intel Architecture processors 3 8 OPERAND SIZE AND ADDRESS SIZE ATTRIBUTES When processor is exec
226. have been added to the EFLAGS register but the function and placement of existing flags have remained the same from one family of the Intel Architecture processors to the next As a result code that accesses or modifies these flags for one family of Intel Architecture processors works as expected when run on later families of processors 3 6 3 1 STATUS FLAGS The status flags bits 0 2 4 6 7 and 11 of the EFLAGS register indicate the results of arith metic instructions such as the ADD SUB MUL and DIV instructions The functions of the status flags are as follows CF bit 0 Carry flag Set if an arithmetic operation generates a carry or a borrow out of the most significant bit of the result cleared otherwise This flag indi cates an overflow condition for unsigned integer arithmetic Itis also used in multiple precision arithmetic BASIC EXECUTION ENVIRONMENT intel e PF bit 2 Parity flag Set if the least significant byte of the result contains an even number of 1 bits cleared otherwise AF bit 4 Adjust flag Set if an arithmetic operation generates a carry or a borrow out of bit 3 of the result cleared otherwise This flag is used in binary coded decimal BCD arithmetic ZF bit 6 Zero flag Set if the result is zero cleared otherwise SF bit 7 Sign flag Set equal to the most significant bit of the result which is the sign bit of a signed integer 0 indicates a positive value and 1 indicates a negative value
227. he CF flag to a known state before an instruction that uses the flag in an operation is executed They are also used in conjunction with the rotate with carry instructions RCL and RCR The STD set direction flag and CLD clear direction flag instructions allow the DF flag in the EFLAGS register to be modified directly The DF flag determines the direction in which index registers ESI and EDI are stepped when executing string processing instructions If the DF flag is clear the index registers are incremented after each iteration of a string instruction if the DF flag is set the registers are decremented 6 13 2 Interrupt Flag Instructions The STI set interrupt flag and CTI clear interrupt flag instructions allow the interrupt IF flag in the EFLAGS register to be modified directly The IF flag controls the servicing of hardware generated interrupts those received at the processor s INTR pin If the IF flag is set the processor services hardware interrupts if the IF flag is clear hardware interrupts are masked 6 13 3 EFLAGS Transfer Instructions The EFLAGS transfer instructions allow groups of flags in the EFLAGS register to be copied to aregister or memory or be loaded from a register or memory The LAHF load AH from flags and SAHF store AH into flags instructions operate on five of the EFLAGS status flags SE ZE AF PF and CF The LAHF instruction copies the status flags to bits 7 6 4 2 and 0 of the AH register r
228. he FPU Table 7 8 gives the precision and range of these data types and Figure 7 17 gives the formats Real Numbers For the single real and double real formats only the fraction part of the significand is encoded The integer is assumed to be 1 for all numbers except 0 and denormalized finite numbers For the extended real format the integer is contained in bit 63 and the most significant fraction bit is bit 62 Here the integer is explicitly set to 1 for normalized numbers infinities and NaNs and to 0 for zero and denormalized numbers Table 7 8 Length Precision and Range of FPU Data Types Data Type Length Precision Approximate Normalized Range Bits Binary Decimal Binary Real Single real 32 24 2 126 to 2127 1 18 x 10738 to 3 40 x 1038 Double real 64 53 271022 to 21023 2 23 x 107308 to 1 79 x 10308 Extended real 80 64 2716382 to 216383 3 37 x 1074982 to 1 18 104932 Binary Integer Word integer 16 15 215 to 215 1 82 768 to 32 767 Short integer 32 31 231 to 23 1 2 14 x 10 to 2 14 x 10 Long integer 64 63 263 to 263 1 9 22 x 10 8 to 9 22 x 1018 Packed BCD 80 18 decimal Not Pertinent 71018 1 to 1018 1 Integers digits The exponent of each real data type is encoded in biased format The biasing constant is 127 for the single real format 1023 for the double real format and 16 383 for the extended real format Table 7 9 shows the encodings for all th
229. he handler procedure An offset into the code segment to the first instruction of the handler procedure The difference between an interrupt gate and a trap gate is as follows If an interrupt or exception handler is called through an interrupt gate the processor clears the interrupt enable IF flag in the EFLAGS register to prevent subsequent interrupts from interfering with the execution of the handler When a handler is called through a trap gate the state of the IF flag is not changed If the code segment for the handler procedure has the same privilege level as the currently executing program or task the handler procedure uses the current stack if the handler executes at a more privileged level the processor switches to the stack for the handler s privilege level PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS Table 4 1 Exceptions and Interrupts Vector No Mnemonic Description Source 0 DE Divide Error DIV and IDIV instructions 1 DB Debug Any code or data reference 2 NMI Interrupt Non maskable external interrupt 3 BP Breakpoint INT 3 instruction 4 OF Overflow INTO instruction 5 BR BOUND Range Exceeded BOUND instruction 6 UD Invalid Opcode UnDefined UD2 instruction or reserved opcode Opcode 7 NM Device Not Available No Math Floating point or WAIT FWAIT instruction Coprocessor 8 DF Double Fault Any instruction that can generate an exception an NMI or an INTR 9 CoProcessor Segment Overrun Float
230. he instruction decoders also handle the decoding of instruction prefixes and looping operations The instruction decoder can generate up to six micro ops per clock cycle one each for the simple instruction decoders and four for the complex instruction decoder The Intel Architecture s register set can cause resource stalls due to register dependencies To solve this problem the processor provides 40 internal general purpose registers which are used for the actual computations These registers can handle both integer and floating point values To allocate the internal registers the enqueued micro ops from the instruction decoder are sent to the register alias table unit where references to the logical Intel Architecture registers are converted into internal physical register references In the final step of the decoding process the allocator in the register alias table unit adds status bits and flags to the micro ops to prepare them for out of order execution and sends the resulting micro ops to the instruction pool 2 5 3 Instruction Pool Reorder Buffer Prior to entering the instruction pool known formally as the reorder buffer the micro op instruction stream is in the same order as the Intel Architecture instruction stream that was sent to the instruction decoder No reordering of instructions has taken place 2 10 intel e INTRODUCTION TO THE INTEL ARCHITECTURE The reorder buffer is an array of content addressable memory arra
231. he same as the segment registers found in the Intel 8086 and Intel 286 processors and the FS and GS registers were introduced into the Intel Architecture with the Intel386 family of processors 3 6 3 EFLAGS Register The 32 bit EFLAGS register contains a group of status flags a control flag and a group of system flags Figure 3 7 defines the flags within this register Following initialization of the processor either by asserting the RESET pin or the INIT pin the state of the EFLAGS register is 00000002H Bits 1 3 5 15 and 22 through 31 of this register are reserved Software should not use or depend on the states of any of these bits Some of the flags in the EFLAGS register can be modified directly using special purpose instructions described in the following sections There are no instructions that allow the whole register to be examined or modified directly However the following instructions can be used to move groups of flags to and from the procedure stack or the EAX register LAHF SAHF PUSHF PUSHED POPE and POPFD After the contents of the EFLAGS register have been transferred to the procedure stack or EAX register the flags can be examined and modified using the processor s bit manipulation instructions BT BTS BTR and BTC When suspending a task using the processor s multitasking facilities the processor automati cally saves the state of the EFLAGS register in the task state segment TSS for the task being suspe
232. he standard formula see Figure D 4 If R1 becomes zero the circuit resistance becomes zero With the divide by zero and precision exceptions masked the processor will produce the correct result FDIV of R1 into 1 gives infinity and then FDIV of infinity R2 R3 into gives zero A 1 1 1 Ry Ro Equivalent Resistance Figure D 4 Arithmetic Example Using Infinity By masking or unmasking specific numeric exceptions in the FPU control word programmers can delegate responsibility for most exceptions to the processor reserving the most severe exceptions for programmed exception handlers Exception handling software is often difficult to write and the masked responses have been tailored to deliver the most reasonable result for each condition For the majority of applications masking all exceptions yields satisfactory results with the least programming effort Certain exceptions can usefully be left unmasked during the debugging phase of software development and then masked when the clean software is actually run An invalid operation exception for example typically indicates a program error that must be corrected The exception flags in the FPU status word provide a cumulative record of exceptions that have occurred since these flags were last cleared Once set these flags can be cleared only by executing the FCLEX FNCLEX clear exceptions instruction by reinitializing the FPU with FINIT ENINIT or FS
233. he value for x shown earlier in this section with the full 66 bit fraction T hight lowr For example the following two values given in scientific notation with the fraction in hexadec imal and the exponent in decimal represent the 33 most significant and the 33 least significant bits of the fraction highz unnormalized 0 C90FDAA20 2 2 lowr unnormalized 0 42D184698 2 3 These values encoded in standard IEEE double real format are as follows hight 400921FB 54400000 lowr 3DE0B461 1A600000 Note that in the IEEE double real format the exponents are biased by 1023 and the fractions are normalized Similar versions of can also be written in extended real format When using this two part value in an algorithm parallel computations should be performed on each part with the results kept separate When all the computations are complete the two results can be added together to form the final result The complications of maintaining a consistent value of x for argument reduction can be avoided either by applying the trigonometric functions only to arguments within the range of the automatic reduction mechanism or by performing all argument reductions down to a magni tude less than 1 4 explicitly in software 7 5 9 Logarithmic Exponential and Scale The following instructions provide two different logarithmic functions an exponential function and a scale function FYL2X Compute log y log x F
234. her allows efficient indexing of a two dimensional array when the elements of the array are 2 4 or 8 bytes in size 5 3 3 3 ASSEMBLER AND COMPILER ADDRESSING MODES At the machine code level the selected combination of displacement base register index register and scale factor is encoded in an instruction All assemblers permit a programmer to use any of the allowable combinations of these addressing components to address operands High level language HLL compilers will select an appropriate combination of these components based on the HHL construct a programmer defines 5 9 DATA TYPES AND ADDRESSING MODES intel e 5 3 4 VO Port Addressing The processor supports an I O address space that contains up to 65 536 8 bit I O ports Ports that are 16 bit and 32 bit may also be defined in the I O address space An I O port can be addressed with either an immediate operand or a value in the DX register See Chapter 9 Input Output for more information about I O port addressing 5 10 Intel Instruction Set summary CHAPTER 6 INSTRUCTION SET SUMMARY This chapter lists all the instructions in the Intel Architecture instruction set divided into three functional groups integer floating point and system It also briefly describes each of the integer instructions Brief descriptions of the floating point instructions are given in Chapter 7 Floating Point Unit brief descriptions of the system instructions are given in the Intel
235. iased exponent 7 5 Binary numbers 1 6 Binary coded decimal see BCD Bit fields secre detainee CES US 5 4 Bitorder seuss olx kd EI oe 1 5 BOUND instruction 4 15 6 34 6 39 BOUND range exceeded exception BR 4 15 BP registi ask e voor ocean EI 3 7 Branch prediction 2 7 Branching FPU condition codes 7 15 7 36 BSF instruction 6 29 BSR instruction 6 29 BSWAP instruction 6 2 6 17 BT instruction 3 10 3 12 6 29 BTC instruction 3 10 3 12 6 29 BTR instruction 3 10 3 12 6 29 BTS instruction 3 10 3 12 6 29 Bus interface 2 8 BX register 3 7 Byle La tere ERR ACCU aq 5 1 Byte order 1 5 C C1 flag FPU status word 7 13 7 48 7 51 7 53 C2 flag FPU status word 7 13 Call Gate Jo eer eere eps 4 7 CALL instruction 3 14 4 4 4 8 6 31 6 39 Calls see Procedure calls CBW instruction 6 21 CDQ instruction 6 22 CF carry flag EFLAGS register 3 11 CH register eee mer P abet 3 7 CLC instruction 3 12 6 37 CLD instruction 3 12 6 37 CLlinstruction 6 38 9 4 CMC instruction
236. ic register stack is extended to memory To extend the FPU stack to memory the invalid exception is unmasked A push to a full register or pop from an empty register sets SF Stack Fault flag and causes an invalid oper ation exception The recovery routine for the exception must recognize this situation fix up the stack then perform the original operation The recovery routine will not work correctly in the first example shown in the figure The problem is that the value of COUNT is incremented before the exception handler is invoked so that the recovery routine will load an incorrect value of COUNT causing the program to fail or behave unreliably D 3 3 3 PROPER EXCEPTION SYNCHRONIZATION IN GENERAL As explained in Section D 2 1 2 Recommended External Hardware to Support the MS DOS Compatibility Mode if the FPU encounters an unmasked exception condition a software exception handler is invoked before execution of the next WAIT or floating point instruction This is because an unmasked floating point exception causes the processor to freeze immedi ately before executing such an instruction unless the IGNNE input is active or it is a no wait FPU instruction Exactly when the exception handler will be invoked in the interval between when the exception is detected and the next WAIT or FPU instruction is dependent on the processor generation the system and which FPU instruction and exception is involved To be safe in exception synchr
237. ices An I O port can be an input port an output port or a bidirectional port Some I O ports are used for transmitting data such as to and from the transmit and receive registers respectively of a serial interface device Other T O ports are used to control peripheral devices such as the control registers of a disk controller This chapter describes the processor s I O architecture The topics discussed include port addressing T O instructions protection mechanism 9 1 I O PORT ADDRESSING The processor allows I O ports to be accessed in either of two ways Through a separate I O address space Through memory mapped I O Accessing I O ports through the I O address space is handled through a set of I O instructions and a special I O protection mechanism Accessing I O ports through memory mapped I O is handled with the processors general purpose move and string instructions with protection provided through segmentation or paging I O ports can be mapped so that they appear in the I O address space or the physical memory address space memory mapped I O or both One benefit of using the I O address space is that writes to I O ports are guaranteed to be completed before the next instruction in the instruction stream is executed Thus I O writes to control system hardware cause the hardware to be set to its new state before any other instruc tions are executed See Section 9 6 Ordering I O for more info
238. icrocode From Instruction Integer Sequencer i Y Y Y Y Y Y x Fu Register Alias Table Y Retirement Retirement Unit I Register File Data Cache TEE 1 Intel Arch i Reorder Buffer Instruction Pool has Y Reservation Station E Floating Floating Memory Point Unit Point Unit Integer Integer Interface lt FPU FPU Unit Unit Unit arget Buffer 9 Y Y Internal Data Results Buses Figure 2 2 Functional Block Diagram of the Pentium Pro Processor Microarchitecture Memory requests to the L2 cache or system memory go through the memory reorder buffer which functions as a scheduling and dispatch station This unit keeps track of all memory requests and is able to reorder some requests to prevent blocks and improve throughput For example the memory reorder buffer allows loads to pass stores It also issues speculative loads Stores are always dispatched in order and speculative stores are never issued 2 9 INTRODUCTION TO THE INTEL ARCHITECTURE intel e 2 5 2 The Fetch Decode Unit The fetch decode unit reads a stream of Intel Architecture instructions from the L1 instruction cache and decodes them into a series of micro operations called micro ops This micro op stream still in the order of the original instruction stream is then sent to the instruction pool The instruction fetch unit fetches one 32 byte cache line per clock from the instruction cache It marks the begi
239. ide access to I O ports through the I O address space These instructions cannot be used to access memory mapped I O ports There are two groups of I O instructions Those which transfer a single item byte word or doubleword between an I O port and a general purpose register Those which transfer strings of items strings of bytes words or doublewords between an I O port and memory The register I O instructions IN input from I O port and OUT output to I O port move data between I O ports and the EAX register 32 bit I O the AX register 16 bit I O or the AL 8 bit I O register The address of the I O port can be given with an immediate value or a value in the DX register The string I O instructions INS input string from I O port and OUTS output string to I O port move data between an I O port and a memory location The address of the I O port being accesses is given in the DX register the source or destination memory address is given in the DS ESI or ES EDI register respectively 9 3 INPUT OUTPUT intel o When used with one of the repeat prefixes such as REP the INS and OUTS instructions perform string or block input or output operations The repeat prefix REP modifies the INS and OUTS instructions to transfer blocks of data between an I O port and memory Here the ESI or EDI register is incremented or decremented according to the setting of the DF flag in the EFLAGS register after each byte word or d
240. ific register Write model specific register Read performance monitoring counters Read time stamp counter 6 15 INSTRUCTION SET SUMMARY intel e 6 3 DATA MOVEMENT INSTRUCTIONS The data movement instructions move bytes words doublewords or quadwords both between memory and the processor s registers and between registers These instructions are divided into four groups General purpose data movement Exchange Stack manipulation Type conversion 6 3 1 General Purpose Data Movement Instructions The MOV move and CMOVcc conditional move instructions transfer data between memory and registers or between registers 6 3 1 1 MOVE INSTRUCTION The MOV instruction performs basic load data and store data operations between memory and the processor s registers and data movement operations between registers It handles data trans fers along the paths listed in Table 6 1 See MOV Move to from Control Registers and MOV Move to from Debug Registers in Chapter 3 of the Intel Architecture Software Devel oper s Manual Volume 2 for information on moving data to and from the control and debug registers The MOV instruction cannot move data from one memory location to another or from one segment register to another segment register Memory to memory moves can be performed with the MOVS string move instruction see Section 6 10 String Operations 6 3 1 2 CONDITIONAL MOVE INSTRUCTIONS The CMOVcc i
241. ify Instructions Instruction Effect on CF Flag Effect on Selected Bit BT Bit Test CF flag Selected Bit No effect BTS Bit Test and Set CF flag Selected Bit Selected Bit lt 1 BTR Bit Test and Reset CF flag Selected Bit Selected Bit 0 BTC Bit Test and Complement CF flag lt Selected Bit Selected Bit NOT Selected Bit 6 8 2 Bit Scan Instructions The BSF bit scan forward and BSR bit scan reverse instructions scan a bit string in a source operand for a set bit and store the bit index of the first set bit found in a destination register The bit index is the offset from the least significant bit bit 0 in the bit string to the first set bit The BSF instruction scans the source operand low to high from bit 0 of the source operand toward the most significant bit the BSR instruction scans high to low from the most significant bit toward the least significant bit 6 8 3 Byte Set On Condition Instructions The SETcc set byte on condition instructions set a destination operand byte to 0 or 1 depending on the state of selected status flags CF OF SE ZF and PF in the EFLAGS register The suffix cc added to the SET mnemonic determines the condition being tested for For example the SETO instruction tests for overflow If the OF flag is set the destination byte is set to 1 if OF is clear the destination byte is cleared to 0 Appendix B EFLAGS Condition Codes lists the conditions it i
242. igher ones The DF flag in 6 35 INSTRUCTION SET SUMMARY intel e the EFLAGS register controls whether the registers are incremented DF 0 or decremented DF 1 The STD and CLD instructions set and clear this flag respectively The following repeat prefixes can be used in conjunction with a count in the ECX register to cause a string instruction to repeat REP Repeat while the ECX register not zero REPE REPZ Repeat while the ECX register not zero and the ZF flag is set REPNE REPNZ Repeat while the ECX register not zero and the ZF flag is clear When a string instruction has a repeat prefix the operation executes until one of the termination conditions specified by the prefix is satisfied The REPE REPZ and REPNE REPNZ prefixes are used only with the CMPS and SCAS instructions Also note that a A REP STOS instruction is the fastest way to initialize a large block of memory 6 11 IO INSTRUCTIONS The IN input from port to register INS input from port to string OUT output from register to port and OUTS output string to port instructions move data between the processor s I O ports and either a register or memory The register I O instructions IN and OUT move data between an I O port and the EAX register 32 bit I O the AX register 16 bit I O or the AL 8 bit I O register The I O port being read or written to is specified with an immediate operand or an address in the DX register The block I O ins
243. ignals the denormal operand exception under the following conditions If an arithmetic instruction attempts to operate on a denormal operand see Section 7 2 3 2 Normalized and Denormalized Finite Numbers Ifan attempt is made to load a denormal single or double real value into an FPU register If the denormal value being loaded is an extended real value the denormal operand exception is not reported The flag DE for this exception is bit 1 of the FPU status word and the mask bit DM is bit 1 of the FPU control word When a denormal operand exception occurs and the exception is masked the FPU sets the DE flag then proceeds with the instruction The denormal operand in single or double real format is automatically normalized when converted to the extended real format Operating on denormal numbers will produce results at least as good as and often better than what can be obtained when denormal numbers are flushed to zero In fact subsequent operations will benefit from the additional precision of the internal extended real format Most programmers mask this excep tion so that a computation may proceed then analyze any loss of accuracy when the final result is delivered When a denormal operand exception occurs and the exception is not masked the DE flag is set and a software exception handler is invoked see Section 7 7 3 Software Exception Handling The top of stack pointer TOP and source operands remain unchang
244. igned or unsigned integer operands The ADD instruction computes the sum of two integer operands The ADC instruction computes the sum of two integer operands plus 1 if the CF flag is set This instruction is used to propagate a carry when adding numbers in stages The SUB instruction computes the difference of two integer operands The SBB instruction computes the difference of two integer operands minus 1 if the CF flag is set This instruction is used to propagate a borrow when subtracting numbers in stages 6 4 2 Increment and Decrement Instructions The INC increment and DEC decrement instructions add 1 to or subtract 1 from an unsigned integer operand respectively A primary use of these instructions is for implementing counters 6 22 intel e INSTRUCTION SET SUMMARY 6 4 3 Comparison and Sign Change Instruction The CMP compare instruction computes the difference between two integer operands and updates the OF SE ZE AF PF and CF flags according to the result The source operands are not modified nor is the result saved The CMP instruction is commonly used in conjunction with a Jcc jump or SETcc byte set on condition instruction with the latter instructions performing an action based on the result of a CMP instruction The NEG negate instruction subtracts a signed integer operand from zero The effect of the NEG instruction is to change the sign of a two s complement operand while keeping its magnitude
245. ill be a delay which depends on the external hardware implementation between the FERR assertion from the processor and the responding INTR assertion to the processor Further the interrupt request to the PICs IRQ13 may be temporarily blocked by the operating system or delayed by higher priority interrupts and processor response to INTR itself is blocked if the operating system has cleared the IF bit in EFLAGS However just as with the Intel486 and Pentium processors if the IGNNE input is inactive a floating point exception which occurred in the previous FPU instruction and is unmasked causes the processor to freeze immediately when encountering the next WAIT or FPU instruction ex cept for no wait instructions This means that if the FPU exception handler has not already been invoked due to the earlier exception and therefore the handler not has cleared that exception state from the FPU the processor is forced to wait for the handler to be invoked and handle the exception before the processor can execute another WAIT or FPU instruction As explained in Section D 2 1 3 No Wait FPU Instructions Can Get FPU Interrupt in Window if a no wait instruction is used outside of the FPU exception handler in the Intel486 and Pentium processors it may accept an unmasked exception from a previous FPU instruction which happens to fall within the external interrupt sampling window that is opened near the beginning of execution of all FPU instru
246. ing Numeric Exceptions Depending on options determined by the software system designer the processor takes one of two possible courses of action when a numeric exception occurs The FPU can handle selected exceptions itself producing a default fix up that is reasonable in most situations This allows the numeric program execution to continue undisturbed Programs can mask individual exception types to indicate that the FPU should generate this safe reasonable result whenever the exception occurs The default exception fix up activity is treated by the FPU as part of the instruction causing the exception no external indication of the exception is given except that the instruction takes longer to execute when it handles a masked exception When masked exceptions are detected a flag is set in the numeric status register but no information is preserved regarding where or when it was set Alternatively a software exception handler can be invoked to handle the exception When a numeric exception is unmasked and the exception occurs the FPU stops further execution of the numeric instruction and causes a branch to a software exception handler The exception handler can then implement any sort of recovery procedures desired for any numeric exception detectable by the FPU D 3 2 1 AUTOMATIC EXCEPTION HANDLING USING MASKED EXCEPTIONS Each of the six exception conditions described above has a corresponding flag bit in the FPU status word and
247. ing an instruction The Intel architecture specifies three classes of exceptions faults traps and aborts The processor responds to interrupts and exceptions in essentially the same way When an inter rupt or exception is signaled the processor halts execution of the current program or task and switches to a handler procedure that has been written specifically to handle the interrupt or exception condition The processor accesses the handler procedure through an entry in the inter rupt descriptor table IDT When the handler has completed handling the interrupt or exception program control is returned to the interrupted program or task The operating system executive and or device drivers normally handle interrupts and excep tions independently from application programs or tasks Application programs can however access the interrupt and exception handlers incorporated in an operating system or executive Intel PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS through assembly language calls The remainder of this section gives a brief overview of the processor s interrupt and exception handling mechanism See Chapter 5 Interrupt and Excep tion Handling in the Intel Architecture Software Developer s Manual Volume 3 for a detailed description of this mechanism The Intel Architecture defines 16 predefined interrupts and exceptions and 224 user defined interrupts which are associated with entries in the IDT Each interrupt and exception in
248. ing point instruction reserved 10 TS Invalid TSS Task switch or TSS access 11 NP Segment Not Present Loading segment registers or accessing system segments 12 SS Stack Segment Fault Stack operations and SS register loads 13 GP General Protection Any memory reference and other protection checks 14 PF Page Fault Any memory reference 15 Intel reserved Do not use 16 MF Floating Point Error Math Fault Floating point or WAIT FWAIT instruction 17 AC Alignment Check Any data reference in memory 3 18 MC Machine Check Error codes if any and source are model dependent 19 31 Intel reserved Do not use 32 255 Maskable Interrupts External interrupt from INTR pin or INT n instruction 1 The UD2 instruction was introduced in the Pentium Pro processor 2 Intel Architecture processors after the Intel386 processor do not generate this exception 3 This exception was introduced in the Intel486 processor 4 This exception was introduced in the Pentium processor and enhanced in the Pentium Pro processor Intel PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS If no stack switch occurs the processor does the following when calling an interrupt or excep tion handler see Figure 4 5 Pushes the current contents of the EFLAGS CS and EIP registers in that order on the stack 2 Pushes an error code if appropriate on the stack 3 Loads the segment selector for the new code segment and the new instruction p
249. inter 7 3 2 2 CONDITION CODE FLAGS The four FPU condition code flags CO through C3 indicate the results of floating point comparison and arithmetic operations Table 7 3 summarizes the manner in which the floating Intel FLOATING POINT UNIT point instructions set the condition code flags These condition code bits are used principally for conditional branching and for storage of information used in exception handling see Section 7 3 3 Branching and Conditional Moves on FPU Condition Codes FPU Busy m Top of Stack Pointer 151413 111098765432 10 eiS tor Condition Code Error Summary Status Stack Fault Exception Flags Precision Underflow Overflow Zero Divide Denormalized Operand Invalid Operation Figure 7 8 FPU Status Word As shown in Table 7 3 the C1 condition code flag is used for a variety of functions When both the IE and SF flags in the FPU status word are set indicating a stack overflow or underflow exception 15 the flag distinguishes between overflow C1 1 and underflow C120 When the PE flag in the status word is set indicating an inexact rounded result the C1 flag is set to 1 if the last rounding by the instruction was upward The FXAM instruction sets C1 to the sign of the value being examined The C2 condition code flag is used by the FPREM and FPREMI instructions to indicate an in
250. ion handling facilities These automatic exception handling facilities permit a high degree of flexibility in numeric processing software without burdening the programmer While performing numeric calcula tions the processor automatically detects exception conditions that can potentially damage a calculation for example X 0 or AX when X lt 0 By default on chip exception logic handles these exceptions so that a reasonable result is produced and execution may proceed without program interruption Alternatively the processor can invoke a software exception handler to provide special results whenever various types of exceptions are detected 7 2 REAL NUMBERS AND FLOATING POINT FORMATS This section describes how real numbers are represented in floating point format in the Intel Architecture FPU It also introduces terms such as normalized numbers denormalized numbers biased exponents signed zeros and NaNs Readers who are already familiar with floating point processing techniques and the IEEE standards may wish to skip this section 7 2 Intel FLOATING POINT UNIT 7 2 4 Real Number System As shown in Figure 7 1 the real number system comprises the continuum of real numbers from minus infinity eo to plus infinity Binary Real Number System 100 10 0 1 10 100 l l l pU 5 Subset of binary real numbers that can be represented with IEEE single precision 32 bit floating p
251. ion handling mechanism and the typical protocol for FPU exception handlers Code examples that demonstrate various levels of FPU exception handlers Discussion of FPU considerations in multitasking environments Discussion of native mode FPU exception handling The information given is oriented toward the most recent generations of Intel architecture processors starting with the Intel486 It is intended to augment the reference information given in Chapter 7 Floating Point Unit A more extensive version of this appendix is available in the application note AP 578 Software and Hardware Considerations for FPU Exception Handlers for Intel Architecture Processors Order Number 242415 001 which is available from Intel 1 Microsoft Windows 95 and Windows 3 1 and earlier versions operating systems use almost the same FPU exception handling interface as the operating system The recommendations in this appendix for a MS DOS compatible exception handler thus apply to all three operating systems D 1 GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS intel D 1 ORIGIN OF THE MS DOS COMPATIBILITY MODE FOR HANDLING FPU EXCEPTIONS The first generations of Intel Architecture processors starting with the Intel 8086 and 8088 processors and going through the Intel 286 and Intel386 processors did not have an on chip floating point unit Instead floating point capability was provided on a separate numeric copro cessor chip The first o
252. ions perform addition subtraction multiplication and multiply add oper ations on packed data types 8 3 2 1 PACKED ADDITION AND SUBTRACTION The PADDSB PADDSW and PADDWD packed add and PSUBB PSUBW and PSUBD packed subtract instructions add or subtract the signed or unsigned data elements of the source operand to or from the destination operand in wrap around mode These instructions support packed byte packed word and packed doubleword data types The PADDSB and PADDSW packed add with saturation and PSUBSB and PSUBSW packed subtract with saturation instructions add or subtract the signed data elements of the source operand to or from the signed data elements of the destination operand and saturate the result to the limits of the signed data type range These instructions support packed byte and packed word data types The PADDUSB and PADDUSW packed add unsigned with saturation and PSUBUSB and PSUBUSW packed subtract unsigned with saturation instructions add or subtract the unsigned data elements of the source operand to or from the unsigned data elements of the destination operand and saturate the result to the limits of the unsigned data type range These instructions support packed byte and packed word data types 8 3 2 2 PACKED MULTIPLICATION Packed multiplication instructions perform four multiplications on pairs of signed 16 bit oper ands producing 32 bit intermediate results Users may choose the low order or high
253. iply and add packed words MMX COMPARISON INSTRUCTIONS Compare packed bytes for equal Compare packed words for equal Compare packed doublewords for equal Compare packed bytes for greater than Compare packed words for greater than Compare packed doublewords for greater than MMX LOGIC INSTRUCTIONS Bitwise logical and Bitwise logical and not Bitwise logical or Bitwise logical exclusive or intel e INSTRUCTION SET SUMMARY 6 2 2 6 MMX SHIFT AND ROTATE INSTRUCTIONS PSLLW Shift packed words left logical PSLLD Shift packed doublewords left logical PSLLQ Shift packed quadword left logical PSRLW Shift packed words right logical PSRLD Shift packed doublewords right logical PSRLQ Shift packed quadword right logical PSRAW Shift packed words right arithmetic PSRAD Shift packed doublewords right arithmetic 6 2 2 7 MMX STATE MANAGEMENT EMMS Empty MMX state 6 2 3 Floating Point Instructions The floating point instructions are those that are executed by the processor s floating point unit FPU These instructions operate on floating point real extended integer and binary coded decimal BCD operands As with the integer instructions the following list of floating point instructions is divided into subgroups 6 2 3 1 DATA TRANSFER FLD FST FSTP FILD FIST FISTP FBLD FBSTP FXCH FCMOVE FCMOVNE FCMOVB Load real Store real Store real and pop Load integer Store integer Store integer and pop Lo
254. iption of 4 1 EIP register return instruction pointer 4 4 maximum size 4 1 number allowed 4 1 passing parameters 4 6 popping valuesfrom 4 1 procedure linking information 4 3 pushing values on 4 1 return instruction pointer 4 4 SS register 4 1 stack pointer 4 1 stack segment 4 1 stack frame base pointer register 4 4 switching osc ee cease ke th hex hme 4 8 top of Stack opere we se ee qoe xS 4 1 A A state Soe 4 3 INDEX Processor identification earlier Intel architecture processors 10 3 using CPUID instruction 10 1 Processor state information saving on a procedure 4 7 Protected mode description 3 4 SES Re Ehe 9 4 Pseudo denormal number 7 29 Pseudo infinity 7 28 Pseudo NaN 7 28 PUSH instruction 4 1 4 3 6 19 6 38 PUSHA instruction 4 7 6 19 PUSHF instruction 3 10 4 7 6 37 PUSHFD instruction 3 10 4 7 6 37 Q QNaN description 7 8 operating on 7 41 rules for generating 7 41 Quadword cada SE
255. ithmetic operation on a SNaN Return a QNaN to the destination operand see Section 7 6 Operating on NaNs Compare and test operations one or both operands are NaNs Set the condition code flags CO C2 and C3 in the FPU status word to 111B not comparable Addition operands are opposite signed infinities Subtraction operands are like signed infinities Return the real indefinite value to the destination operand Multiplication e by 0 0 by oo Return the real indefinite value to the destination operand Division e by O by 0 Return the real indefinite value to the destination operand Remainder instructions FPREM FPREM1 modulus divisor is 0 or dividend is ee Return the real indefinite clear condition code flag C2 to 0 Trigonometric instructions FCOS FPTAN FSIN FSINCOS source operand is oo Return the real indefinite clear condition code flag C2 to 0 FSQRT negative operand except FSQRT 0 0 FYL2X negative operand except FYL2X 0 FYL2XP1 operand more negative than 1 Return the real indefinite value to the destination operand FBSTP source register is empty or it contains a NaN eo or a value that cannot be represented in 18 decimal digits Store BCD integer indefinite value in the destination operand FXCH one or both registers are tagged empty Load empty registers with the real indefinite value then pe
256. its caches it has the same two on chip 8 KByte L1 caches as does the Pentium processor and also has a 256 K Byte L2 cache that s in the same package as and closely coupled to the CPU using a dedicated 64 bit backside full clock speed bus The L1 cache is dual ported the L2 cache supports up to 4 concurrent accesses and the 64 bit external data bus is transaction oriented meaning that each access is handled as a separate request and response with numerous requests allowed while awaiting a response These parallel features for data access work with the parallel execution capabilities to provide a non blocking architec ture in which the processor is more fully utilized and performance is enhanced The Pentium Pro processor also has an expanded 36 bit address bus giving a maximum physical address space of 64 GBytes Since the Pentium Pro processor is currently the most advanced of the Intel Architecture family a more detailed description of its architecture is provided in Sections 2 4 and 2 5 More detailed hardware and architectural information on each of the generations of the Intel Architecture family is available in the separate data books for the processor generations see Section 1 5 Related Literature 2 3 INTRODUCTION TO THE INTEL ARCHITECTURE intel e 2 2 INCREASING INTEL ARCHITECTURE PERFORMANCE AND MOORE S LAW In the mid 1960s Intel Chairman of the Board Gordon Moore deduced a principle or law which h
257. k and establish it as the current stack the program or operating system executive must do the following 1 Establish a stack segment 2 Load the segment selector for the stack segment into SS register using a MOV POP or LSS instruction 4 2 Intel PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS 3 Load the stack pointer for the stack into the ESP register using a MOV POP or LSS instruction The LSS instruction can be used to load the SS and ESP registers in one operation See Segment Descriptors in Chapter 3 of the Intel Architecture Software Developer s Manual Volume 3 for information on how to set up a segment descriptor and segment limits for a stack segment 4 2 2 Stack Alignment The stack pointer for a stack segment should be aligned on 16 bit word or 32 bit double word boundaries depending on the width of the stack segment The D flag in the segment descriptor for the current code segment sets the stack segment width see Segment Descriptors in Chap ter 3 of the Intel Architecture Software Developer s Manual Volume 3 The PUSH and POP in structions use the D flag to determine how much to decrement or increment the stack pointer on a push or pop operation respectively When the stack width is 16 bits the stack pointer is incre mented or decremented in 16 bit increments when the width is 32 bits the stack pointer is in cremented or decremented in 32 bit increments If a 16 bit value is pushed onto a 32
258. l Architecture processors e g Pentium processor Pentium processor with MMX technology and Pentium Pro processor may contain design defects or errors known as errata which may cause the product to deviate from published specifications Such errata are not covered by Intel s warranty Current characterized errata are available on request Contact your local Intel sales office or your distributor to obtain the latest specifications and before placing your product order Copies of documents which have an ordering number and are referenced in this document or other Intel literature may be obtained from Intel Corporation P O Box 7641 Mt Prospect IL 60056 7641 or call 1 800 879 4683 or visit Intel s website at httpNwww intel com Copyright O Intel Corporation 1996 1997 Third party brands and names are the property of their respective owners intel TABLE OF CONTENTS PAGE CHAPTER 1 ABOUT THIS MANUAL 1 1 OVERVIEW OF THE NTEL ARCHITECTURE SOFTWARE DEVELOPER S MANUAL VOLUME 1 BASIC ARCHITECTURE 1 1 1 2 OVERVIEW OF THE INTEL ARCHITECTURE SOFTWARE DEVELOPER S MANUAL VOLUME 2 INSTRUCTION SET REFERENCE 1 2 1 3 OVERVIEW OF THE INTEL ARCHITECTURE SOFTWARE DEVELOPER S MANUAL VOLUME 3 SYSTEM PROGRAMMING GUIDE 1 3 1 4 NOTATIONAL CONVENTIONS 0 00 00 eee eee n 1 5 1 4 1 Bitand Byte Orders lect pa sche tl e Ur FR CX sa ae 1 5 1 4 2 Reserved Bits and Software Compatibility
259. l Moves CMOVGE CMOVNL SF xor OF 0 Greater or equal not less CMOVL CMOVNGE SF xor OF 1 Less not greater or equal CMOVLE CMOVNG SF xor OF or ZF 1 Less or equal not greater CMOVO OF 1 Overflow CMOVNO OF 0 Not overflow CMOVS SF 1 Sign negative CMOVNS SF 0 Not sign non negative The XADD exchange and add instruction swaps two operands and then stores the sum of the two operands in the destination operand The status flags in the EFLAGS register indicate the result of the addition This instruction can be combined with the LOCK prefix see LOCK Assert LOCK Signal Prefix in Chapter 3 of the Intel Architecture Software Devel oper s Manual Volume 2 in a multiprocessing system to allow multiple processors to execute one DO loop The CMPXCHG compare and exchange and CMPXCHG8B compare and exchange 8 bytes instructions are used to synchronize operations in systems that use multiple processors The CMPXCHG instruction requires three operands a source operand in a register another source operand in the EAX register and a destination operand If the values contained in the destination operand and the EAX register are equal the destination operand is replaced with the value of the other source operand the value not in the EAX register Otherwise the original value of the destination operand is loaded in the EAX register The status flags in the EFLAGS register 6 18 intel e INSTRUCTION SET SUMMARY reflect the result
260. l Number and Non number 5 7 5 7 2 3 1 Sigried Zeros adi pp ae a HE REA 7 6 7 2 3 2 Normalized and Denormalized Finite Numbers 7 6 viii intel e TABLE OF CONTENTS PAGE 7 2 3 8 Signed Infinities ic EMT AA 7 8 7 2 3 4 NEM EP 7 8 7 2 4 Indefihite nenie es AR ab daa qud red 7 8 7 3 FPU ARCHITECTURE be ntn Ur M Ree 7 8 7 3 1 The FPU Data 7 9 7 3 1 1 Parameter Passing With the FPU Register 7 11 7 3 2 FPU Status Register cese cere gs i she s uqa d halia tusa 7 12 7 3 2 1 Top of Stack TOP 7 12 7 3 2 2 Condition Code Flags 7 12 7 3 2 8 Exception Flags 9eesleie l9 Deme ew RR VW RR Rees 7 14 7 3 2 4 Stack Fault Flag assasi Gee wld ACER SR 7 15 7 3 3 Branching and Conditional Moves on FPU Condition Codes 7 15 7 3 4 FPU Control Word u l usu I Hh 7 16 7 3 4 1 Exception Flag Masks Ta eee eee eens 7 17 7 3 4 2 Precision Control 7 17 7 3 4 8 Rounding Control Field 2 2 0 ce ne 7 18 7 3 5 Infinity Control Flag 0 eee 7 20 7 3 6 EPUl Tag Word vec ft boli ea mL
261. l features can be exploited to identify the processor The settings of bits 12 and 13 IOPL 14 NT and 15 reserved in the EFLAGS register see Figure 3 7 is different for Intel s 32 bit processors than for the Intel 8086 and Intel 286 proces sors By examining the settings of these bits with the PUSHF PUSHFD and POP POPFD instructions an application program can determine whether the processor is an 8086 Intel286 or one of the Intel 32 bit processors 8086 processor Bits 12 through 15 of EFLAGS register are always set Intel 286 processor Bits 12 through 15 are always clear in real address mode 32 bit processors In real address mode bit 15 is always clear and bits 12 through 14 have the last value loaded into them In protected mode bit 15 is always clear bit 14 has the last value loaded into it and the IOPL bits depends on the current privilege level CPL The IOPL field can be changed only if the CPL is 0 Other EFLAG register bits that can be used to differentiate between the 32 bit processors Bit 18 AC Implemented only on the Pentium Pro Pentium and Intel486 processors The inability to set or clear this bit distinguishes an Intel386 processor from the other Intel 32 bit processors Bit 21 ID Determines if the processor is able to execute the CPUID instruction The ability to set and clear this bit indicates that the processor is a Pentium Pro Pentium or later version Inte
262. l486 processor To determine whether an FPU or NPX is present in a system applications can write to the FPU NPX status and control registers using the FNINIT instruction and then verify the correct values are read back using the FNSTENV instruction After determining that an FPU or NPX is present its type can then be determined In most cases the processor type will determine the type of FPU or NPX however an Intel386 processor is compatible with either an Intel 287 or Intel 387 math coprocessor The method the coprocessor uses to represent co after the execution of the FINIT FNINIT or RESET instruction indicates which coprocessor is present The Intel 287 math coprocessor uses the same bit representation for and ee whereas the Intel 387 math coprocessor uses different representations for and oo 10 3 Intel A EFLAGS Cross Reference APPENDIX A EFLAGS CROSS REFERENCE The cross reference in Table A 1 summarizes how the flags in the processor s EFLAGS register are affected by each instruction For detailed information on how flags are affected see Chapter 3 Instruction Set Reference in the Intel Architecture Software Developer s Manual Volume 2 The following codes describe the how the flags are affected T Instruction tests flag M Instruction modifies flag either sets or resets depending on operands 0 Instruction resets flag 1 Instruction sets flag Instruction s effect o
263. lag for the exception and delivers a predefined default response and continues executing instructions The masked default responses to exceptions have been chosen to deliver a reasonable result for each exception condition and are generally satisfactory for most floating point applications By masking or unmasking specific floating point excep tions in the FPU control word programmers can delegate responsibility for most exceptions to the FPU and reserve the most severe exception conditions for software exception handlers Because the exception flags are sticky they provide a cumulative record of the exceptions that have occurred since they were last cleared A programmer can thus mask all exceptions run a calculation and then inspect the exception flags to see if any exceptions were detected during the calculation 7 43 FLOATING POINT UNIT Table 7 19 Arithmetic and Non arithmetic Instructions Non arithmetic Instructions Arithmetic Instructions FABS F2XM1 FCHS FADD FADDP FCLEX FBLD FDECSTP FBSTP FFREE FCOM FCOMP FCOMPP FINCSTP FCOS FINIT FNINIT FDIV FDIVP FDIVR FDIVRP FLD register to register FIADD FLD extended format from memory FICOM FICOMP FLD constant FIDIV FIDIVR FLDCW FILD FLDENV FIMUL FNOP FIST FISTP FRSTOR FISUB FISUBR FSAVE FNSAVE FLD conversion FST FSTP register to register FMUL FMULP FSTP extended format to memory FPATAN FSTCW FNSTCW FPREM FPREM1 FSTENV FNSTEN
264. lectors to and from segment registers DS ES FS GS and SS The transfers are always made to or from a segment register and a general purpose register or memory Transfers between segment registers are not supported 6 38 intel e INSTRUCTION SET SUMMARY The POP and MOV instructions cannot place a value in the CS register Only the far control transfer versions of the JMP CALL and RET instructions see Section 6 14 2 Far Control Transfer Instructions affect the CS register directly 6 14 2 Far Control Transfer Instructions The JMP and CALL instructions see Section 6 9 Control Transfer Instructions both accept a far pointer as a source operand to transfer program control to a segment other than the segment currently being pointed to by the CS register When a far call is made with the CALL instruction the current values of the EIP and CS registers are both pushed on the stack The RET instruction see Section 6 9 1 2 Call and Return Instructions can be used to execute a far return Here program control is transferred from a code segment that contains a called procedure back to the code segment that contained the calling procedure The RET instruction restores the values of the CS and EIP registers for the calling procedure from the stack 6 14 3 Software Interrupt Instructions The software interrupt instructions INT INTO BOUND and IRET see Section 6 9 3 Soft ware Interrupts can also call and return
265. leword used to access the variables of a procedure The set of stack frame pointers used by a procedure to access the variables of other procedures is called the display The first doubleword in the display is a pointer to the previous stack frame This pointer is used by a LEAVE instruction to undo the effect of an ENTER instruction by discarding the current stack frame After the ENTER instruction creates the display for a procedure it allocates the dynamic local variables for the procedure by decrementing the contents of the ESP register by the number of bytes specified in the first parameter This new value in the ESP register serves as the initial top of stack for all PUSH and POP operations within the procedure 4 16 Intel PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS To allow a procedure to address its display the ENTER instruction leaves the EBP register pointing to the first doubleword in the display Because stacks grow down this is actually the doubleword with the highest address in the display Data manipulation instructions that specify the EBP register as a base register automatically address locations within the stack segment instead of the data segment The ENTER instruction can be used in two ways nested and non nested If the lexical level is 0 the non nested form is used The non nested form pushes the contents of the EBP register on the stack copies the contents of the ESP register into the EBP register and subtracts th
266. lip Flop 1 and also clocks out the IGNNE signal active from Flip Flop 2 The assertion of IGNNE may be used by the handler if needed to execute any FPU instruction while ignoring the pending FPU errors The problem here is that the state of Flip Flop 2 is effectively an additional but hidden status bit that can affect processor behavior and so ideally should be saved upon entering SMM and restored before resuming to normal operation If this is not done and also the SMM code saves the FPU state AND an FPU error handler is being used which relies on IGNNE assertion then very rarely the FPU handler will nest inside itself and malfunction The following example shows how this can happen Suppose that the FPU exception handler includes the following sequence FNSTSW save sw Save the FPU status word using a no wait FPU instruction OUT OFOH AL clears IRQ13 amp activates IGNNE FLDCW new cw loads new CW ignoring FPU errors Since IGNNE is assumed active or any other FPU instruction that is not a no wait type will cause the same problem D 20 Intel GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS FCLEX clear the FPU error conditions amp thus turn off FERR amp reset the IGNNE FF The problem will only occur if the processor enters SMM between the OUT and the FLDCW instructions But if that happens AND the SMM code saves the FPU state using FNSAVE then the IGNNE Flip Flop will be cleared b
267. lmost any threat to numerical integrity If double precision data and results were at issue a better formula would have to be used and once again the FPU s default evaluation of that formula would provide substantially enhanced numerical integrity over mere double precision evaluation On most machines straightforward algorithms will not deliver consistently correct results and will not indicate when they are incorrect To obtain correct results on traditional machines under all conditions usually requires sophisticated numerical techniques that go beyond typical programming practice General application programmers using straightforward algorithms will produce much more reliable programs using the Intel architectures This simple fact greatly reduces the software investment required to develop safe accurate computation based products Beyond traditional numeric support for scientific applications the Intel architectures have built in facilities for commercial computing They can process decimal numbers of up to 18 digits without round off errors performing exact arithmetic on integers as large as 2 or 10 5 Exact arithmetic is vital in accounting applications where rounding errors may introduce mone tary losses that cannot be reconciled The Intel FPU s contain a number of optional numerical facilities that can be invoked by sophis ticated users These advanced features include directed rounding gradual underflow and programmed except
268. m or task must be less than or equal to the I O privilege level to access the I O address space This field can only be modified by the POPF and IRET instructions when operating at a CPL of 0 Nested task flag Controls the chaining of interrupted and called tasks Set when the current task is linked to the previously executed task cleared when the current task is not linked to another task Resume flag Controls the processor s response to debug exceptions Virtual 8086 mode flag Set to enable virtual 8086 mode clear to return to protected mode Alignment check flag Set this flag and the AM bit in the CRO register to enable alignment checking of memory references clear the AC flag and or the AM bit to disable alignment checking Virtual interrupt flag Virtual image of the IF flag Used in conjunc tion with the VIP flag To use this flag and the VIP flag the virtual mode extensions are enabled by setting the VME flag in control register CR4 Virtual interrupt pending flag Set to indicate to that an interrupt is pending clear when no interrupts are pending Software sets and clears this flag The processor only reads it Used in conjunction with the VIF flag Identification flag The ability of a program to set or clear this flag indicates support for the CPUID instruction See Chapter 3 Protected Mode Memory Management in the Intel Architecture Software Devel oper s Manual Volume 3 for a detail description of these fla
269. ment Order Number 231630 376 Embedded Processor Programmer s Reference Manual Order Number 240314 60387 DX User s Manual Programmer s Reference Order Number 231917 376 High Performance 32 Bit Embedded Processor Order Number 240182 Intel386 SX Microprocessor Order Number 240187 Microprocessor and Peripheral Handbook Vol 1 Order Number 230843 Intel Architecture Optimization Manual Order Number 242816 Intel 2 Introduction to the Intel Architecture Intel CHAPTER 2 INTRODUCTION TO THE INTEL ARCHITECTURE A strong case can be made that the exponential growth of both the power and breadth of usage of the computer has made it the most important force that is reshaping human technology busi ness and society in the second half of the twentieth century Further the computer promises to continue to dominate technological growth well into the twenty first century in part since other powerful technological forces that are just emerging are strongly dependent on the growth of computing power for their own existence and growth such as the Internet and genetics devel opments like recombinant DNA research and development The Intel Architecture is clearly today s preferred computer architecture as measured by number of computers in use and total computing power available in the world Thus it is hard to overestimate the importance of the Intel Architecture 2 1 BRIEF HISTORY OF THE INTEL ARCHITECTURE The developments l
270. mode more efficient and to allow for 4 MByte as well as 4 KByte pages The main registers are still 32 bits but internal data paths of 128 and 256 bits have been added to speed internal data transfers and the burstable external data bus has been increased to 64 bits The Advanced Programmable Interrupt Controller APIC has been added to support systems with multiple Pentium processors and new pins and a special mode dual processing has been designed in to support glueless two processor systems The Intel Pentium Pro processor is the latest and most powerful member of the Intel Architec ture It has three way superscalar architecture which means that it can execute three instructions per CPU clock It does this by incorporating even more parallelism than the Pentium processor The Pentium Pro processor provides Dynamic Execution micro data flow analysis out of order execution superior branch prediction and speculative execution in a superscalar imple mentation Three instruction decode units work in parallel to decode object code into smaller operations called micro ops These go into an instruction pool and when interdependencies don t prevent can be executed out of order by the five parallel execution units two integer two FPU and one memory interface unit The Retirement Unit retires completed micro ops in their original program order taking account of any branches The power of the Pentium Pro processor is further enhanced by
271. n flag is undefined R Instruction restores prior value of flag Blank Instruction does not affect flag Table A 1 EFLAGS Cross Reference Instruction OF SF ZF AF PF CF TF IF DF NT RF AAA TM x M AAD M M M AAM M M M AAS TM M ADC M M M M M TM ADD M M M M M M AND 0 M M M 0 ARPL M BOUND BSF BSR M BSWAP BT BTS BTR BTC M A 1 EFLAGS CROSS REFERENCE Table A 1 EFLAGS Cross Reference Contd Instruction OF SF ZF AF PF CF TF DF NT RF CALL CBW CLC CLD CLI CLTS CMC CMOV cc CMP CMPS CMPXCHG CMPXCHG8B lt Zj lt Zz Z lt S A E mod lt E S A CPUID CWD DAA DAS E Z TM TM E TM TM DEC DIV ENTER ESC FCMOVcc FCOMI FCOMIP FUCOMI FUCOMIP HLT IDIV IMUL IN INC INS INT INTO INVD INVLPG A 2 Intel EFLAGS CROSS REFERENCE Table A 1 EFLAGS Cross Reference Contd Instruction OF SF ZF AF PF CF TF IF DF NT RF IRET R R R R R R R R R T Jcc JCXZ JMP LAHF LAR M LDS LES LSS LFS LGS LEA LEAVE LGDT LIDT LLDT LMSW LOCK LODS T LOOP LOOPE LOOPNE T LSL LTR MOV MOV control debug test MOVS T MOVSX MOVZX MUL M M NEG M M M M M M NOP NOT OR 0 M M M 0 OUT OUTS T POP POPA
272. n procedure A and MAIN by using the stack frame pointers in its display Old EBP Main s EBP Main s EBP Main s EBP Procedure A s EBP Procedure A s EBP EBP y Main s EBP Display Procedure A s EBP Procedure B s EBP Dynamic Storage lt ESP Figure 4 9 Stack Frame after Entering Procedure B When procedure B calls procedure C the ENTER instruction creates a new display for proce dure C see Figure 4 10 The first doubleword holds a copy of the last value in procedure B s EBP register This is used by the LEAVE instruction to restore procedure B s stack frame The second and third doublewords are copies of the two stack frame pointers in procedure A s display If procedure C were at the next deeper lexical level from procedure B a fourth double word would be copied which would be the stack frame pointer to procedure B s local variables Note that procedure B and procedure C are at the same level so procedure C is not intended to access procedure B s variables This does not mean that procedure C is completely isolated from procedure B procedure C is called by procedure B so the pointer to the returning stack frame is a pointer to procedure B s stack frame In addition procedure B can pass parameters to proce dure C either on the stack or through variables global to both procedures that is variables in the scope of both procedures
273. n thus be granted to individual tasks If in protected mode and the CPL is less than or equal to the current IOPL the processor allows all I O operations to proceed If the CPL is greater than the IOPL or if the processor is operating 9 5 INPUT OUTPUT intel o in virtual 8086 mode the processor checks the I O permission bit map to determine if access to a particular I O port is allowed Each bit in the map corresponds to an I O port byte address For example the control bit for I O port address 29H in the I O address space is found at bit position 1 ofthe sixth byte in the bit map Before granting I O access the processor tests all the bits corre sponding to the I O port being addressed For a doubleword access for example the processors tests the four bits corresponding to the four adjacent 8 bit port addresses If any tested bit is set a general protection exception GP is signaled If all tested bits are clear the I O operation is allows to proceed Because I O port addresses are not necessarily aligned to word and doubleword boundaries the processor read two bytes from the I O permission bit map for every access to an I O port To prevent exceptions from being generated when the ports with the highest addresses are accessed an extra byte needs to included in the TSS immediately after the table This byte must have all of its bits set and it must be within the segment limit Itis not necessary for the I O permission bit map to r
274. nded When binding itself to a new task the processor loads the EFLAGS register with data from the new task s TSS When a call is made to an interrupt or exception handler procedure the processor automatically saves the state of the EFLAGS registers on the procedure stack When an interrupt or exception is handled with a task switch the state of the EFLAGS register is saved in the TSS for the task being suspended 3 10 intel e BASIC EXECUTION ENVIRONMENT 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 131211109 8 7 65 4 3 21 0 VIV 10 no DI TIS F F TIN n no A V P 0 1 plp CM F F ID Flag ID Virtual Interrupt Pending VIP Virtual Interrupt Flag VIF Alignment Check AC Virtual 8086 Mode VM Resume Flag RF Nested Task NT I O Privilege Level IOPL Overflow Flag OF Direction Flag DF Interrupt Enable Flag IF Trap Flag TF Sign Flag SF Zero Flag ZF Auxiliary Carry Flag AF Parity Flag PF Carry Flag CF Indicates a Status Flag Indicates a Control Flag Indicates a System Flag XON O0 0 0 O0 O0 X X X X X X X X X xxx Reserved bit positions DO NOT USE Always set to values previously read Figure 3 7 EFLAGS Register As the Intel Architecture has evolved flags
275. ng floating point instruction is followed by one or more non floating point instruc tions it may not be useful to re execute the faulting instruction See Section 7 9 Floating Point Exception Synchronization for more information on synchronizing floating point excep tions In cases where the handler needs to restart program execution with the faulting instruction the IRET instruction cannot be used directly The reason for this is that because the exception is not generated until the next floating point or WAIT FWAIT instruction following the faulting floating point instruction the return instruction pointer on the stack may not point to the faulting instruction To restart program execution at the faulting instruction the exception handler must obtain a pointer to the instruction from the saved FPU state information load it into the return instruction pointer location on the stack and then execute the IRET instruction In lieu of writing recovery procedures the exception handler can do the following Increment an exception counter for later display or printing Print or display diagnostic information such as the FPU environment and registers Halt further program execution See Section D 3 4 FPU Exception Handling Examples for general examples of floating point exception handlers and for specific examples of how to write a floating point exception handler when using the MS DOS compatibility mode 7 8 FLOATING PO
276. ng thread and the registers of the resuming thread are loaded The FPU state does not need to be saved at this point If the resuming thread does not use the FPU before it is itself suspended then both a save and a load of the FPU state has been avoided It is often the case that several threads may be executed without any usage of the FPU The processor supports speculative deferral of FPU saves via interrupt 7 Device Not Available DNA used in conjunction with CRO bit 3 the Task Switched bit TS See Control Regis ters in Chapter 2 of the ntel Architecture Software Developer s Manual Volume 3 Every task switch via the hardware supported task switching mechanism see Task Switching in Chapter 6 of the Intel Architecture Software Developer s Manual Volume 3 sets TS Multi threaded kernels that use software task switching can set the TS bit by reading CRO ORing a 1 into bit 3 and writing back CRO Any subsequent floating point instructions now being executed in a new thread context will fault via interrupt 7 before execution This allows a DNA handler to save the old floating point context and reload the FPU state for the current thread The handler should clear the TS bit before exit using the CLTS instruction On return from the handler the faulting thread will proceed with its floating point computation Some operating systems save the FPU context on every task switch typically because they also change the linea
277. nged into 40 micro op regis ters It contains micro ops that are waiting to be executed as well as those that have already been executed but not yet committed to machine state The dispatch execute unit can execute instruc tions from the reorder buffer in any order 2 5 4 Dispatch Execute Unit The dispatch execute unit is an out of order unit that schedules and executes the micro ops stored in the reorder buffer according to data dependencies and resource availability and tempo rarily stores the results of these speculative executions The scheduling and dispatching of micro ops from the reorder buffer is handled by the reserva tion station It continuously scans the reorder buffer for micro ops that are ready to be executed that is all the source operands are available and dispatches them to the available execution units The results of a micro op execution are returned to the reorder buffer and stored along with the micro op until it is retired This scheduling and dispatching process supports classic out of order execution where micro ops are dispatched to the execution units strictly according to data flow constraints and execution resource availability without regard to the original ordering of the instructions When two or more micro ops of the same type for example integer opera tions are available at the same time they are executed in a pseudo FIFO order in the reorder buffer Execution of micro ops is handled by two integer
278. nit the retire unit and the instruc tion pool Instructions and data are supplied to these units through the bus interface unit System Bus L2 Cache Cache Bus Bus Interface Unit L1 Instruction Cache L1 Data Cache Fetch Load I Store Fetch Decode Dispatch intel Unit Execute Unit Retire Unit lt gt Architecture Registers Instruction Pool Figure 2 1 The Processing Units in the Pentium Pro Processor Microarchitecture and Their Interface with the Memory Subsystem To insure a steady supply of instructions and data to the instruction execution pipeline the Pentium Pro processor microarchitecture incorporates two cache levels The L1 cache provides an 8 KByte instruction cache and an 8 KByte data cache both closely coupled to the pipeline The L2 cache is a 256 KByte static RAM that is coupled to the core processor through a full clock speed 64 bit cache bus 2 6 intel e INTRODUCTION TO THE INTEL ARCHITECTURE The centerpiece of the Pentium Pro processor microarchitecture is an innovative out of order execution mechanism called dynamic execution Dynamic execution incorporates three data processing concepts Deep branch prediction Dynamic data flow analysis Speculative execution Branch prediction is a concept found in most mainframe and high speed microprocessor archi tectures It allows the processor to decode instructions
279. nning and end of the Intel Architecture instructions in the cache lines and trans mits 16 aligned bytes to the decoder The instruction fetch unit computes the instruction pointer based on inputs from the branch target buffer the exception interrupt status and branch misprediction indications from the integer execution units The most important part of this process is the branch prediction performed by the branch target buffer Using an extension of Yeh s algorithm the 512 entry branch target buffer looks many instructions ahead of the retirement program counter Within this instruction window there may be numerous branches procedure calls and returns that must be correctly predicted if the dispatch execute unit is to do useful work The instruction decoder contains three parallel decoders two simple instruction decoders and one complex instruction decoder Each decoder converts an Intel Architecture instruction into one or more triadic micro ops two logical sources and one logical destination per micro op Micro ops are primitive instructions that are executed by the processor s six parallel execution units Many Intel Architecture instructions are converted directly into single micro ops by the simple instruction decoders and some instructions are decoded into from one to four micro ops The more complex Intel Architecture instructions are decoded into sequences of preprogrammed micro ops obtained from the microcode instruction sequencer T
280. nstruction When the support for MMX technology is detected by the CPUID instruction it is signaled by setting bit 23 MMX technology bit in the feature flags to 1 In general two versions of the routine can be created one with scalar instructions and one with MMX instructions The appli cation will call the appropriate routine depending on the results of the CPUID instruction If support for MMX technology is detected then the MMX routine is called if no support for the MMX technology exists the application calls the scalar routine NOTE The CPUID instruction will continue to report the existence of the MMX technology if the CRO EM bit is set which signifies that the CPU is configured to generate exception interrupt 7 that can be used to emulate floating point instructions In this case executing an MMX instruction results in an invalid opcode exception Example 8 1 illustrates how to use the CPUID instruction This example does not represent the entire CPUID sequence but shows the portion used for detection of MMX technology Example 8 1 Partial Routine for Detecting MMX Technology with the CPUID Instruction identify existence of CPUID instruction identify Intel processor mov EAX 1 request for feature flags CPUID OFh 0A2h CPUID instruction test EDX 00800000h Is IA MMX technology bit Bit 23 of EDX in feature flags set jnz MX Technology Found 8 5 2 Using the EMMS Instruction When int
281. nstructions are a group of instructions that check the state of the status flags in the EFLAGS register and perform a move operation if the flags are in a specified state or condi tion These instructions can be used to move a 16 or 32 bit value from memory to a general purpose register or from one general purpose register to another The flag state being tested for each instruction is specified with a condition code cc that is associated with the instruction If the condition is not satisfied a move is not performed and execution continues with the instruc tion following the CMOVzcc instruction 6 16 intel e INSTRUCTION SET SUMMARY Table 6 1 Move Instruction Operations Type of Data Movement Source gt Destination From memory to a register Memory location General purpose register Memory location Segment register From a register to memory General purpose register Memory location Segment register Memory location Between registers General purpose register General purpose register General purpose register Segment register Segment register gt General purpose register General purpose register Control register Control register General purpose register General purpose register Debug register Debug register gt General purpose register Immediate data to a register Immediate General purpose register Immediate data to memory Immediate Memory location Table
282. ntal instructions FSCALE FXTRACT FPREM and others and all exceptions except precision when caused by FPU store instructions Like deferred error reporting immediate error reporting will cause the processor to freeze just before executing the next WAIT or FPU instruction if the error condition has not been cleared by that time Note that in general whether deferred or immediate error reporting is used for an FPU exception depends both on which exception occurred and which instruction caused that exception A com plete specification of these cases which applies to both the Pentium and the Intel486 processors is given in Section 5 1 21 in the Pentium Processor Family Developer s Manual Volume 1 D 4 Intel GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS If NE 0 but the IGNNE input is active while an unmasked FPU exception is in effect the processor disregards the exception does not assert FERR and continues If IGNNE is then de asserted and the FPU exception has not been cleared the processor will respond as described above That is an immediate exception case will assert FERR immediately A deferred excep tion case will assert FERR and freeze just before the next FPU or WAIT instruction The asser tion of IGNNE is intended for use only inside the FPU exception handler where it is needed if one wants to execute non control FPU instructions for diagnosis before clearing the exception condition When IGNNE is asserted inside
283. ntel Architecture mainstream with the Intel486 SL Enhanced processors These features were developed in the Intel386 SL and Intel486 SL processors which were specialized for the rapidly growing battery operated note 1 Requires ony one 32 bit address component to access anywhere in the address space 2 2 intel e INTRODUCTION TO THE INTEL ARCHITECTURE book PC market The features include the new System Management Mode triggered by its own dedicated interrupt pin which allows complex system management features such as power management of various subsystems within the PC to be added to a system transparently to the main operating system and all applications The Stop Clock and Auto Halt Powerdown features allow the CPU itself to execute at a reduced clock rate to save power or to be shut down with state preserved to save even more power The Intel Pentium processor added a second execution pipeline to achieve superscalar perfor mance two pipelines known as u and v together can execute two instructions per clock The on chip L1 cache has also been doubled with 8 KBytes devoted to code and another 8 KBytes to data The data cache uses the MESI protocol to support the more efficient write back mode as well as the write through mode that is used by the Intel486 processor Branch prediction with an on chip branch table has been added to increase performance in looping constructs Exten sions have been added to make the virtual 8086
284. o change the IOPL does not result in an exception the IOPL simply remains unchanged The POPF instruction also may be used to change the state of the IF flag as can the CLI and STI instructions however the POPF instruction in this case is also I O sensitive A procedure may use the POPF instruction to change the setting of the IF flag only if the CPL is less than or equal to the current IOPL An attempt by a less privileged procedure to change the IF flag does not result in an exception the IF flag simply remains unchanged 9 5 2 WO Permission Bit Map The I O permission bit map is a device for permitting limited access to I O ports by less privi leged programs or tasks and for tasks operating in virtual 8086 mode The I O permission bit map is located in the TSS see Figure 9 2 for the currently running task or program The address of the first byte of the I O permission bit map is given in the I O map base address field of the TSS The size of the I O permission bit map and its location in the TSS are variable Task State Segment TSS 31 24 23 0 T TOT T d Last byte of bit map must be followed by a byte with all bits set 1 0 Permission Bit Map 64H 1 0 base map must gt VO Map not exceed DFFFH lt lt 0 Figure 9 2 I O Permission Bit Map Because each task has its own TSS each task has its own I O permission bit map Access to indi vidual I O ports ca
285. ocedure stack Stack frame base pointer register 4 4 Status flags EFLAGS register 3 11 7 15 7 16 7 35 STC instruction 3 12 6 37 STD instruction 3 12 6 37 6 37 6 38 9 4 STOS instruction 3 12 6 35 SIbigs Contes e chee eat ane as 5 4 ST 0 top of stack register 7 11 SUB instruction 6 22 Superscaler 2 6 Synchronization of floating point exceptions 7 54 System flags EFLAGS register 3 13 System management mode SSM 3 4 T Tangent FPU operation 7 37 TASK Gate ue enone oe wes eme 4 14 Task state segment see TSS Tasks exception handler 4 14 interrupt handler 4 14 TEST instruction 6 30 TF trap flag EFLAGS register 3 13 Tiny number see pee Rte 7 7 TOP stack TOP field FPU status word 7 9 Transcendental instruction accuracy 7 39 Trap gat uu ss eb ide mE RR Vase ane 4 11 TSS VO 9 5 permission bit map 9 5 saving state of EFLAGS register 3 10 UD2 instruction 6 2 6 40 UE numeric overflow exception flag FPU status word 7 14 7 52 Underflow FPU exception see Numeric underflow exception Un
286. ocessor does not offer any control flow instructions that branch on the setting of the condition code flags CO C2 and C3 in the FPU status word To branch on the state of these flags the FPU status word must first be moved to the AX register in the integer unit The FSTSW AX store status word instruction can be used for this purpose When these flags are in the AX register the TEST instruction can be used to control conditional branching as follows 1 Check for an unordered result Use the TEST instruction to compare the contents of the AX register with the constant 0400H see Table 7 17 This operation will clear the ZF flag in the EFLAGS register if the condition code flags indicate an unordered result otherwise the ZF flag will be set The JNZ instruction can then be used to transfer control if necessary to a procedure for handling unordered operands Table 7 17 TEST Instruction Constants for Conditional Branching Order Constant Branch ST 0 Source Operand 4500H JZ ST 0 Source Operand 0100H JNZ ST 0 Source Operand 4000H JNZ Unordered 0400H JNZ 2 Check ordered comparison result Use the constants given in Table 7 17 in the TEST instruction to test for a less than equal to or greater than result then use the corresponding conditional branch instruction to transfer program control to the appropriate procedure or section of code If a program or procedure has been thoroughly tested and it incorpora
287. ogramming environment of the Intel 8086 processor with a few extensions such as the ability to switch to protected or system management mode The processor is placed in real address mode following power up or a reset System management mode A standard architectural feature unique to all Intel processors beginning with the Intel386 SL processor This mode provides an operating system or executive with a transparent mechanism for implementing platform specific functions such as power management and system security The processor enters SMM when the external SMM interrupt pin SMI is activated or an SMI is received from the advanced programmable interrupt controller APIC In SMM the processor switches to a separate address space while saving the entire context of the currently running program or task SMM specific code may then be executed transparently Upon returning from SMM the processor is placed back into its state prior to the system management interrupt The basic execution environment is the same for each of these operating modes as is described in the remaining sections of this chapter 3 1 BASIC EXECUTION ENVIRONMENT intel e 3 2 OVERVIEW OF THE BASIC EXECUTION ENVIRONMENT Any program or task running on an Intel Architecture processor is given a set of resources for executing instructions and for storing code data and state information These resources shown in Figure 3 1 include an address space of up to 2 bytes a s
288. oint format 100 10 1 0 1 10 100 dex xo a s sas sau sa nois aep e Re Rs os 10 4 4 di 10 0000000000000000000000 1 11111111111111111111111 Precision 24 Binary Digits Numbers within this range cannot be represented Figure 7 1 Binary Real Number System Because the size and number of registers that any computer can have is limited only a subset of the real number continuum can be used in real number calculations As shown at the bottom of Figure 7 1 the subset of real numbers that a particular FPU supports represents an approxima tion of the real number system The range and precision of this real number subset is determined by the format that the FPU uses to represent real numbers 7 2 2 Floating Point Format To increase the speed and efficiency of real number computations computers or FPUs typically represent real numbers in a binary floating point format In this format a real number has three 7 3 FLOATING POINT UNIT intel parts a sign a significand and an exponent Figure 7 2 shows the binary floating point format that the Intel Architecture FPU uses This format conforms to the IEEE standard The sign is a binary value that indicates whether the number is positive 0 or negative 1 The significand has two parts a 1 bit binary integer also referred to as the J bit and a binary fraction The J bit is often not represented but instead
289. ointer from the interrupt gate or trap gate into the CS and EIP registers respectively 4 If the call is through an interrupt gate clears the IF flag in EFLAGS register 5 Begins execution of the handler procedure at the new privilege level Stack Usage with No Privilege Level Change Interrupted Procedure s and Handler s Stack lt ESP Before EFLAGS Transfer to Handler CS EIP Error Code lt ESP After Transfer to Handler Stack Usage with Privilege Level Change Interrupted Procedure s Handler s Stack Stack lt ESP Before Transfer to Handler SS ESP EFLAGS CS EIP ESP After Error Code Transfer to Handler Figure 4 5 Stack Usage on Transfers to Interrupt and Exception Handling Routines If a stack switch does occur the processor does the following Temporarily saves internally the current contents of the SS ESP EFLAGS CS and EIP registers 4 13 PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS intel 2 Loads the segment selector and stack pointer for the new stack that is the stack for the privilege level being called from the TSS into the SS and ESP registers and switches to the new stack 3 Pushes the temporarily saved SS ESP EFLAGS CS and EIP values for the interrupted procedure s stack onto the new stack 4 Pushes an error code on the new stack if appr
290. oller APIC Chapter 8 Processor Management and Initialization Defines the state of an Intel Archi tecture processor and its floating point unit after reset initialization This chapter also explains 1 3 ABOUT THIS MANUAL intel how to set up an Intel Architecture processor for real address mode operation and protected mode operation and how to switch between modes Chapter 9 Memory Cache Control Describes the general concept of caching and the caching mechanisms supported by the Intel Architecture This chapter also describes the memory type range registers MTRRs and how they can be used to map memory types of phys ical memory MTRRs were introduced into the Intel Architecture with the Pentium Pro processor Chapter 10 MMX Technology System Programming Model Describes those aspects of the Intel MMX technology that must be handled and considered at the system programming level including task switching exception handling and compatibility with existing system envi ronments Chapter 11 System Management Mode SMM Describes the Intel Architecture s system management mode SMM which can be used to implement power management functions Chapter 12 Machine Check Architecture Describes the machine check architecture which was introduced into the Intel Architecture with the Pentium processor Chapter 13 Code Optimization Discusses general optimization techniques for program ming an Intel Architec
291. ommended method of enforcing program ordering of memory mapped I O accesses with the Pentium Pro processor is to use the MTRRs to make the memory mapped I O address space uncacheable for the Pentium and Intel486 processors either the KEN pin or the PCD flags can be used for this purpose see Section 9 3 1 Memory Mapped I O When the target of a read or write is in an uncacheable region of memory memory reordering does not occur externally at the processor s pins that is reads and writes appear in order Designating a memory mapped I O region of the address space as uncacheable insures that reads and writes of I O devices are carried out in program order See Chapter 9 Memory Cache Control in the Intel Architecture Software Developer s Manual Volume 3 for more information on using MTRRs 9 6 intel INPUT OUTPUT Another method of enforcing program order is to insert one of the serializing instructions such as the CPUID instruction between operations See Chapter 7 Multiple Processor Management in the Intel Architecture Software Developer s Manual Volume 3 for more information on seri alization of instructions It should be noted that the chip set being used to support the processor bus controller memory controller and or I O controller may post writes to uncacheable memory which can lead to out of order execution of memory accesses In situations where out of order processing of memory accesses by the chip set can p
292. ompatibility FPU Exception Handling I m D 6 Behavior of Signals During FPU Exception Handling D 7 Timing of Receipt of External D 8 Arithmetic Example Using Infinily D 12 General Program Flow for DNA Exception Handler D 25 Program Flow for a Numeric Exception Dispatch Routine D 25 intel Table 2 1 Table 3 1 Table 4 1 Table 5 1 Table 6 1 Table 6 2 Table 6 3 Table 6 4 Table 6 5 Table 7 1 Table 7 2 Table 7 3 Table 7 4 Table 7 5 Table 7 6 Table 7 7 Table 7 8 Table 7 9 Table 7 10 Table 7 11 Table 7 12 Table 7 13 Table 7 14 Table 7 15 Table 7 16 Table 7 17 Table 7 18 Table 7 19 Table 7 20 Table 7 21 Table 7 22 Table 8 1 Table 8 2 Table 8 3 Table 9 1 Table A 1 Table B 1 Table C 1 TABLE OF TABLES PAGE Processor Performance Over Time and Other Key Features of the Intel Architecture ee leu else Rr e yup EYE 2 4 Effective Operand and Address Size Attributes 3 15 Exceptions and Interrupts 4 12 Default Segment Selection Rules 5 7 Move Instruction Operations 6 17 Conditional Move Instructions 6 18 Bit Test and Modify Instructions
293. on Masks Precision Underflow Overflow Zero Divide Denormalized Operand Invalid Operation Reserved Figure 7 10 FPU Control Word 7 3 4 1 EXCEPTION FLAG MASKS The exception flag mask bits bits 0 through 5 of the FPU control word mask the 6 exception flags in the FPU status word also bits 0 through 5 When one of these mask bits is set its corre sponding floating point exception is blocked from being generated 7 3 4 2 PRECISION CONTROL FIELD The precision control PC field bits 8 and 9 of the FPU control word determines the precision 64 53 or 24 bits of floating point calculations made by the FPU see Table 7 4 The default precision is extended precision which uses the full 64 bit significand available with the extended real format of the FPU data registers This setting is best suited for most applications because it allows applications to take full advantage of the precision of the extended real format Table 7 4 Precision Control Field PC Precision PC Field Single Precision 24 Bits 00B Reserved 01B Double Precision 53 Bits 10B Extended Precision 64 Bits 11B NOTE Includes the implied integer bit FLOATING POINT UNIT intel The double precision and single precision settings reduce the size of the significand to 53 bits and 24 bits respectively These settings are provided to support the IEEE
294. on event may be generated A soft exception may occur In this case floating point code continues to execute but generates incorrect results This happens when the floating point exceptions are masked and no visible exceptions occur The internal exception handler microcode not user visible loads a NaN Not a Number with an exponent of 11 11B onto the floating point stack The NaN is used for further calculations yielding incorrect results A potential error may occur only if the operating system does NOT manage floating point context across task switches These operating systems are usually cooperative operating systems It is imperative that the EMMS instruction execute at the end of all the MMXTM routines that may enable a task switch immediately after they end execution explicit yield API or implicit yield API 8 5 3 Interfacing with MMX Code The MMX technology enables direct access to all the MMX registers This means that all existing interface conventions that apply to the use of the processor s general purpose registers EAX EBX etc also apply to use of MMX register An efficient interface to MMX routines might pass parameters and return values through the MMX registers or through a combination of memory locations via the stack and MMX regis ters Such an interface would have to be written in assembly language since passing parameters through MMX registers is not currently supported by any existing C compilers Do
295. on pushes the address in the EIP register onto the current stack This address is then called the return instruction pointer and it points to the instruction where execution of the calling procedure should resume following a return from the called procedure Upon returning from a called procedure the RET instruction pops the return instruction pointer from the stack back into the EIP register Execution of the calling procedure then resumes The processor does not keep track of the location of the return instruction pointer It is thus up to the programmer to insure that stack pointer is pointing to the return instruction pointer on the stack prior to issuing a RET instruction A common way to reset the stack pointer to the point to the return instruction pointer is to move the contents of the EBP register into the ESP register If the EBP register is loaded with the stack pointer immediately following a procedure call it should point to the return instruction pointer on the stack The processor does not require that the return instruction pointer point back to the calling proce dure Prior to executing the RET instruction the return instruction pointer can be manipulated in software to point to any address in the current code segment near return or another code segment far return Performing such an operation however should be undertaken very cautiously using only well defined code entry points 4 3 CALLING PROCEDURES USING CALL AND RET
296. on takes precedence in order to support handling the numeric exception in its own context D 24 Intel GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS DNA Handler Entry Y other handler set up code Current Thread same as gt FPU Owner Yes FPU Owner Kernel Y FNSAVE to Old Thread s si FP Save Area may cause numeric exception handler final clean up Y Y FRSTOR from Current Thread s CLTS clears CRO TS FP Save Area Y Y lt other handler code gt Exit DNA Handler Y FPU Owner Current Thread Y Figure D 5 General Program Flow for DNA Exception Handler Numeric Exception Entry Is Kernel FPU Owner Yes No Normal Dispatch to Numeric Exception Handler Exit Figure D 6 Program Flow for a Numeric Exception Dispatch Routine D 25 GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS intel When the FNSAVE starts it will trigger an interrupt via FERR because of the pending numeric exception After some system dependent delay the numeric exception handler is entered It may be entered before the FNSAVE starts to execute or it may be entered shortly after execution of the FNSAVE Since the FPU Owner is the kernel the numeric exception handler simply exits discarding the exception The DNA handler resumes execution completing the FNSAVE o
297. onality and power of accurate numeric computation into the hands of the general user The IEEE 754 standard specifically addresses this issue recognizing the funda mental importance of making numeric computations both easy and safe to use For example some processors can overflow when two single precision floating point numbers are multiplied together and then divided by a third even if the final result is a perfectly valid 32 bit number The Intel Architecture FPUs deliver the correctly rounded result Other typical examples of undesirable machine behavior in straightforward calculations occur when computing financial rate of return which involves the expression 1 i or when solving for roots of a quadratic equation 7 1 FLOATING POINT UNIT intel bib Ad 2a If a does not equal 0 the formula is numerically unstable when the roots are nearly coincident or when their magnitudes are wildly different The formula is also vulnerable to spurious over underflows when the coefficients a b and c are all very big or all very tiny When single precision 4 byte floating point coefficients are given as data and the formula is evaluated in the FPU s normal way keeping all intermediate results in its stack the FPU produces impeccable single precision roots This happens because by default and with no effort on the programmer s part the FPU evaluates all those sub expressions with so much extra precision and range as to overwhelm a
298. onization one should assume the handler will be invoked at the end of the interval Thus the program should not change any value that might be needed by the handler such as COUNT in the above example until after the next FPU instruction following an FPU instruction that could cause an error If the program needs to modify such a value before the next FPU instruction or if the next FPU instruction could also cause an error then a WAIT instruction should be inserted before the value is modified This will force the handling of any exception before the value is modified A WAIT instruction should also be placed after the last floating point instruction in an application so that any unmasked exceptions will be serviced before the task completes D 3 4 FPU Exception Handling Examples There are many approaches to writing exception handlers One useful technique is to consider the exception handler procedure as consisting of prologue body and epilogue sections of code In the transfer of control to the exception handler due to an INTR NMI or SMI external inter rupts have been disabled by hardware The prologue performs all functions that must be protected from possible interruption by higher priority sources Typically this involves saving registers and transferring diagnostic information from the FPU to memory When the critical processing has been completed the prologue may re enable interrupts to allow higher priority inter
299. opriate Loads the segment selector for the new code segment and the new instruction pointer from the interrupt gate or trap gate into the CS and EIP registers respectively 6 Ifthe call is through an interrupt gate clears the IF flag in the EFLAGS register 7 Begins execution of the handler procedure at the new privilege level A return from an interrupt or exception handler is initiated with the IRET instruction The IRET instruction is similar to the far RET instruction except that it also restores the contents of the EFLAGS register for the interrupted procedure When executing a return from an interrupt or exception handler from the same privilege level as the interrupted procedure the processor performs these actions 1 Restores the CS and EIP registers to their values prior to the interrupt or exception 2 Restores the EFLAGS register 3 Increments the stack pointer appropriately 4 Resumes execution of the interrupted procedure When executing a return from an interrupt or exception handler from a different privilege level than the interrupted procedure the processor performs these actions Performs a privilege check 2 Restores the CS and EIP registers to their values prior to the interrupt or exception 3 Restores the EFLAGS register 4 Restores the SS and ESP registers to their values prior to the interrupt or exception resulting in a stack switch back to the stack of the interrupted procedure 5 Resume
300. or conditions Load FPU environment Save FPU state after checking error conditions Save FPU state without checking error conditions Restore FPU state Store FPU status word after checking error conditions Store FPU status word without checking error conditions Wait for FPU FPU no operation Intel INSTRUCTION SET SUMMARY 6 2 1 System Instructions The following system instructions are used to control those functions of the processor that are provided to support for operating systems and executives LGDT SGDT LLDT SLDT LTR STR LIDT SIDT MOV LMSW SMSW CLTS ARPL LAR LSL VERR VERW MOV INVD WBINVD INVLPG LOCK prefix HLT RSM RDMSR WRMSR RDPMC RDTSC Load global descriptor table GDT register Store global descriptor table GDT register Load local descriptor table LDT register Store local descriptor table LDT register Load task register Store task register Load interrupt descriptor table IDT register Store interrupt descriptor table IDT register Load and store control registers Load machine status word Store machine status word Clear the task switched flag Adjust requested privilege level Load access rights Load segment limit Verify segment for reading Verify segment for writing Load and store debug registers Invalidate cache no writeback Invalidate cache with writeback Invalidate TLB Entry Lock Bus Halt processor Return from system management mode SSM Read model spec
301. or operations that produce NaN results 7 19 FLOATING POINT UNIT intel 7 3 5 Infinity Control Flag The infinity control flag bit 12 of the FPU control word is provided for compatibility with the Intel 287 Math Coprocessor it is not meaningful for the Pentium Pro processor FPU or for the Pentium processor FPU the Intel486 processor FPU or Intel 387 processor NPX See Section 7 2 3 3 Signed Infinities for information on how the Intel Architecture FPUs handle infinity values 7 3 6 FPU Tag Word The 16 bit tag word see in Figure 7 11 indicates the contents of each the 8 registers in the FPU data register stack one 2 bit tag per register The tag codes indicate whether a register contains a valid number zero or a special floating point number NaN infinity denormal or unsup ported format or whether it is empty The FPU tag word is cached in the FPU in the FPU tag word register When the FPU is initialized with either an FINIT FNINIT or FSAVE FNSAVE instruction the FPU tag word is set to FFFFH which marks all the FPU data registers as empty 15 0 TAG 7 TAG 6 TAG 5 TAG 4 TAG 3 TAG 2 TAG 1 TAG 0 TAG Values 00 Valid 01 Zero 10 Special invalid NaN unsupported infinity or denormal 11 Empty Figure 7 11 FPU Tag Word Each tag in the FPU tag word corresponds to a physical register numbers 0 through 7 The current top of sta
302. or to c according to the rounding mode selected in the RC field Rounding introduces an error in a result that is less than one unit in the last place to which the result is rounded The rounded result is called the inexact result When the FPU produces an inexact result the floating point precision inexact flag PE is set in the FPU status word When the overflow exception is masked and the infinitely precise result is between the largest positive finite value allowed in a particular format and FPU rounds the result as shown in Table 7 6 Table 7 6 Rounding of Positive Numbers With Masked Overflow Rounding Mode Result Rounding to nearest even Rounding toward zero Truncate Maximum positive finite value Rounding up toward Rounding down toward ee Maximum positive finite value When the overflow exception is masked and the infinitely precise result is between the largest negative finite value allowed in a particular format and oo FPU rounds the result as shown in Table 7 7 Table 7 7 Rounding of Negative Numbers With Masked Overflow Rounding Mode Result Rounding to nearest even Rounding toward zero Truncate Maximum negative finite value Rounding up toward Maximum negative finite value Rounding down toward ee The rounding modes have no effect on comparison operations operations that produce exact results
303. orst case error on transcendental func tions is less than 1 ulp when rounding to the nearest even and less than 1 5 ulps when rounding in other modes The functions are guaranteed to be monotonic with respect to the input oper ands throughout the domain supported by the instruction With the Intel486 processor and Intel 387 math coprocessor the worst case transcendental function error is typically 3 or 3 5 ulps but is sometimes as large as 4 5 ulps 7 5 11 FPU Control Instructions The following instructions control the state and modes of operation of the FPU They also allow the status of the FPU to be examined FINIT FNINIT Initialize FPU FLDCW Load FPU control word FSTCW FNSTCW Store FPU control word FSTSW FNSTSW Store FPU status word FCLEX FNCLEX Clear FPU exception flags FLDENV Load FPU environment FSTENV FNSTENV Store FPU environment FRSTOR Restore FPU state FSAVE FNSAVE Save FPU state FINCSTP Increment FPU register stack pointer FDECSTP Decrement FPU register stack pointer FFREE Free FPU register FNOP No operation WAIT FWAIT Check for and handle pending unmasked FPU exceptions 7 39 FLOATING POINT UNIT intel The FINIT FNINIT instructions initialize the FPU and its internal registers to default values The FLDCW instructions loads the FPU control word register with a value from memory The FSTCW FNSTCW and FSTSW FNSTSW instructions store the FPU control and status words respectively in memory or for an FS
304. otentially cause faulty memory mapped I O processing code must be written to force synchronization and ordering of I O operations Serializing instructions can often be used for this purpose When the I O address space is used instead of memory mapped I O the situation is different in two respects The processor never buffers I O writes Therefore strict ordering of I O operations is enforced by the processor As with memory mapped I O it is possible for a chip set to post writes in certain I O ranges The processor synchronizes I O instruction execution with external bus activity see Table 9 1 Table 9 1 1 0 Instruction Serialization Processor Delays Execution of Until Completion of Next Instruction Pending Stores Current Store IN Yes Yes INS Yes Yes REP INS Yes Yes OUT Yes Yes Yes OUTS Yes Yes Yes REP OUTS Yes Yes Yes 9 7 Intel 10 Processor Identification and Feature Determination Intel CHAPTER 10 PROCESSOR IDENTIFICATION AND FEATURE DETERMINATION When writing software intended to run on several different types of Intel Architecture proces sors it is generally necessary to identify the type of processor present in a system and the processor features that are available to an application This chapter describes how to identify the processor that is executing the code and determine the features the processor supports It also shows how to
305. oubleword is transferred between the selected I O port and memory See the individual references for the IN INS OUT and OUTS instructions in Chapter 3 Instruction Set Reference of the Intel Architecture Software Developer s Manual Volume 2 for more information on these instructions 9 5 PROTECTED MODE I O When the processor is running in protected mode the following protection mechanisms regulate access to I O ports When accessing I O ports through the I O address space two protection devices control access The I O privilege level IOPL field in the EFLAGS register The I O permission bit map of a task state segment TSS When accessing memory mapped I O ports the normal segmentation and paging protection and the MTRRs in processors that support them also affect access to I O ports See Chapter 4 Protection and Chapter 9 Memory Cache Control in the Intel Architecture Software Developer s Manual Volume 3 for a complete discussion of memory protection The following sections describe the protection mechanisms available when accessing I O ports in the I O address space with the I O instructions 9 5 1 IO Privilege Level In systems where I O protection is used the IOPL field in the EFLAGS register controls access to the I O address space by restricting use of selected instructions This protection mechanism permits the operating system or executive to set the privilege level needed to perform I O In a
306. ove the ends of the real number line This space includes any value with the maximum allowable biased exponent and a non zero fraction The sign bit is ignored for NaNs The IEEE standard defines two classes of NaN quiet NaNs QNaNs and signaling NaNs SNaNs A QNaN is a NaN with the most significant fraction bit set an SNaN is a NaN with the most significant fraction bit clear QNaNs are allowed to propagate through most arithmetic operations without signaling an exception SNaNs generally signal an invalid operation excep tion whenever they appear as operands in arithmetic operations Exceptions are discussed in Section 7 7 Floating Point Exception Handling See Section 7 6 Operating on NaNs for detailed information on how the FPU handles NaNs 7 2 4 Indefinite For each FPU data type one unique encoding is reserved for representing the special value indefinite For example when operating on real values the real indefinite value is a QNaN see Section 7 4 1 Real Numbers The FPU produces indefinite values as responses to masked floating point exceptions 7 3 FPU ARCHITECTURE From an abstract architectural view the FPU is a coprocessor that operates in parallel with the processor s integer unit see Figure 7 4 The FPU gets its instructions from the same instruction decoder and sequencer as the integer unit and shares the system bus with the integer unit Other than these connections the integer unit and FP
307. pages and mapping the pages into virtual memory If an operating system executive uses the Intel Architecture s paging mechanism the existence of the pages is transparent to an application program The real address mode model uses the memory model for the Intel 8086 processor the first Intel Architecture processor It was provided in all the subsequent Intel Architecture processors for compatibility with existing programs written to run on the Intel 8086 processor The real address mode uses a specific implementation of segmented memory in which the linear address space for the program and the operating system executive consists of an array of segments of up to 64K bytes in size each The maximum size of the linear address space in real address mode is 220 bytes See Chapter 15 8086 Emulation in the Intel Architecture Software Developer s Manual Volume 3 for more information on this memory model 3 4 4 MODES OF OPERATION When writing code for the Pentium Pro processor a programmer needs to know the operating mode the processor is going to be in when executing the code and the memory model being used The relationship between operating modes and memory models is as follows Protected mode When in protected mode the processor can use any of the memory models described in this section The real addressing mode memory model is ordinarily used only when the processor is in the virtual 8086 mode The memory model used depends on the design
308. perands 5 5 index a eto E eiie as a ayata 5 8 index times scale plus displacement 5 9 memory operands 5 6 register operands 5 5 scalo factor seit sr epe lane es 5 8 specifying a segment selector 5 6 specifying an offset 5 7 Addressing segments 1 7 Advanced programmable interrupt controller see APIC AF adjust flag EFLAGS register 3 11 AH tegister iic uakis rqa sea pia ed 3 7 Alignment of words doublewords quadwords 5 1 AND instruction 6 25 APIC presence of 10 1 Arctangent FPU operation 7 37 Arithmetic instructions FPU 7 43 Assembler addressing modes 5 9 AX register _ 3 7 INDEX B B default size flag segment descriptor 3 14 4 3 Base operand addressing 5 8 5 9 Basic execution environment 3 2 B bit FPU status 7 15 BD vieille ere ee e NUR edi 5 4 BCD integers 5 4 FPU 7 28 7 29 Packed remate 5 4 6 24 relationship to status flags 3 12 unpacked 5 4 6 24 BH register iis oppo da deg 3 7 Bias value numeric overfloW 7 51 numeric underfloW 7 52 B
309. pin by the no wait floating point instruction via the INTR pin during this window then the interrupt is serviced first before resuming the execution of the no wait floating point instruction Case 2 If the system responds via the INTR pin after the window has closed then the inter rupt is recognized only at the next instruction boundary D 8 Intel GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS There are two other ways in addition to Case 1 above in which a no wait floating point instruc tion can service a numeric exception inside its interrupt window First the first floating point error condition could be of the immediate category as defined in Section D 2 1 1 Basic Rules When FERR Is Generated that asserts FERR immediately If the system delay before asserting INTR is long enough relative to the time elapsed before the no wait floating point in struction INTR can be asserted inside the interrupt window for the latter Second consider two no wait FPU instructions in close sequence and assume that a previous FPU instruction has caused an unmasked numeric exception Then if the INTR timing is too long for an FERR sig nal triggered by the first no wait instruction to hit the first instruction s interrupt window it could catch the interrupt window of the second The possible malfunction of a no wait FPU instruction explained above cannot happen if the in struction is being used in the manner for which Intel originally
310. pose registers on the stack in the following order EAX ECX EDX EBX ESP the value prior to executing the PUSHA instruction EBP ESI and EDI The POPA instruction pops all the register values saved with a PUSHA instruction except the ESI value from the stack to their respective registers If a called procedure changes the state of any of the segment registers explicitly it should restore them to their former value before executing a return to the calling procedure If a calling procedure needs to maintain the state of the EFLAGS register it can save and restore all or part of the register using the PUSHF PUSHFD and POPF POPFD instructions The PUSHF instruction pushes the lower word of the EFLAGS register on the stack while the PUSHFD instruction pushes the entire register The POPF instruction pops a word from the stack into the lower word of the EFLAGS register while the POPFD instruction pops a double word from the stack into the register 4 3 5 Calls to Other Privilege Levels The Intel Architecture s protection mechanism recognizes four privilege levels numbered from 0 to 3 where greater numbers mean lesser privileges The primary reason to use these privilege levels is to improve the reliability of operating systems For example Figure 4 3 shows how privilege levels can be interpreted as rings of protection In this example the highest privilege level 0 at the center of the diagram is used for segments that contain the most
311. present For future feature identification flags a value of 1 may indicate that the specific feature is not present e Test feature identification flags individually and do not make assumptions about undefined bits Note that the CPUID instruction will cause the invalid opcode exception UD if executed on a processor that does not support it The CPUID instruction application note provides a code sequence to test the validity of the CPUID instruction Also this test code for CPUID valid is not reliable when executed in virtual 8086 mode To avoid this if the test code is written to run in real address mode the SMSW instruction must be used to read the PE bit from the MSW lower half of CRO If PE flag is set to 1 the Real Mode code is actually being executed in virtual 8086 mode and the test sequence cannot be guaranteed to return reliable information Note that the new version of the CPUID application note AP 485 Intel Processor Identifica tion and the CPUID Instruction Order Number 241618 005 explains this virtual 8086 problem but the older versions of the application note do not 10 2 Intel PROCESSOR IDENTIFICATION AND FEATURE DETERMINATION 10 2 IDENTIFICATION OF EARLIER INTEL ARCHITECTURE PROCESSORS The CPUID instruction is only available in the Pentium Pro Pentium and recent Intel486 processors For the earlier Intel Architecture processors including the earlier Intel486 proces Sors several other architectura
312. r Interface with the Memory 2 6 Functional Block Diagram of the Pentium Pro Processor Microarchitecture 2 9 Pentium Pro Processor Basic Execution 3 2 Three Memory Management Models 3 3 Application Programming Registers 3 6 Alternate General Purpose Register Names 3 7 Use of Segment Registers for Flat Memory Model 3 8 Use of Segment Registers in Segmented Memory Model 3 9 EFLAGS Register ou paki a Sarit Somes a ae ant 3 11 Stack Structures sk co ee eae ee ee RES URS eater RES 4 2 Stack on Near and Far 4 5 Protection RiNS iis iir tie nee pee See eee Rb xa 4 8 Stack Switch on a Call to a Different Privilege Level 4 9 Stack Usage on Transfers to Interrupt and Exception Handling Routines 4 13 Nested 4 18 Stack Frame after Entering the MAIN Procedure 4 19 Stack Frame after Entering Procedure 4 19 Stack Frame after Entering 4 20 Stack Frame after Entering Procedure 4 21 Fundamental Data
313. r address space between tasks The problem and solution discussed in the fol lowing sections apply to these operating systems also D 3 6 2 TRACKING FPU OWNERSHIP Since the contents of the FPU may not belong to the currently executing thread the thread iden tifier for the last FPU user needs to be tracked separately This is not complicated the kernel should simply provide a variable to store the thread identifier of the FPU owner separate from the variable that stores the identifier for the currently executing thread This variable is updated in the DNA exception handler and is used by the DNA exception handler to find the FPU save areas of the old and new threads A simplified flow for a DNA exception handler is then 1 Use the FPU Owner variable to find the FPU save area of the last thread to use the FPU 1 In a software task switch the operating system uses a sequence of instructions to save the suspending thread s state and restore the resuming thread s state instead of the single long non interruptible task Switch operation provided by the Intel Architecture 2 Although CRO bit 2 the emulation flag EM also causes a DNA exception do not use the EM bit as a surrogate for TS EM means that no floating point unit is available and that floating point instructions must be emulated Using EM to trap on task switches is not compatible with the Intel Architecture s MMX technology If the EM flag is set MMX instructions rais
314. r all Intel Architecture instructions and gives the allowable encodings of prefixes the operand iden tifier byte ModR M byte the addressing mode specifier byte SIB byte and the displacement and immediate bytes intel ABOUT THIS MANUAL Chapter 3 Instruction Set Reference Describes each of the Intel Architecture instructions in detail including an algorithmic description of operations the effect on flags the effect of operand and address size attributes and the exceptions that may be generated The instructions are arranged in alphabetical order The FPU and MMX instructions are included in this chapter Appendix A Opcode Map Gives an opcode map for the Intel Architecture instruction set Appendix B Instruction Formats and Encodings Gives the binary encoding of each form of each Intel Architecture instruction 1 3 OVERVIEW OF THE NTEL ARCHITECTURE SOFTWARE DEVELOPER S MANUAL VOLUME 3 SYSTEM PROGRAMMING GUIDE The contents of the ntel Architecture Software Developer s Manual Volume 3 are as follows Chapter 1 About This Manual Gives an overview of all three volumes of the Intel Archi tecture Software Developer s Manual It also describes the notational conventions in these manuals and lists related Intel manuals and documentation of interest to programmers and hard ware designers Chapter 2 System Architecture Overview Describes the modes of operation of an Intel Architecture processor and the me
315. r no wait instructions which will be executed as described in Section D 4 1 Origin With the Intel 286 and Intel 287 and Intel386 and Intel 387 Processors An unmasked numerical exception causes the FERR output to be activated even with NE 1 and at exactly the same point in the program flow as it would have been asserted if NE were zero However the system would not connect FERR to a PIC to generate INTR when operating in the native internal mode If the hardware of a system has FERR connected to trigger IRQ13 in order to support MS DOS but an O S using the native mode is actually running the system it is the O S s responsibility to make sure that IRQ13 is not enabled in the slave PIC With this configuration a system is immune to the problem discussed in Section D 2 1 3 No Wait FPU Instructions Can Get FPU Interrupt in Window where for Intel486 and Pentium processors a no wait FPU instruction can get an FPU exception D 27 GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS intel D 4 3 Considerations When FPU Shared Between Tasks Using Native Mode The protocols recommended in Section D 3 6 Considerations When FPU Shared Between Tasks for MS DOS compatibility FPU exception handlers that are shared between tasks may be used without change with the native mode However the protocols for a handler written specifically for native mode can be simplified because the problem of a spurious floating point exception interrupt
316. rand is used with the XCHG instruction the processor s LOCK signal is automatically asserted This instruction is thus useful for implementing semaphores or similar data structures for process synchronization See Bus Locking in Chapter 7 of the Intel Architecture Software Developer s Manual Volume 3 for more information on bus locking The BSWAP byte swap instruction reverses the byte order in a 32 bit register operand Bit posi tions 0 through 7 are exchanged with 24 through 31 and bit positions 8 through 15 are exchanged with 16 through 23 Executing this instruction twice in a row leaves the register with the same INSTRUCTION SET SUMMARY intel e value as before The BSWAP instruction is useful for converting between big endian and little endian data formats This instruction also speeds execution of decimal arithmetic The XCHG instruction can be used two swap the bytes in a word Table 6 2 Conditional Move Instructions Instruction Mnemonic Status Flag States Condition Description Unsigned Conditional Moves CMOVA CMOVNBE CF or ZF 0 Above not below or equal CMOVAE CMOVNB CF 0 Above or equal not below CMOVNC CF 0 Not carry CMOVB CMOVNAE CF 1 Below not above or equal CMOVC CF 1 Carry CMOVBE CMOVNA CF or ZF 1 Below or equal not above CMOVE CMOVZ ZF 1 Equal zero CMOVNE CMOVNZ ZF 0 Not equal not zero CMOVP CMOVPE PF 1 Parity parity even CMOVNP CMOVPO PF 0 Not parity parity odd Signed Conditiona
317. re information about the SF flag 7 47 FLOATING POINT UNIT intel 7 8 1 1 STACK OVERFLOW OR UNDERFLOW EXCEPTION IS The FPU tag word keeps track of the contents of the registers in the FPU register stack see Section 7 3 6 FPU Tag Word It then uses this information to detect two different types of stack faults Stack overflow an instruction attempts to write a value into a non empty FPU register Stack underflow an instruction attempts to read a value from an empty FPU register When the FPU detects stack overflow or underflow it sets the IE flag bit 0 and the SF flag bit 6 in the FPU status word to 1 It then sets condition code flag C1 bit 9 in the FPU status word to 1 if stack overflow occurred or to 0 if stack underflow occurred If the invalid operation exception is masked the FPU then returns the real integer or BCD integer indefinite value to the destination operand depending on the instruction being executed This value overwrites the destination register or memory location specified by the instruction If the invalid operation exception is not masked a software exception handler is invoked see Section 7 7 3 Software Exception Handling and the top of stack pointer TOP and source operands remain unchanged The term stack overflow comes from the condition where the a program has pushed eight values onto the FPU register stack and the next value pushed on the stack causes a stack wraparound
318. re of the status flags as condition codes and test them for branch set byte or end loop conditions 3 6 3 2 DF FLAG The direction flag DF located in bit 10 of the EFLAGS register controls the string instructions MOVS CMPS SCAS LODS and STOS Setting the DF flag causes the string instructions to auto decrement that is to process strings from high addresses to low addresses Clearing the DF flag causes the string instructions to auto increment process strings from low addresses to high addresses The STD and CLD instructions set and clear the DF flag respectively intel BASIC EXECUTION ENVIRONMENT 3 6 4 System Flags and IOPL Field The system flags and IOPL field in the EFLAGS register control operating system or executive operations They should not be modified by application programs The functions of the status flags are as follows IF bit 9 TF bit 8 IOPL bits 12 and 13 NT bit 14 RF bit 16 VM bit 17 AC bit 18 VIF bit 19 VIP bit 20 ID bit 21 Interrupt enable flag Controls the response of the processor to maskable interrupt requests Set to respond to maskable interrupts cleared to inhibit maskable interrupts Trap flag Set to enable single step mode for debugging clear to disable single step mode I O privilege level field Indicates the I O privilege level of the currently running program or task The current privilege level CPL of the currently running progra
319. real types as required by IEEE Std 854 With the exception of the 80 bit extended real format all of these data types exist in memory only When they are loaded into FPU data registers they are converted into extended real format and operated on in that format Single Real Sin 8130 23 22 Implied Integer 0 Double Real Sign 9 Exponent e 6362 52 51 Implied Integer 0 Sign Y Extended Real Exponent Fraction o 7978 6463 62 Integer Word Integer Sign 15 14 Short Integer o Sign x 3130 Long Integer o Sign Sign 63 62 o Packed BCD Integers X D17 016 015 D14 D13 D12 011 D10 D9 07 D D5 04 03 D2 Dt DO 7978 7271 4 Bits 1 BCD Digit o Figure 7 17 Floating Point Unit Data Type Formats 7 24 FLOATING POINT UNIT intel When stored in memory the least significant byte of an FPU data type value is stored at the initial address specified for the value Successive bytes from the value are then stored in succes sively higher addresses in memory The floating point instructions load and store memory oper ands using only the initial address of the operand 7 4 1 The FPU s three real data types single real double real and extended real correspond directly to the single precision double precision and double extended precision formats in the IEEE standard The extended precision format is the format used by the data registers in t
320. red Between Tasks discusses an important example of this type of problem and gives a solution D 2 2 MS DOS Compatibility Mode in the Pentium Pro Processor When bit NE 0 in CRO the MS DOS compatibility mode of the Pentium Pro processor provides FERR and IGNNE functionality that is almost identical to the Intel486 and Pentium proces sors The same external hardware described in Section D 2 1 2 Recommended External Hard ware to Support the MS DOS Compatibility Mode is recommended for the Pentium Pro processor as well as the two previous generations The only change to MS DOS compatibility FPU exception handling with the Pentium Pro processor is that all exceptions for all FPU instructions cause immediate error reporting That is FERR is asserted as soon as the FPU detects an unmasked exception there are no cases in which error reporting is deferred to the next FPU or WAIT instruction As is discussed in Section D 2 1 1 Basic Rules When FERR Is Generated most exception cases in the Intel486 and Pentium processors are of the deferred type D 9 GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS intel Although FERR is asserted immediately upon detection of an unmasked FPU error this certainly does not mean that the requested interrupt will always be serviced before the next instruction in the code sequence is executed To begin with the Pentium Pro processor executes several instructions simultaneously There also w
321. remove parameters from the stack that were pushed on the stack by the calling procedure See Section 4 3 Calling Procedures Using CALL and RET for more information on the mechanics of making procedure calls with the CALL and RET instructions 6 9 1 3 RETURN FROM INTERRUPT INSTRUCTION When the processor services an interrupt it performs an implicit call to an interrupt handling procedure The IRET return from interrupt instruction returns program control from an inter rupt handler to the interrupted procedure that is the procedure that was executing when the interrupt occurred The IRET instruction performs a similar operation to the RET instruction see Section 6 9 1 2 Call and Return Instructions except that it also restores the EFLAGS register from the stack The contents of the EFLAGS register are automatically stored on the stack along with the return instruction pointer when the processor services an interrupt 6 9 2 Conditional Transfer Instructions The conditional transfer instructions execute jumps or loops that transfer program control to another instruction in the instruction stream if specified conditions are met The conditions for control transfer are specified with a set of condition codes that define various states of the status flags CE ZF OF PF and SF in the EFLAGS register 6 31 INSTRUCTION SET SUMMARY intel e 6 9 2 1 CONDITIONAL JUMP INSTRUCTIONS The Jcc conditional jump instructions tr
322. ress spaces called segments When using this model code data and stacks are typically contained in separate segments To address a byte in a segment a program must issue a logical address which consists of a segment selector and an offset A logical address is often referred to as a far pointer The segment selector identifies the segment to be accessed and the offset identifies a byte in the address space of the segment The programs running on an Intel Archi tecture processor can address up to 16 383 segments of different sizes and types and each segment can be as large as 222 bytes 3 3 BASIC EXECUTION ENVIRONMENT intel e Internally all the segments that are defined for a system are mapped into the processor s linear address space So the processor translates each logical address into a linear address to access a memory location This translation is transparent to the application program The primary reason for using segmented memory is to increase the reliability of programs and systems For example placing a program s stack in a separate segment prevents the stack from growing into the code or data space and overwriting instructions or data respectively And placing the operating system s or executive s code data and stack in separate segments protects them from the application program and vice versa With either the flat or segmented model the Intel Architecture provides facilities for dividing the linear address space into
323. rform the exchange 7 8 2 Divide By Zero Exception 2 The FPU reports a floating point zero divide exception whenever an instruction attempts to divide a finite non zero operand by 0 The flag ZE for this exception is bit 2 of the FPU status word and the mask bit ZM is bit 2 of the FPU control word The FDIV FDIVP FDIVR FDIVRP FIDIV and FIDIVR instructions and the other instructions that perform division inter nally FYL2X and FXTRACT can report the divide by zero exception When a divide by zero exception occurs and the exception is masked the FPU sets the ZE flag and returns the values shown in Table 7 21 If the divide by zero exception is not masked the ZE flag is set a software exception handler is invoked see Section 7 7 3 Software Exception Handling and the top of stack pointer TOP and source operands remain unchanged 7 49 FLOATING POINT UNIT intel Table 7 21 Divide By Zero Conditions and the Masked Responses to Them Condition Masked Response Divide or reverse divide operation Returns an ee signed with the exclusive OR of the sign of the two with a 0 divisor operands to the destination operand FYL2X instruction Returns an ee signed with the opposite sign of the non zero operand to the destination operand FXTRACT instruction ST 1 is set to ST 0 is set to O with the same sign as the Source operand 7 8 83 Denormal Operand Exception 0 The FPU s
324. ries among instructions but can never be greater than the maximum value of an unsigned doubleword integer 232 5 3 2 Register Operands Source and destination operands can be located in any of the following registers depending on the instruction being executed The 32 bit general purpose registers EAX EBX ECX EDX ESI EDI ESP or EBP The 16 bit general purpose registers AX BX CX DX SI DI SP or BP 5 5 DATA TYPES AND ADDRESSING MODES intel e The 8 bit general purpose registers AH BH CH DH AL BL CL or DL The segment registers CS DS SS ES FS and GS The EFLAGS register e System registers such as the global descriptor table GDTR or the interrupt descriptor table register IDTR Some instructions such as the DIV and MUL instructions use quadword operands contained in a pair of 32 bit registers Register pairs are represented with a colon separating them For example in the register pair EDX EAX EDX contains the high order bits and EAX contains the low order bits of a quadword operand Several instructions such as the PUSHFD and POPFD instructions are provided to load and store the contents of the EFLAGS register or to set or clear individual flags in this register Other instructions such as the Jcc instructions use the state of the status flags in the EFLAGS register as condition codes for branching or other decision making operations The processor contains a selection o
325. rithmetic operation so that the operation will produce a valid BCD result These instructions operate only on both packed and unpacked BCD values 6 23 INSTRUCTION SET SUMMARY intel e 6 5 1 Packed BCD Adjustment Instructions The DAA decimal adjust after addition and DAS decimal adjust after subtraction instructions adjust the results of operations performed on packed BCD integers see Section 5 2 3 BCD Integers Adding two packed BCD values requires two instructions an ADD instruction followed by a DAA instruction The ADD instruction adds binary addition the two values and stores the result in the AL register The DAA instruction then adjusts the value in the AL register to obtain a valid 2 digit packed BCD value and sets the CF flag if a decimal carry occurred as the result of the addition Likewise subtracting one packed BCD value from another requires a SUB instruction followed by a DAS instruction The SUB instruction subtracts binary subtraction one BCD value from another and stores the result in the AL register The DAS instruction then adjusts the value in the AL register to obtain a valid 2 digit packed BCD value and sets the CF flag if a decimal borrow occurred as the result of the subtraction 6 5 2 Unpacked BCD Adjustment Instructions The AAA ASCII adjust after addition AAS ASCII adjust after subtraction AAM ASCII adjust after multiplication and AAD ASCII adjust before division instructions
326. rmation on serializing of I O operations 9 2 I O PORT HARDWARE From a hardware point of view I O addressing is handled through the processor s address lines For Pentium Pro processors a special memory I O transaction on the system bus indicates whether the address lines are being driven with a memory address or an I O address for Pentium and earlier Intel Architecture processors the M IO pin indicates a memory address 1 or an I O address 0 When the separate I O address space is selected it is the responsibility of the hard ware to decode the memory I O bus transaction to select I O ports rather than memory Data is transmitted between the processor and an I O device through the data lines 9 1 INPUT OUTPUT intel o 9 3 I O ADDRESS SPACE The processor s I O address space is separate and distinct from the physical memory address space The I O address space consists of 2 6 64K individually addressable 8 bit I O ports numbered 0 through FFFFH I O port addresses OF8H through OFFH are reserved Do not assign I O ports to these addresses The result of an attempt to address beyond the I O address space limit of FFFFH is implementation specific see the Developer s Manuals for specific processors for more details Any two consecutive 8 bit ports can be treated as a 16 bit port and any four consecutive ports can be a 32 bit port In this manner the processor can transfer 8 16 or 32 bits to or from a device in the I O addr
327. rom port Input string from port Input word string from port Input string from port Input doubleword string from port Output string to port Output byte string to port Output string to port Output word string to port Output string to port Output doubleword string to port 6 2 1 9 FLAG CONTROL INSTRUCTIONS STC CLC CMC CLD STD LAHF SAHF PUSHF PUSHFD POPF POPFD STI CLI 6 2 1 10 LDS LES LFS 6 8 Set carry flag Clear the carry flag Complement the carry flag Clear the direction flag Set direction flag Load flags into AH register Store AH register into flags Push EFLAGS onto stack Pop EFLAGS from stack Set interrupt flag Clear the interrupt flag SEGMENT REGISTER INSTRUCTIONS Load far pointer using DS Load far pointer using ES Load far pointer using FS intel e INSTRUCTION SET SUMMARY LGS Load far pointer using GS LSS Load far pointer using SS 6 2 1 11 MISCELLANEOUS INSTRUCTIONS LEA Load effective address NOP No operation UB2 Undefined instruction XLAT XLATB Table lookup translation CPUID Processor Identification 6 2 2 MMX Technology Instructions The MMX instructions execute on those Intel Architecture processors that implement the Intel MMX technology These instructions operate on packed byte packed word packed double word and quadword operands As with the integer instructions the following list of MMX instructions is divided into subgroups 6 2 2 1 MMX DATA TRANSFER INS
328. rrors can thus be diagnosed and corrected in one test run In embedded applications which use computed results in further computations an undetected QNaN can invalidate all subsequent results Such applications should therefore periodically check for QNaNs and provide a recovery mechanism to be used if a QNaN result is detected 7 7 FLOATING POINT EXCEPTION HANDLING The FPU detects six classes of exception conditions while executing floating point instructions Invalid operation 1 Stack overflow or underflow IS Invalid arithmetic operation IA Divide by zero Z Denormalized operand 0 Numeric overflow 0 Numeric underflow 0 nexact result precision HP 7 42 Intel FLOATING POINT UNIT The nomenclature of symbol followed by one or two letters for example 15 is used in this manual to indicate exception conditions It is merely a short hand form and is not related to assembler mnemonics Each of the six exception classes has a corresponding flag bit in the FPU status word and a mask bit in the FPU control word see Section 7 3 2 FPU Status Register and Section 7 3 4 FPU Control Word respectively In addition the exception summary ES flag in the status word indicates when any of the exceptions has been detected and the stack fault SF flag also in the status word distinguishes between the two types of invalid operation exceptions When the FPU detects a flo
329. rs fit into the real number continuum The encodings shown here are for the IEEE single precision 32 bit format where the term S indicates the sign bit E the biased exponent and F the fraction The exponent values are given in decimal FLOATING POINT UNIT The FPU can operate on and or return any of these values depending on the type of computation being performed The following sections describe these number and non number classes 7 2 3 1 SIGNED ZEROS Zero can be represented as a 0 or a 0 depending on the sign bit Both encodings are equal in value The sign of a zero result depends on the operation being performed and the rounding mode being used Signed zeros have been provided to aid in implementing interval arithmetic The sign of a zero may indicate the direction from which underflow occurred or it may indicate the sign of an that has been reciprocated 7 2 3 2 NORMALIZED AND DENORMALIZED FINITE NUMBERS Non zero finite numbers are divided into two classes normalized and denormalized The normalized finite numbers comprise all the non zero finite values that can be encoded in a normalized real number format between zero and oe In the single real format shown in Figure 7 3 this group of numbers includes all the numbers with biased exponents ranging from 1 to 254 unbiased the exponent range is from 126 to 127 0
330. rted to extended real format automatically Reverse versions of the subtract and divide instructions are provided to foster efficient coding For example the FSUB instruction subtracts the value in a specified FPU register ST 1 from the value in register ST 0 whereas the FSUBR instruction subtracts the value in ST 0 from the value in ST 1 The results of both operations are stored in register ST 0 These instructions eliminate the need to exchange values between register ST 0 and another FPU register to perform a subtraction or division The pop versions of the add subtract multiply and divide instructions pop the FPU register stack following the arithmetic operation The FPREM instruction computes the remainder from the division of two operands in the manner used by the Intel 8087 and Intel 287 math coprocessors the FPREM1 instructions computes the remainder is the manner specified in the IEEE specification The FSQRT instruction computes the square root of the source operand The FRNDINT instructions rounds a real value to its nearest integer value according to the current rounding mode specified in the RC field of the FPU control word This instruction performs a function similar to the FIST FISTP instructions except that the result is saved in a real format The FABS FCHS and FXTRACT instructions perform convenient arithmetic operations The FABS instruction produces the absolute value of the source operand The FCHS instruc
331. ruction is usually used in a loop where other instructions process each element of the string after they are loaded into the target register The STOS instruction stores the source string element from the EAX doubleword string AX word string or AL byte string register into the memory location identified with the EDI register The short forms for this instruction are STOSB store byte string STOSW store word string and STOSD store doubleword string This instruction is also normally used in a loop Here a string is commonly loaded into the register with a LODS instruction operated on by other instructions and then stored again in memory with a STOS instruction The I O instructions see Section 6 11 I O Instructions also perform operations on strings in memory 6 10 1 Repeating String Operations The string instructions described in Section 6 10 String Operations perform one iteration of a string operation To operate strings longer than a doubleword the string instructions can be combined with a repeat prefix REP to create a repeating instruction or be placed in a loop When used in string instructions the ESI and EDI registers are automatically incremented or decremented after each iteration of an instruction to point to the next element byte word or doubleword in the string String operations can thus begin at higher addresses and work toward lower ones or they can begin at lower addresses and work toward h
332. rupt handlers to preempt the exception handler The standard prologue not only saves the registers and transfers diagnostic information from the FPU to memory but also clears the floating point exception flags in the status word Alternatively when it is not necessary for the handler to be re entrant another technique may also be used In this technique the exception flags are not cleared in the prologue and the body of the handler must not contain any floating point instructions that cannot complete execution when there is a pending floating point excep Intel GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS tion The no wait instructions are discussed in Section 7 5 12 Waiting Vs Non waiting Instructions Note that the handler must still clear the exception flag s before executing the IRET If the exception handler uses neither of these techniques the system will be caught in an endless loop of nested floating point exceptions and hang The body of the exception handler examines the diagnostic information and makes a response that is necessarily application dependent This response may range from halting execution to displaying a message to attempting to repair the problem and proceed with normal execution The epilogue essentially reverses the actions of the prologue restoring the processor so that normal execution can be resumed The epilogue must not load an unmasked exception flag into the FPU or another exception will be r
333. s 1 6 Instruction pointer EIP register 3 14 Instruction pointer 7 21 Instruction pool reorder buffer 2 10 Instruction prefixes see Prefixes Instruction set binary arithmetic instructions 6 22 bit scan instructions 6 29 bit test and modify instructions 6 29 byte set on condition instructions 6 29 control transfer instructions 6 30 data movement instructions 6 16 decimal arithmetic instructions 6 23 EFLAGS instructi0nS 6 37 floating point instructions 6 9 6 11 integer instructions 6 3 I O instructions 6 36 liStS Of sees sian big Ete SER bed ane rex em 6 2 logical instructions 6 25 MMX instructions 8 4 intel processor identification instruction 6 40 repeating string operations 6 35 rotate instructions 6 27 segment register instructions 6 38 shift instructions 6 25 software interrupt instructions 6 34 string operation instructions 6 34 Sumtrnaly fe u ed IRSE EE 6 1 system instructions 6 15 testinstruction 6 30 type conversion instructions 6 21 INT instruction 4 15 6 39 Inteders 4e Lev 5 2 6
334. s some details on writing software exception handlers are provided in Chapter 5 Interrupt and Exception Handling in the Intel Architecture Software Developer s Manual Volume 3 as well as in Section D 3 4 FPU Exception Handling Examples in this appendix As discussed in Section D 2 1 2 Recommended External Hardware to Support the MS DOS Compatibility Mode some early FERR to INTR hardware interface implementations are less robust than the recommended circuit This is because they depended on the exception handler to clear the FPU exception interrupt request to the PIC by accessing port OFOH before the handler causes FERR to be de asserted by clearing the exception from the FPU itself To elim inate the chance of a problem with this early hardware Intel recommends that FPU exception handlers always access port OFOH before clearing the error condition from the FPU D 3 3 Synchronization Required for Use of FPU Exception Handlers Concurrency or synchronization management requires a check for exceptions before letting the processor change a value just used by the FPU It is important to remember that almost any numeric instruction can under the wrong circumstances produce a numeric exception D 3 3 1 EXCEPTION SYNCHRONIZATION WHAT WHY AND WHEN Exception synchronization means that the exception handler inspects and deals with the excep tion in the context in which it occurred If concurrent execution is allowed the state of
335. s 2 bytes 16 bits a doubleword is 4 bytes 32 bits and a quadword is 8 bytes 64 bits 7 0 Byte N 15 87 0 High Low Byte Byte Word N 1 N 31 1615 0 High Word Low Word Doubleword N 2 N 63 32 31 0 High Doubleword Low Doubleword Quadword N 4 N Figure 5 1 Fundamental Data Types Figure 5 2 shows the byte order of each of the fundamental data types when referenced as oper ands in memory The low byte bits 0 through 7 of each data type occupies the lowest address in memory and that address is also the address of the operand 5 1 1 Alignment of Words Doublewords and Quadwords Words doublewords and quadwords do not need to be aligned in memory on natural bound aries The natural boundaries for words double words and quadwords are even numbered addresses addresses evenly divisible by four and addresses evenly divisible by eight respec tively However to improve the performance of programs data structures especially stacks should be aligned on natural boundaries whenever possible The reason for this is that the processor requires two memory accesses to make an unaligned memory access whereas 5 1 DATA TYPES AND ADDRESSING MODES intel e aligned accesses require only one memory access A word or doubleword operand that crosses a 4 byte boundary or a quadword operand that crosses an 8 byte boundary is considered unaligned and requires two separate memory bus cycles
336. s Manual Volume 1 describes the basic architecture and programming environment of an Intel Architecture processor the Intel Architecture Soft ware Developer s Manual Volume 2 describes the instruction set of the processor and the opcode structure These two volumes are aimed at application programmers who are writing programs to run under existing operating systems or executives The Intel Architecture Software Developer s Manual Volume 3 describes the operating system support environment of an Intel Architecture processor including memory management protection task management interrupt and exception handling and system management mode It also provides Intel Architecture processor compatibility information This volume is aimed at operating system and BIOS designers and programmers 1 1 OVERVIEW OF THE INTEL ARCHITECTURE SOFTWARE DEVELOPER S MANUAL VOLUME 1 BASIC ARCHITECTURE The contents of this manual are as follows Chapter 1 About This Manual Gives an overview of all three volumes of the Intel Archi tecture Software Developer s Manual It also describes the notational conventions in these manuals and lists related Intel manuals and documentation of interest to programmers and hard ware designers Chapter 2 Introduction to the Intel Architecture Introduces the Intel Architecture and the families of Intel processors that are based on this architecture It also gives an overview of the common features found in these pro
337. s calls inter privilege level return 4 9 near reti pa a ec ep US 4 5 RF resume flag EFLAGS register 3 13 ROL instruction 6 28 ROR instruction 6 28 Rounding control RC field of FPU control word 7 18 modes bici i Ae aa 7 18 results FPU s urs meg 7 19 RSMinstruction 6 2 s SAHF instruction 3 10 6 37 SAL instruction 6 25 SAR instruction 6 26 Saturation arithmetic MMX instructions 8 5 Saving the FPU state 7 21 SBBiinstruction 6 22 Scale operand addressing 5 8 5 9 Scale FPU operation 7 38 Scaling bias value 7 51 7 52 SCAS instruction 3 12 6 35 Segment registers description of 3 5 3 7 Segment selector description of 3 3 3 7 specifying 5 6 Segmented addressing 1 7 Segmented memory model 3 3 3 8 Segments defined jel ee Ee eee big 3 3 maximum number 3 3 Serialization of I O instructions 9 6 SETcc instructions 3 12 6 29 SF sign flag EFLAGS register 3 11 SF stack fault flag FPU status word 7 15 7 48 SHL instructiON ooooocoocoooo 6
338. s execution of the interrupted procedure 4 4 2 Calls to Interrupt or Exception Handler Tasks Interrupt and exception handler routines can also be executed in a separate task Here an inter rupt or exception causes a task switch to a handler task The handler task is given its own address space and optionally can execute at a higher protection level than application programs or tasks The switch to the handler task is accomplished with an implicit task call that references a task gate descriptor The task gate provides access to the address space for the handler task As part 4 14 Intel PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS of the task switch the processor saves complete state information for the interrupted program or task Upon returning from the handler task the state of the interrupted program or task is restored and execution continues See Chapter 5 Interrupt and Exception Handling in the Intel Architecture Software Developer s Manual Volume 3 for a detailed description of the processor s mechanism for handling interrupts and exceptions through handler tasks 4 4 3 Interrupt and Exception Handling in Real Address Mode When operating in real address mode the processor responds to an interrupt or exception with an implicit far call to an interrupt or exception handler The processor uses the interrupt or excep tion vector number as an index into an interrupt table The interrupt table contains instruction pointers
339. s possible to test for with this instruction 6 29 INSTRUCTION SET SUMMARY intel e 6 8 4 Test Instruction The TEST instruction performs a logical AND of two operands and sets the SF ZF and PF flags according to the results The flags can then be tested by the conditional jump or loop instructions or the SETcc instructions The TEST instruction differs from the AND instruction in that it does not alter either of the operands 6 9 CONTROL TRANSFER INSTRUCTIONS The processor provides both conditional and unconditional control transfer instructions to direct the flow of program execution Conditional transfers are taken only for specified states of the status flags in the EFLAGS register Unconditional control transfers are always executed 6 9 1 Unconditional Transfer Instructions The JMP CALL RET INT and IRET instructions transfer program control to another location destination address in the instruction stream The destination can be within the same code segment near transfer or in a different code segment far transfer 6 9 1 1 JUMP INSTRUCTION The JMP jump instruction unconditionally transfers program control to a destination instruc tion The transfer is one way that is a return address is not saved A destination operand spec ifies the address the instruction pointer of the destination instruction The address can be a relative address or an absolute address A relative address is a displacement offset
340. saturation causes a color to remain pure black or pure white without allowing for and inversion MMX instructions do not indicate overflow or underflow occurrence by generating exceptions or setting flags 8 5 PROGRAMMING WITH THE INTEL MMX TECHNOLOGY intel 8 2 2 Instruction Operands All MMX instructions except the EMMS instruction reference and operate on two operands the source and destination operands The first operand is the destination and the second operand is the source The destination operand may also be a second source operand for the operation The instruction overwrites the destination operand with the result For example a two operand instruction would be decoded as DEST first operand lt DEST first operand OPERATION SRC second operand The source operand for all the MMX instructions except the data transfer instructions can reside either in memory or in an MMX register The destination operand resides in an MMX register For data transfer instructions the source and destination operands can also be an integer register for the MOVD instruction or memory location for both the MOVD and MOVQ instructions 8 3 OVERVIEW OF THE MMX INSTRUCTION SET Table 8 2 shows the instructions in the MMX instruction set The following sections give a brief overview of each group of instructions in the MMX instruction set and the instructions within each group 8 3 1 Data Transfer Instructions The MOVD Mo
341. segment descriptor for the currently executing code segment defines the default address and operand size A segment descriptor is a system data structure not normally visible to application code Assembler directives allow the default addressing and operand size to be chosen for a program The assembler and other tools then set up the segment descriptor for the code segment appropriately When operating in real address mode the default addressing and operand size is 16 bits An address size override can be used in real address mode to enable 32 bit addressing however the maximum allowable 32 bit address is still 0000FFFFH 21 3 6 REGISTERS The processor provides 16 registers for use in general system and application programing As shown in Figure 3 3 these registers can be grouped as follows General purpose data registers These eight registers are available for storing operands and pointers Segment registers These registers hold up to six segment selectors Status and control registers These registers report and allow modification of the state of the processor and of the program being executed 3 6 1 General Purpose Data Registers The 32 bit general purpose data registers EAX EBX ECX EDX ESI EDI EBP and ESP are provided for holding the following items Operands for logical and arithmetic operations Operands for address calculations Memory pointers 3 5 BASIC EXECUTION ENVIRONMENT intel e
342. simplify procedure entry and exit in compiler generated code 4 5 1 ENTER Instruction The ENTER instruction creates a stack frame compatible with the scope rules typically used in block structured languages In block structured languages the scope of a procedure is the set of variables to which it has access The rules for scope vary among languages They may be based on the nesting of procedures the division of the program into separately compiled files or some other modularization scheme The ENTER instruction has two operands The first specifies the number of bytes to be reserved on the stack for dynamic storage for the procedure being called Dynamic storage is the memory allocated for variables created when the procedure is called also known as automatic variables The second parameter is the lexical nesting level from 0 to 31 of the procedure The nesting level is the depth of a procedure in a hierarchy of procedure calls The lexical level is unrelated to either the protection privilege level or to the I O privilege level of the currently running program or task The ENTER instruction in the following example allocates 2K bytes of dynamic storage on the stack and sets up pointers to two previous stack frames in the stack frame for this procedure ENTER 2048 3 The lexical nesting level determines the number of stack frame pointers to copy into the new stack frame from the preceding frame A stack frame pointer is a doub
343. standard and to allow exact replication of calculations which were done using the lower precision data types Using these settings nullifies the advantages of the extended real format s 64 bit significand length When reduced precision is specified the rounding of the significand value clears the unused bits on the right to zeros The precision control bits only affect the results of the following floating point instructions FADD FADDP FSUB FSUBP FSUBR FSUBRP FMUL FMULP FDIV FDIVP FDIVR FDIVRP and FSQRT 7 3 4 3 ROUNDING CONTROL FIELD The rounding control RC field of the FPU control register bits 10 and 11 controls how the results of floating point instructions are rounded Four rounding modes are supported see Table 7 5 round to nearest round up round down and round toward zero Round to nearest is the default rounding mode and is suitable for most applications It provides the most accurate and statistically unbiased estimate of the true result Table 7 5 Rounding Control Field RC Rounding RC Field Mode Setting Description Round to 00B Rounded result is the closest to the infinitely precise result If two values nearest even are equally close the result is the even value that is the one with the least significant bit of zero Round down 01B Rounded result is close to but no greater than the infinitely precise toward ee result Round up 10B Rounded result is close to but no less th
344. sters For all the Intel Architecture FPUs and NPXs except the 8087 the FPU instruction pointer points to any prefixes that preceded the instruction For the 8087 the FPU instruction pointer points only to the actual opcode 7 3 8 Last Instruction Opcode The FPU stores the opcode of the last non control instruction executed in an 11 bit FPU opcode register This information provides state information for exception handlers Only the first and second opcode bytes after all prefixes are stored in the FPU opcode register Figure 7 12 shows the encoding of these two bytes Since the upper 5 bits of the first opcode byte are the same for all floating point opcodes 11011B only the lower 3 bits of this byte are stored in the opcode register 7 3 9 Saving the FPU s State The FSTENV FNSTENV and FSAVE FNSAVE instructions store FPU state information in memory for use by exception handlers and other system and application software The FSTENV FNSTENV instruction saves the contents of the status control tag FPU instruction pointer FPU operand pointer and opcode registers The FSAVE FNSAVE instruction stores that 7 21 FLOATING POINT UNIT intel information plus the contents of the FPU data registers Note that the FSAVE FNSAVE instruc tion also initializes the FPU to default values just as the FINIT FNINIT instruction does after it has saved the original state of the FPU 1st Instruction Byte 2nd Instruction Byte 7 2 0 7 0
345. support far transfers however far transfers can be accomplished with a combination of a and a JMP instruction see Jcc Jump if Condition Is Met in Chapter 3 of the Intel Archi tecture Software Developer s Manual Volume 2 6 32 intel e INSTRUCTION SET SUMMARY Table 6 4 shows the mnemonics for the Jcc instructions and the conditions being tested for each instruction The condition code mnemonics are appended to the letter J to form the mnemonic for a Jcc instruction The instructions are divided into two groups unsigned and signed condi tional jumps These groups correspond to the results of operations performed on unsigned and signed integers respectively Those instructions listed as pairs for example JA JNBE are alter nate names for the same instruction The assembler provides these alternate names to make it easier to read program listings The JCXZ and JECXZ instructions test the CX and ECX registers respectively instead of one or more status flags See Section 6 9 2 3 Jump If Zero Instructions for more information about these instructions 6 9 2 2 LOOP INSTRUCTIONS The LOOP LOOPE loop while equal LOOPZ loop while zero LOOPNE loop while not equal and LOOPNZ loop while not zero instructions are conditional jump instructions that use the value of the ECX register as a count for the number of times to execute a loop All the loop instructions decrement the count in the ECX register each time th
346. t 16 Bit Protected Mode Format 15 0 Control Word Status Word Tag Word FPU Instruction Pointer Offset FPU Instruction Pointer Selector FPU Operand Pointer Offset FPU Operand Pointer Selector 12 gt O O N O Figure 7 15 Protected Mode FPU State Image in Memory 16 Bit Format 16 Bit Real Address Mode and Virtual 8086 Mode Format 15 Control Word 0 Status Word 2 Tag Word 4 FPU Instruction Pointer 15 00 6 IP 19 16 0 Opcode 10 00 8 FPU Operand Pointer 15 00 10 OP 19 16 000000000000 12 Figure 7 16 Real Mode FPU State Image in Memory 16 Bit Format 7 23 FLOATING POINT UNIT intel The FLDENV and FRSTOR instructions allow FPU state information to be loaded from memory into the FPU Here the FLDENV instruction loads only the status control tag FPU instruction pointer FPU operand pointer and opcode registers and the FRSTOR instruction loads all the FPU registers including the FPU stack registers 7 4 FLOATING POINT DATA TYPES AND FORMATS The Intel Architecture FPU recognizes and operates on seven data types divided into three groups reals integers and packed BCD integers Figure 7 17 shows the data formats for each of the FPU data types Table 7 8 gives the length precision and approximate normalized range that can be represented of each FPU data type Denormal values are also supported in each of the
347. t through the CF flag This instruc tion treats the CF flag as a one bit extension on the upper end of the operand Each bit which exits from the most significant bit location of the operand moves into the CF flag At the same time the bit in the CF flag enters the least significant bit location of the operand The RCR instruction rotates the bits in the operand to the right through the CF flag For all the rotate instructions the CF flag always contains the value of the last bit rotated out of the operand even if the instruction does not use the CF flag as an extension of the operand The value of this flag can then be tested by a conditional jump instruction JC or JNC 6 28 intel e INSTRUCTION SET SUMMARY 6 8 BIT AND BYTE INSTRUCTIONS The bit and byte instructions operate on bit or byte strings They are divided into four groups Bittest and modify instructions Bit scan instructions Byte set on condition Test 6 8 1 Bit Test and Modify Instructions The bit test and modify instructions see Table 6 3 operate on a single bit which can be in an operand The location of the bit is specified as an offset from the least significant bit of the operand When the processor identifies the bit to be tested and modified it first loads the CF flag with the current value of the bit Then it assigns a new value to the selected bit as determined by the modify operation for the instruction Table 6 3 Bit Test and Mod
348. te code In this case the error code is zero as shown below for a general protection exception GP 0 See Chapter 5 Interrupt and Exception Handling in the Intel Architecture Software Developer s Manual Volume 3 for a list of exception mnemonics and their descriptions 1 5 RELATED LITERATURE The following books contain additional material related to Intel processors Intel Pentium Pro Processor Specification Update Order Number 242689 1 7 ABOUT THIS MANUAL intel Intel Pentium Processor Specification Update Order Number 242480 AP 485 Intel Processor Identification and the CPUID Instruction Order Number 241618 AP 578 Software and Hardware Considerations for FPU Exception Handlers for Intel Architecture Processors Order Number 242415 001 Pentium Pro Processor Family Developer s Manual Volume 1 Specifications Order Number 242690 001 Pentium Processor Family Developer s Manual Order Number 241428 Intel486 Microprocessor Data Book Order Number 240440 Intel4S6 SX CPU Intel487 SX Math Coprocessor Data Book Order Number 240950 Intel486 DX2 Microprocessor Data Book Order Number 241245 Intel486 Microprocessor Product Brief Book Order Number 240459 Intel386 Processor Hardware Reference Manual Order Number 231732 Intel3861M Processor System Software Writer s Guide Order Number 231499 Intel386 High Performance 32 Bit CHMOS Microprocessor with Integrated Memory Manage
349. ted to a QNaN by setting the most signifi cant fraction bit of the value to 1 The result is then stored in the destination operand and the floating point invalid operation flag is set If the invalid operation mask is clear a floating point invalid operation fault is signaled and no result is stored in the destination operand When a real operation or exception delivers a QNaN result the value of the result depends on the source operands as shown in Table 7 18 Except for the rules given at the beginning of this section for encoding SNaNs and QNaNs soft ware is free to use the bits in the significand of a NaN for any purpose Both SNaNs and QNaNs can be encoded to carry and store data such as diagnostic information Table 7 18 Rules for Generating QNaNs Source Operands QNaN Result An SNaN and a QNaN The QNaN source operand Two SNaNs The SNaN with the larger significand converted into a QNaN Two QNaNs The QNaN with the larger significand An SNaN and a real value The SNaN converted into a QNaN A QNaN and a real value The QNaN source operand Neither source operand is a NaN and a floating The default QNaN real indefinite point invalid operation exception is signaled 7 41 FLOATING POINT UNIT intel 7 6 1 Uses for Signaling NANs By unmasking the invalid operation exception the programmer can use signaling NaNs to trap to the exception handler The generality of this approach and the large number of
350. ter for that data register is marked empty 11B if the tag register image indicates that the data register is non empty the FPU reads the actual value in the data register and sets the tag for the register accordingly This action prevents a program from setting the values in the tag register to incor rectly represent the actual contents of non empty data registers 7 3 7 The FPU Instruction and Operand Data Pointers The FPU stores pointers to the instruction and operand data for the last non control instruction executed in two 48 bit registers the FPU instruction pointer and FPU operand data pointer registers see Figure 7 5 This information is saved to provide state information for exception handlers The contents of the FPU instruction and operand pointer registers remain unchanged when any of the control instructions FINIT FNINIT FCLEX FNCLEX FLDCW FSTCW FNSTCW FSTSW ENSTSW FSTENV ENSTENV FLDENV FSAVE FNSAVE FRSTOR and WAIT FWAIT are executed The contents of the FPU operand register are undefined if the prior non control instruction did not have a memory operand The pointers stored in the FPU instruction and operand pointer registers consist of an offset stored in bits 0 through 31 and a segment selector stored in bits 32 through 47 These registers can be accessed by the FSTENV FNSTENV FLDENV FINIT FNINIT FSAVE FNSAVE and FRSTOR instructions The FINIT FNINIT and FSAVE ENSAVE instruc tions clear these regi
351. ter in the FPU it is automatically converted to the extended real format All decimal inte gers are exactly representable in extended real format The packed decimal indefinite encoding is stored by the FBSTP instruction in response to a masked floating point invalid operation exception Attempting to load this value with the FBLD instruction produces an undefined result 7 4 4 Unsupported Extended Real Encodings The extended real format permits many encodings that do not fall into any of the categories shown in Table 7 9 Table 7 12 shows these unsupported encodings Some of these encodings were supported by the Intel 287 math coprocessor however most of them are not supported by the Intel 387 math coprocessor or the internal FPUs in the Intel486 Pentium or Pentium Pro processors These encodings are no longer supported due to changes made in the final version of IEEE Std 754 that eliminated these encodings The categories of encodings formerly known as pseudo NaNs pseudo infinities and un normal numbers are not supported The Intel 387 math coprocessor and the internal FPUs in the Intel486 Pentium and Pentium Pro processors generate the invalid operation exception when they are encountered as operands 7 28 Intel FLOATING POINT UNIT The encodings formerly known as pseudo denormal numbers are not generated by the Intel 387 math coprocessor and the internal FPUs in the Intel486 Pentium and Pentium Pro processors however th
352. tes periodic checks for QNaN results then it is not necessary to check for the unordered result every time a comparison is made See Section 7 3 3 Branching and Conditional Moves on FPU Condition Codes for another technique for branching on FPU condition codes Some non comparison FPU instructions update the condition code flags in the FPU status word To ensure that the status word is not altered inadvertently store it immediately following a comparison operation 7 36 Intel FLOATING POINT UNIT 7 5 7 Trigonometric Instructions The following instructions perform four common trigonometric functions FSIN Sine FCOS Cosine FSINCOS Sine and cosine FPTAN Tangent FPATAN Arctangent These instructions operate on the top one or two registers of the FPU register stack and they return their results to the stack The source operands must be given in radians The FSINCOS instruction returns both the sine and the cosine of a source operand value It oper ates faster than executing the FSIN and FCOS instructions in succession The FPATAN instruction computes the arctangent of ST 1 divided by ST 0 It is useful for converting rectangular coordinates to polar coordinates 7 5 8 Pi When the argument source operand of a trigonometric function is within the range of the func tion the argument is automatically reduced by the appropriate multiple of 27 through the same reduction mechanism used by and FPREMI ins
353. the I O privilege level IOPL is greater than or equal to the CPL numerically the IF flag bit 9 also is affected The POPFD instruction pops a doubleword into the EFLAGS register This instruction can change the state of the AC bit bit 18 and the ID bit bit 21 as well as the bits affected by a POPF instruction The restrictions for changing the IOPL bits and the IF flag that were given for the POPF instruction also apply to the POPFD instruction 6 13 4 Interrupt Flag Instructions The CLI clear interrupt flag and STI set interrupt flag instructions clear and set the interrupt flag IF in the EFLAGS register respectively Clearing the IF flag causes external interrupts to be ignored The ability to execute these instructions depends on the operating mode of the processor and the current privilege level CPL of the program or task attempting to execute these instructions 6 14 SEGMENT REGISTER INSTRUCTIONS The processor provides a variety of instructions that address the segment registers of the processor directly These instructions are only used when an operating system or executive is using the segmented or the real address mode memory model 6 14 1 Segment Register Load and Store Instructions The MOV instruction introduced in Section 6 3 1 General Purpose Data Movement Instruc tions and the PUSH and POP instructions introduced in Section 6 3 2 Stack Manipulation Instructions can transfer 16 bit segment se
354. the IDT is identified with a number called a vector Table 4 1 lists the interrupts and exceptions with entries in the IDT and their respective vector numbers Vectors 0 through 8 10 through 14 and 16 through 18 are the predefined interrupts and exceptions and vectors 32 through 255 are the user defined interrupts called maskable interrupts Note that the processor defines several additional interrupts that do not point to entries in the IDT the most notable of these interrupts is the SMI interrupt See Exception and Interrupt Vectors in Chapter 5 of the Intel Architecture Software Developer s Manual Volume 3 for more information about the interrupts and exceptions that the Intel Architecture supports When the processor detects an interrupt or exception it does one of the following things Executes an implicit call to a handler procedure Executes an implicit call to a handler task 4 4 1 Call and Return Operation for Interrupt or Exception Handling Procedures A call to an interrupt or exception handler procedure is similar to a procedure call to another protection level see Section 4 3 6 CALL and RET Operation Between Privilege Levels Here the interrupt vector references one of two kinds of gates an interrupt gate or a trap gate Interrupt and trap gates are similar to call gates in that they provide the following information Access rights information The segment selector for the code segment that contains t
355. the effects of floating point and MMX instructions on the floating point tag word For details on floating point tag word see Section 7 3 6 FPU Tag Word 8 4 2 Effect of Instruction Prefixes on MMX Instructions Table 8 3 details the effect of an instruction prefix on an MMX instruction Table 8 3 Effect of Prefixes on MMX Instructions Prefix Type Effect of Prefix Address size 67H Affects MMX instructions with a memory operand Ignored by MMX instructions without a memory operand Operand size 66H Ignored Segment override Affects MMX instructions with a memory operand Ignored by MMX instructions without a memory operand Repeat Ignored Lock FOH Generates an invalid opcode exception See the section titled Instruction Prefixes in Chapter 2 of the Intel Architecture Software Developer s Manual Volume 2 for detailed information on prefixes 8 5 WRITING APPLICATIONS WITH MMX CODE The following sections give guidelines for writing applications code uses the MMX technology 8 10 Intel PROGRAMMING WITH THE INTEL MMX TECHNOLOGY 8 5 1 Detecting Support for MMX Technology Using the CPUID Instruction Use the CPUID instruction to determine whether the processor supports the MMX instruction set see the section titled CPUID CPU Identification in Chapter 3 of the Intel Architecture Software Developer s Manual Volume 2 for a detailed description of the CPUID i
356. the exception handler a preceding FPU exception has already caused FERR to be asserted and the external interrupt hardware has responded but IGNNE assertion still prevents the freeze at FPU instructions Note that if IGNNE is left active outside of the FPU exception handler additional FPU instructions may be executed after a given instruction has caused an FPU exception In this case if the FPU exception handler ever did get invoked it could not determine which instruction caused the exception To properly manage the interface between the processor s FERR output its IGNNE input and the IRQ13 input of the PIC additional external hardware is needed A recommended configu ration is described in the following section D 2 1 2 RECOMMENDED EXTERNAL HARDWARE TO SUPPORT THE MS DOS COMPATIBILITY MODE Figure D 1 provides an external circuit that will assure proper handling of FERR and IGNNE when an FPU exception occurs In particular it assures that IGNNE will be active only inside the FPU exception handler without depending on the order of actions by the exception handler Some hardware implementations have been less robust because they have depended on the exception handler to clear the FPU exception interrupt request to the PIC FP IRQ signal before the handler causes FERR to be de asserted by clearing the exception from the FPU itself Figure D 2 shows the details of how IGNNE will behave when the circuit in Figure D 1 is implemented
357. the four processing units and the memory subsystem shown in Figure 2 1 Memory subsystem System bus L2 cache bus interface unit instruction cache L1 data cache unit L1 memory interface unit and memory reorder buffer Fetch decode unit Instruction fetch unit branch target buffer instruction decoder microcode sequencer and register alias table Instruction pool Reorder buffer Dispatch execute unit Reservation station two integer units two floating point units and two address generation units Retire unit Retire unit and retirement register file 2 5 1 Memory Subsystem The memory subsystem for the Pentium Pro processor consists of main system memory the primary cache L1 and the secondary cache L2 The bus interface unit accesses system memory through the external system bus This 64 bit bus is a transaction oriented bus meaning that each bus access is handled as separate request and response operations While the bus inter face unit is waiting for a response to one bus request it can issue numerous additional requests The bus interface unit accesses the close coupled L2 cache through a 64 bit cache bus This bus is also transactional oriented supporting up to four concurrent cache accesses and operates at the full clock speed of the processor Access to the L1 caches is through internal buses also at full clock speed The 8 KByte L1 instruction cache is four way set associative the 8
358. the more general Intel MultiProcessor Specification for systems with multiple Pentium or Pentium Pro processors support FPU exception handling only in the native mode Intel does not recommend using the MS DOS compatibility mode for systems using more than one processor D 28 Intel Index intel Numerics 16 bit address size 3 5 operand size 3 5 32 bit address size 3 5 operand size 3 5 A AAAinstruction 6 24 AAD instruction 6 24 AAM instruction 6 24 AASinstruction 6 24 AC alignment check flag EFLAGS register eer ex HEX 3 13 Access rights segment descriptor 4 8 4 11 ADC instruction 6 22 ADD instruction 6 22 Address size attribute code segment 3 14 description of 3 14 E re aaa aa E ash 4 3 Address sizes 3 5 Addressing modes assembler r Remi whe 5 9 5 8 5 9 base plus displacement 5 9 base plus index plus displacement 5 9 base plus index time scale plus displacement 5 9 displacement 5 8 effective address 5 8 immediate o
359. though MIPs are no longer considered a preferred measure of CPU performance they are the only benchmarks that span all six generations of the Intel Architecture The MIPs and frequency values given here correspond to the maximum CPU frequency available at product introduction 2 Main CPU register size and external data bus size are given in bits Note also that there are 8 and 16 bit data registers in all of the CPUs there are eight 80 bit registers in the FPUs integrated into the Intel386TM chip and beyond and there are internal data paths that are 2 to 4 times wider than the external data bus for each processor 3 In addition to the large general purpose caches listed in the table for the Intel486 processor 8 KBytes of combined code and data and the Intel Pentium and Pentium Pro processors 8 KBytes each for sep arate code cache and data cache there are smaller special purpose caches The Intel 286 has 6 byte descriptor caches for each segment register The Intel386 has 8 byte descriptor caches for each segment register and also a 32 entry 4 way set associative Translation Lookaside Buffer cache to store access information for recently used pages on the chip The Intel486 has the same caches described for the Intel386 as well as its 8K L1 general purpose cache The Intel Pentium and Pentium Pro processors have their general purpose caches descriptor caches and two Translation Lookaside Buffers each one for each 8K L1 cache 2 4
360. tion changes the sign of the source operand The FXTRACT instruction separates the source operand into its exponent and fraction and stores each value in a register in real format 7 5 6 Comparison and Classification Instructions The following instructions compare or classify real values FCOM FCOMP FCOMPP Compare real and set FPU condition code flags FUCOM FUCOMP FUCOMPP Unordered compare real and set FPU condition code flags FICOM FICOMP Compare integer and set FPU condition code flags FCOMI FCOMIP Compare real and set EFLAGS status flags FUCOMI FUCOMIP Unordered compare real and set EFLAGS status flags FTST Test compare real with 0 0 FXAM Examine Comparison of real values differ from comparison of integers because real values have four rather than three mutually exclusive relationships less than equal greater than and unordered The unordered relationship is true when at least one of the two values being compared is a NaN or in an undefined format This additional relationship is required because by definition NaNs are not numbers so they cannot have less than equal or greater than relationships with other real values 7 34 Intel FLOATING POINT UNIT The FCOM FCOMP and FCOMPP instructions compare the value in register ST 0 with a real source operand and set the condition code flags CO C2 and C3 in the FPU status word according to the results see Table 7 15 If an unordered condition is detected one or
361. tion instruction converts signed words into signed bytes or signed doublewords into signed words in signed saturation mode The PACKUSWB packed with unsigned saturation instruction converts signed words into unsigned bytes in unsigned saturation mode The PUNPCKHBW PUNPCKHWD and PUNPCKHDQ unpack high packed data and PUNPCKLBW PUNPCKLWD and PUNPCKLDQ unpack low packed data instructions convert bytes to words words to doublewords or doublewords to quadwords 8 3 5 Logical Instructions The PAND bitwise logical AND PANDN bitwise logical AND NOT POR bitwise logical OR and PXOR bitwise logical exclusive OR instructions perform bitwise logical operations on 64 bit quantities 8 3 6 Shift Instructions The logical shift left logical shift right and arithmetic shift right instructions shift each element by a specified number of bits The logical left and right shifts also enable a 64 bit quantity quad word to be shifted as one block assisting in data type conversions and alignment operations The PSLLW and PSLLD packed shift left logical and PSRLW and PSRLD packed shift right logical instructions perform a logical left or right shift and fill the empty high or low order bit positions with zeros These instructions support packed word packed doubleword and quad word data types The PSRAW and PSRAD packed shift right arithmetic instruction performs an arithmetic right shift copying the sign bit into empty bit
362. tions 6 9 6 2 2 2 MMX Conversion Instructions 6 9 6 2 2 3 MMX Packed Arithmetic Instructions 6 10 6 2 2 4 MMX Comparison Instructions 6 10 6 2 2 5 MMX Logic Instructions lisse BI 6 10 6 2 2 6 MMX Shift and Rotate Instructions 6 11 6 2 2 7 MMX State 6 11 6 2 3 Floating Point 5 6 11 6 2 3 1 Data Transfer u asi r Me exp ERU DR Run 6 11 6 2 3 2 Basic Arithmetic 0 o euet et repe ge E eden teni n top ae 6 12 6 2 3 3 Comparison kb ied etur a ea a P Pe usu as ed 6 13 6 2 3 4 Transcendental ice deine Sas Fas Ra uU E OE eee kate 6 13 6 2 3 5 Load Gonstants uere ene eu ee We Y c Y aas yg 6 13 6 2 3 6 Control sz a pora A UA 6 14 6 2 4 System Instructions solet emm eer oe EE eie 6 15 6 3 DATA MOVEMENT INSTRUCTIONS 6 16 6 3 1 General Purpose Data Movement Instructions 6 16 6 3 1 1 Move Instruction i i erreur auge Y E RA 6 16 6 3 1 2 Conditional Move Instructions 6 16 6 3 1 3 Exchange 1 51 5 6 17 6 3 2 Stack Manipulation Instructions
363. tions FENI and FDISI and the Intel 287 math coprocessor instruction FSETPM perform no function in the Intel 387 math coprocessor or the Intel486 Pentium or Pentium Pro processors If these opcodes are detected in the instruction stream the FPU performs no specific operation and no internal FPU states are affected 7 6 OPERATING ON NANS As was described in Section 7 2 3 4 NaNs the FPU supports two types of NaNs SNaNs and QNaNs An SNaN is any NaN value with its most significant fraction bit set to 0 and at least one other fraction bit set to 1 If all the fraction bits are set to 0 the value is an A QNaN is any NaN value with the most significant fraction bit set to 1 The sign bit of a NaN is not interpreted As a general rule when a QNaN is used in one or more arithmetic floating point instructions it is allowed to propagate through a computation An SNaN on the other hand causes a floating point invalid operation exception to be signaled SNaNs are typically used to trap or invoke an exception handler They must be inserted by software that is the FPU never generates an SNaN as a result The floating point invalid operation exception has a flag and a mask bit associated with it in the FPU status and control registers respectively see Section 7 7 Floating Point Exception Handling The mask bit determines how the FPU handles an SNaN value If the floating point invalid operation mask bit is set the SNaN is conver
364. tive integers and cleared for positive integers and zero Integer values range from 128 to 127 for a byte integer from 32 768 to 32 767 for a word integer and from 2 to 23 1 for a doubleword integer 5 2 DATA TYPES AND ADDRESSING MODES Byte Signed Integer Sign 76 Word Signed Integer Sign 15 14 Doubleword Signed Integer Sign 31 30 o Byte Unsigned Integer 7 Word Unsigned Integer o 15 Doubleword Unsigned Integer o 31 0 BCD Integers x scp X sop 7 43 0 Packed BCD Integers BCD BCD LBCD BCD BCD BCD 7 43 0 Near Pointer Offset or Linear Address 31 0 Far Pointer or Logical Address Segment Selector Offset 47 32 31 0 Bit Field Field Length Least Significant Bit Figure 5 3 Numeric Pointer and Bit Field Data Types 5 3 DATA TYPES AND ADDRESSING MODES intel e 5 2 2 Unsigned Integers Unsigned integers are unsigned binary numbers contained in a byte word or doubleword Unsigned integer values range from 0 to 255 for an unsigned byte integer from 0 to 65 535 for an unsigned word integer and from 0 to 2 1 for an unsigned doubleword integer Unsigned integers are sometimes referred to as ordinals 5 2 8 BCD Integers Binary coded decimal integers BCD integers are unsigned 4 bit integers with valid values ranging from 0 to 9 BCD integers can be unpacked one BCD digit per byte or packed two
365. to the interrupt and exception handler procedures The processor saves the state of the EFLAGS register the EIP register the CS register and an optional error code on the stack before switching to the handler procedure A return from the interrupt or exception handler is carried out with the IRET instruction See Chapter 15 8086 Emulation in the Intel Architecture Software Developer s Manual Volume 3 for more information on handling interrupts and exceptions in real address mode 4 4 4 INT n INTO INT 3 and BOUND Instructions The INT n INTO INT 3 and BOUND instructions allow a program or task to explicitly call an interrupt or exception handler The INT 7 instruction uses an interrupt vector as an argument which allows a program to call any interrupt handler The INTO instruction explicitly calls the overflow exception OF handler if the overflow flag OF in the EFLAGS register is set The OF flag indicates overflow on arithmetic instructions but it does not automatically raise an overflow exception An overflow exception can only be raised explicitly in either of the following ways Execute the INTO instruction Test the OF flag and execute the INT n instruction with an argument of 4 the vector number of the overflow exception if the flag is set Both the methods of dealing with overflow conditions allow a program to test for overflow at specific places in the instruction stream The INT 3 instruction explicitly
366. tructions 3 12 6 33 LSSinstruction 6 39 M Maskable interrupts 4 11 Masked responses to denormal operand exception 7 50 to division by zero exception 7 50 to FPU stack overflow or underflow exception 7 48 to inexact result precision exception 7 53 to invalid arithmetic operation 7 49 to numeric overflow exception 7 51 to numeric underflow exception 7 52 Masks exception flags FPU control word 7 17 Memory orderbuffer 2 9 organization 3 2 3 3 subsystem fs eee eed eee tende 2 8 Memory interface 2 8 Memory operands 5 6 Memory mapped l 0 9 1 9 2 MESI modified exclusive shared invalid cache protocol 7 ex Rm en 2 8 Microarchitecture detailed description 2 8 OVEIVIOW 2 5 Micro Ops yaoa aqu ps We ete 2 10 INDEX MM1 MM2 MM4 MM5 MM6 registers 8 2 MMX technology arithmetic instructions 8 8 comparison instructions 8 8 compatibility with FPU architecture 8 10 conversion instructions 8 9 CPUID instruction 10 2 data transfer instructions 8 6 data
367. tructions INS and OUTS instructions move blocks of data strings between an I O port and memory These instructions operate similar to the string instructions see Section 6 10 String Operations The ESI and EDI registers are used to specify string elements in memory and the repeat prefixes REP are used to repeat the instructions to implement block moves The assembler recognizes the following alternate mnemonics for these instructions INSB input byte INSW input word and INSD input doubleword and OUTB output byte OUTW output word and OUTD output doubleword The INS and OUTS instructions use an address in the DX register to specify the I O port to be read or written to 6 12 ENTER AND LEAVE INSTRUCTIONS The ENTER and LEAVE instructions provide machine language support for procedure calls in block structured languages such as C and Pascal These instructions and the call and return mechanism that they support are described in detail in Section 4 5 Procedure Calls for Block Structured Languages 6 36 intel e INSTRUCTION SET SUMMARY 6 13 EFLAGS INSTRUCTIONS The EFLAGS instructions allow the state of selected flags in the EFLAGS register to be read or modified 6 13 1 Carry and Direction Flag Instructions The STC set carry flag CLC clear carry flag and CMC complement carry flag instructions allow the CF flags in the EFLAGS register to be modified directly They are typically used to initialize t
368. tructions The internal value of that the Intel Architecture FPU uses for argument reduction and other computations is as follows n 0 f 22 where f C90FDAA2 2168C234 C The spaces in the fraction above indicate 32 bit boundaries This internal value has a 66 bit mantissa which is 2 bits more than is allowed in the significand of an extended real value Since 66 bits is not an even number of hexadecimal digits two addi tional zeros have been added to the value so that it can be represented in hexadecimal format The least significant hexadecimal digit C is thus 1100B where the two least significant bits represent bits 67 and 68 of the mantissa This value of x has been chosen to guarantee no loss of significance in a source operand provided the operand is within the specified range for the instruction If the results of computations that explicitly use 7 are to be used in the FSIN FCOS FSINCOS or FPTAN instructions the full 66 bit fraction of x should be used This insures that the results are consistent with the argument reduction algorithms that these instructions use Using a rounded version of 7 can cause inaccuracies in result values which if propagated through several calculations might result in meaningless results 7 37 FLOATING POINT UNIT intel common method of representing the full 66 bit fraction of is to separate the value into two numbers hight and lowr that when added together give t
369. ture processor Chapter 14 Debugging and Performance Monitoring Describes the debugging registers and other debug mechanism provided in the Intel Architecture This chapter also describes the time stamp counter and the performance monitoring counters Chapter 15 8086 Emulation Describes the real address and virtual 8086 modes of the Intel Architecture Chapter 16 Mixing 16 Bit and 32 Bit Code Describes how to mix 16 bit and 32 bit code modules within the same program or task Chapter 17 Intel Architecture Compatibility Describes the programming differences between the Intel 286 Intel386 Intel486 Pentium and Pentium Pro processors The differ ences among the 32 bit Intel Architecture processors the Intel386 Intel486 Pentium and Pentium Pro processors are described throughout the three volumes of the Intel Architecture Software Developer s Manual as relevant to particular features of the architecture This chapter provides a collection of all the relevant compatibility information for all Intel Architecture processors and also describes the basic differences with respect to the 16 bit Intel Architecture processors the Intel 8086 and Intel 286 processors Appendix A Performance Monitoring Counters Lists the events that can be counted with the performance monitoring counters and the codes used to select these events Appendix B Model Specific Registers MSRs Lists the MSRs available in the Pentium Pro
370. turn calls procedure C then procedure C will have access to the variables of the MAIN procedure and procedure A but not those of procedure B because they are at the same lexical level The following definition describes the access to variables for the nested procedures in Figure 4 6 1 MAIN has variables at fixed locations 2 Procedure A can access only the variables of MAIN 3 Procedure B can access only the variables of procedure A and MAIN Procedure B cannot access the variables of procedure C or procedure D 4 Procedure C can access only the variables of procedure A and MAIN procedure C cannot access the variables of procedure B or procedure D 5 Procedure D can access the variables of procedure C procedure A and MAIN Procedure D cannot access the variables of procedure B In Figure 4 7 an ENTER instruction at the beginning of the MAIN procedure creates three doublewords of dynamic storage for MAIN but copies no pointers from other stack frames The first doubleword in the display holds a copy of the last value in the EBP register before the ENTER instruction was executed The second doubleword holds a copy of the contents of the EBP register following the ENTER instruction After the instruction is executed the EBP register points to the first doubleword pushed on the stack and the ESP register points to the last doubleword in the stack frame When MAIN calls procedure A the ENTER instruction creates a new display see Fig
371. typical protection ring model access to the I O address space is restricted to privilege levels 0 and 1 Here kernel and the device drivers are allowed to perform I O while less privileged device drivers and application programs are denied access to the I O address space Application programs must then make calls to the operating system to perform I O The following instructions can be executed only if the current privilege level CPL of the program or task currently executing is less than or equal to the IOPL IN INS OUT OUTS CLI clear interrupt enable flag and STI set interrupt enable flag These instructions are called I O sensitive instructions because they are sensitive to the IOPL field Any attempt by a less privileged program or task to use an I O sensitive instruction results in a general protection exception GP being signaled Because each task has its own copy of the EFLAGS register each task can have a different IOPL 9 4 intel INPUT OUTPUT The I O permission bit map in the TSS can be used to modify the effect of the IOPL on I O sensi tive instructions allowing access to some I O ports by less privileged programs or tasks see Section 9 5 2 I O Permission Bit Map A program or task can change its IOPL only with the POPF and IRET instructions however such changes are privileged No procedure may change the current IOPL unless it is running at privilege level 0 An attempt by a less privileged procedure t
372. uct 5 6 x 2 4 3 8 x 10 3 Code FLD valuel valuel 5 6 FMUL value2 b value2 2 4 FLD value3 value3 3 8 FMUL value4 c value4 10 3 FADD ST 1 d a b c d R7 R7 R7 R7 R6 R6 R6 R6 R5 R5 R5 R5 R4 5 6 ST 0 R4 13 44 ST 0 R4 13 44 ST 1 R4 13 44 ST 1 R3 R3 R3 39 14 ST 0 R3 52 58 ST 0 R2 R2 R2 R2 R1 R1 R1 R1 RO RO RO RO Figure 7 7 Example FPU Dot Product Computation 7 3 2 FPU Status Register The 16 bit FPU status register see in Figure 7 8 indicates the current state of the FPU The flags in the FPU status register include the FPU busy flag top of stack TOP pointer condition code flags error summary status flag stack fault flag and exception flags The FPU sets the flags in this register to show the results of operations The contents of the FPU status register referred to as the FPU status word can be stored in memory using the FSTSW FNSTSW FSTENV FNSTENV and FSAVE ENS AVE instructions It can also be stored in the AX register of the integer unit using the FSTSW ENSTSW instructions 7 3 2 1 TOP OF STACK TOP POINTER A pointer to the FPU data register that is currently at the top of the FPU register stack is contained in bits 11 through 13 of the FPU status word This pointer which is commonly referred to as TOP for top of stack is a binary value from 0 to 7 See Section 7 3 1 The FPU Data Registers for more information about the TOP po
373. uction by adding an operand size and or address size prefix to an instruction see Instruction Prefixes in Chapter 2 of the Intel Architecture Software Developer s Manual Volume 3 The effect of this prefix applies only to the instruction it is attached to Table 3 1 shows effective operand size and address size when executing in protected mode depending on the settings of the B flag and the operand size and address size prefixes 3 14 intel e BASIC EXECUTION ENVIRONMENT Table 3 1 Effective Operand and Address Size Attributes D Flag in Code Segment Descriptor 0 0 0 0 1 1 1 1 Operand Size Prefix 66H N N Y Y N N Y Y Address Size Prefix 67H N Y N Y N Y N Y Effective Operand Size 16 16 32 32 32 32 16 16 Effective Address Size 16 32 16 32 32 16 32 16 NOTES Y Yes this instruction prefix is present N No this instruction prefix is not present 3 15 Intel Procedure Calls Interrupts and Exceptions Intel CHAPTER 4 PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS This chapter describes the facilities in the Intel Architecture for executing calls to procedures or subroutines It also describes how interrupts and exceptions are handled from the perspective of an application programmer 4 1 PROCEDURE CALL TYPES The processor supports procedure calls in two different ways CALL and RET instructions ENTER and LEAVE instructions in conjunction with the CALL and
374. uction to be executed The CS register cannot be loaded explicitly by an appli cation program Instead it is loaded implicitly by instructions or internal processor operations that change program control such as procedure calls Interrupt handling or task switchine The DS ES FS and GS registers point to four data segments The availability of four data segments permits efficient and secure access to different types of data structures For example four separate data segments might be created one for the data structures of the current module another for the data exported from a higher level module a third for a dynamically created data structure and a fourth for data shared with another program To access additional data segments the application program must load segment selectors for these segments into the DS ES FS and GS registers as needed The SS register contains the segment selector for a stack segment where the procedure stack is stored for the program task or handler currently being executed All stack operations use the SS register to find the stack segment Unlike the CS register the SS register can be loaded explic itly which permits application programs to set up multiple stacks and switch among them See Section 3 3 Memory Organization for an overview of how the segment registers are used in real address mode 3 9 BASIC EXECUTION ENVIRONMENT intel e The four segment registers CS DS SS and ES are t
375. uctions in response to a masked floating point invalid operation exception see Table 7 20 Table 7 9 Real Number and NaN Encodings Class Sign Biased Exponent Significand Integer Fraction Positive 0 11 11 1 00 00 Normals 0 11 10 1 11 11 0 00 01 1 00 00 Denormals 0 00 00 0 11 11 0 00 00 0 00 01 Zero 0 00 00 0 00 00 Negative Zero 1 00 00 0 00 00 Denormals 1 00 00 0 00 01 1 00 00 0 11 11 Normals 1 00 01 1 00 00 1 11 10 1 11 11 oo 1 11 11 1 00 00 NaNs SNaN X 11 11 1 0X XX QNaN X 11 11 1 1X XX Real Indefinite 1 11 11 1 10 00 QNaN Single Real lt 8 Bits gt lt 23 Bits gt Double Real lt 11 Bits gt lt 52 Bits gt Extended Real lt 15 Bits gt lt 63 Bits gt NOTES 1 Integer bit is implied and not stored for single real and double real formats 2 The fraction for SNaN encodings must be non zero 7 26 Intel FLOATING POINT UNIT 7 4 2 Binary Integers The FPU s three binary integer data types word short and long have identical formats except for length Table 7 8 gives the precision and range of these data types and Figure 7 17 gives the formats Table 7 10 gives the encodings of the three binary integer types Table 7 10 Binary Integer Encodings Class Sign Magnitude Positive Largest 0 11 11 Smallest 0 00 01 Zero 0 00 00 Negative Smallest 1 11 11 Lar
376. umeric coprocessor provided the same hardware mechanism for signaling and handling floating point exceptions as the Intel 286 and 287 processors And again to maintain compatibility with existing MS DOS software basically the same MS DOS compatibility floating point exception handling mechanism that was used in the PC AT was used in PCs based on the Intel386 D 2 Intel GUIDELINES FOR WRITING FPU EXCEPTION HANDLERS D 2 IMPLEMENTATION OF THE MS DOS COMPATIBILITY MODE IN THE INTEL486 PENTIUM AND PENTIUM PRO PROCESSORS Beginning with the Intel486 processor the Intel Architecture provided a dedicated mechanism for enabling the MS DOS compatibility mode for FPU exceptions and for generating external FPU exception signals while operating in this mode The following sections describe the imple mentation of the MS DOS compatibility mode in Intel486 and Pentium processors and in the Pentium Pro processor Also described is the recommended external hardware to support this mode of operation D 2 1 MS DOS Compatibility Mode in the Intel486 and Pentium Processors In the Intel486 several things were done to enhance and speed up the numeric coprocessor now called the floating point unit FPU The most important enhancement was that the FPU was included in the same chip as the processor for increased speed in FPU computations and reduced latency for FPU exception handling Also for the first time the MS DOS compatibility mode was
377. ure An offset into the code segment that is the instruction pointer for the called procedure The processor switches to a new stack to execute the called procedure Each privilege level has its own stack The segment selector and stack pointer for the privilege level 3 stack are stored in the SS and ESP registers respectively and are automatically saved when a call to a more privileged level occurs The segment selectors and stack pointers for the privilege level 2 1 and 0 stacks are stored in a system segment called the task state segment TSS The use of a call gate and the TSS during a stack switch are transparent to the calling procedure except when a general protection exception is raised Intel PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS 4 3 6 CALL and RET Operation Between Privilege Levels When making a call to a more privileged protection level the processor does the following see Figure 4 4 Performs an access rights check privilege check 2 Temporarily saves internally the current contents of the SS ESP CS and EIP registers Stack for Stack for Calling Procedure Called Procedure Calling SS Calling ESP Stack Frame Faram 1 Param 1 Before Call Param 2 Param 2 Stack Frame Param 3 m ESP Before Call Param 3 After Call Calling CS ESP After Call gt Calling EIP Callin
378. ure 4 8 The first doubleword is the last value held in MAIN s EBP register The second doubleword is a pointer to MAIN s stack frame which is copied from the second doubleword in MAIN s display 4 18 Intel PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS This happens to be another copy of the last value held in MAIN s EBP register Procedure A can access variables in MAIN because MAIN is at level 1 Therefore the base address for the dynamic storage used in MAIN is the current address in the EBP register plus four bytes to account for the saved contents of MAIN s EBP register All dynamic variables for MAIN are at fixed positive offsets from this value Old EBP lt Display ee ee Main s EBP Dynamic Storage lt ESP Figure 4 7 Stack Frame after Entering the MAIN Procedure Old EBP Main s EBP Main s EBP Display Main s Procedure A s Dynamic Storage lt ESP Figure 4 8 Stack Frame after Entering Procedure A 4 19 PROCEDURE CALLS INTERRUPTS AND EXCEPTIONS intel When procedure A calls procedure B the ENTER instruction creates a new display see Figure 4 9 The first doubleword holds a copy of the last value in procedure A s EBP register The second and third doublewords are copies of the two stack frame pointers in procedure A s display Procedure B can access variables i
379. uting in protected mode every code segment has a default operand size attribute and address size attribute These attributes are selected with the D default size flag in the segment descriptor for the code segment see Chapter 3 Protected Mode Memory Manage ment in the Intel Architecture Software Developer s Manual Volume 3 When the D flag is set the 32 bit operand size and address size attributes are selected when the flag is clear the 16 bit size attributes are selected When the processor is executing in real address mode virtual 8086 mode or SMM the default operand size and address size attributes are always 16 bits The operand size attribute selects the sizes of operands that instructions operate on When the 16 bit operand size attribute is in force operands can generally be either 8 bits or 16 bits and when the 32 bit operand size attribute is in force operands can generally be 8 bits or 32 bits The address size attribute selects the sizes of addresses used to address memory 16 bits or 32 bits When the 16 bit address size attribute is in force segment offsets and displacements are 16 bits This restriction limits the size of a segment that can be addressed to 64 KBytes When the 32 bit address size attribute is in force segment offsets and displacements are 32 bits allowing segments of up to 4 GBytes to be addressed The default operand size attribute and or address size attribute can be overridden for a partic ular instr
380. value so that when the value is divided with the DIV instruction a valid unpacked BCD result is obtained The instruction converts the BCD value in registers AH most significant digit and AL least significant digit into a binary value and stores the result in register AL When the value in AL is divided by an unpacked BCD value the quotient and remainder will be automatically encoded in unpacked BCD format 6 24 intel e INSTRUCTION SET SUMMARY 6 6 LOGICAL INSTRUCTIONS The logical instructions AND OR XOR exclusive or and NOT perform the standard Boolean operations for which they are named The AND OR and XOR instructions require two oper ands the NOT instruction operates on a single operand 6 7 SHIFT AND ROTATE INSTRUCTIONS The shift and rotate instructions rearrange the bits within an operand These instructions fall into the following classes Shift Double shift Rotate 6 7 1 Shift Instructions The SAL shift arithmetic left SHL shift logical left SAR shift arithmetic right SHR shift logical right instructions perform an arithmetic or logical shift of the bits in a byte word or doubleword The SAL and SHL instructions perform the same operation see Figure 6 6 They shift the source operand left by from 1 to 31 bit positions Empty bit positions are cleared The CF flag is loaded with the last bit shifted out of the operand Initial State CF Operand X 10001000100010001000100010
381. values have full extended real precision 64 bits and are accurate to approximately 19 decimal digits They are stored internally in a format more precise than extended real When loading the constant the FPU rounds the more precise internal constant according to the RC rounding control field of the FPU control word See Section 7 5 8 Pi for information on the x constant 7 5 5 Basic Arithmetic Instructions The following floating point instructions perform basic arithmetic operations on real numbers Where applicable these instructions match IEEE Standard 754 FADD FADDP Add real FIADD Add integer to real FSUB FSUBP Subtract real FISUB Subtract integer from real FSUBR FSUBRP Reverse subtract real FISUBR Reverse subtract real from integer FMUL FMULP Multiply real FIMUL Multiply integer by real FDIV FDIVP Divide real FIDIV Divide real by integer FDIVR FDIVRP Reverse divide FIDIVR Reverse divide integer by real FABS Absolute value FCHS Change sign FSQRT Square root FPREM Partial remainder FPREMI IEEE partial remainder FRNDINT Round to integral value FXTRACT Extract exponent and significand The add subtract multiply and divide instructions operate on the following types of operands Two FPU register values A register value and a real or integer value in memory 7 33 FLOATING POINT UNIT intel Operands in memory can be in single real double real short integer or word integer format They are conve
382. ve 32 Bits instruction transfers 32 bits of packed data from memory to MMX registers and visa versa or from integer registers to MMX registers and visa versa The MOVQ Move 64 Bits instruction transfers 64 bits of packed data from memory to MMX registers and vise versa or transfers data between MMX registers 8 6 intel Table 8 2 MMX Instruction Set Summary Category Wraparound Signed Unsigned Saturation Saturation Arithmetic Addition PADDB PADDW PADDSB PADDUSB PADDD PADDSW PADDUSW Subtraction PSUBB PSUBW PSUBSB PSUBUSB PSUBD PSUBSW PSUBUSW Multiplication PMULL PMULH Multiply and Add PMADD Comparison Compare for Equal PCMPEQB PCMPEQW PCMPEQD Compare for PCMPGTPB Greater Than PCMPGTPW PCMPGTPD Conversion Pack PACKSSWB PACKUSWB PACKSSDW Unpack High PUNPCKHBW PUNPCKHWD PUNPCKHDQ Unpack Low PUNPCKLBW PUNPCKLWD PUNPCKLDQ Packed Full Quadword Logical And PAND And Not PANDN Or POR Exclusive OR PXOR Shift Shift Left Logical PSLLW PSLLD PSLLQ Shift Right Logical PSRLW PSRLD PSRLQ Shift Right PSRAW PSRAD Arithmetic Doubleword Transfers Quadword Transfers Data Transfer Register to Register MOVD MOVQ Load from Memory MOVD MOVQ Store to Memory MOVD MOVQ Empty EMMS MMX State PROGRAMMING WITH THE INTEL MMX TECHNOLOGY 8 7 PROGRAMMING WITH THE INTEL MMX TECHNOLOGY intel 8 3 2 Arithmetic Instructions The arithmetic instruct
383. with respect to the address in the EIP register The destination address a near pointer is formed by adding the displacement to the address in the EIP register The displacement is specified with a signed integer allowing jumps either forward or backward in the instruction stream An absolute address is a offset from address 0 of a segment It can be specified in either of the following ways An address in a general purpose register This address is treated as a near pointer which is copied into the EIP register Program execution then continues at the new address within the current code segment An address specified using the standard addressing modes of the processor Here the address can be a near pointer or a far pointer If the address is for a near pointer the address is translated into an offset and copied into the EIP register If the address is for a far pointer the address is translated into a segment selector which is copied into the CS register and an offset which is copied into the EIP register In protected mode the JMP instruction also allows jumps to a call gate a task gate and a task state segment 6 30 intel e INSTRUCTION SET SUMMARY 6 9 1 2 CALL AND RETURN INSTRUCTIONS The CALL call procedure and RET return from procedure instructions allow a jump from one procedure or subroutine to another and a subsequent jump back return to the calling procedure The CALL instruction transfers program
384. word and the mask bit OM is bit 3 of the FPU control word When a numeric overflow exception occurs and the exception is masked the FPU sets the OE flag and returns one of the values shown in Table 7 22 The value returned depends on the current rounding mode of the FPU see Section 7 3 4 3 Rounding Control Field Table 7 22 Masked Responses to Numeric Overflow Rounding Mode Sign of True Result Result To nearest oo Toward co Largest finite positive number Toward _ Largest finite negative number Toward zero Largest finite positive number _ Largest finite negative number The action that the FPU takes when numeric overflow occurs and the numeric overflow excep tion is not masked depends on whether the instruction is supposed to store the result in memory or on the register stack If the destination is a memory location the OE flag is set and a software exception handler is invoked see Section 7 7 3 Software Exception Handling The top of stack pointer TOP and source and destination operands remain unchanged If the destination is the register stack the exponent of the rounded result is divided by 224576 and the result is stored along with the significand in the destination operand Condition code bit C1 in the FPU status word called in this situation the round up bit is set if the significand was rounded upward and cleared if the result was rounded toward 0 A
385. xecution Unit executes the microcode instructions the Segment Unit trans lates logical addresses to linear addresses and does protection checks and the Paging Unit translates linear addresses to physical addresses does page based protection checks and contains a cache with information for up to 32 most recently accessed pages The Intel486 processor added more parallel execution capability by basically expanding the Intel386 processor s Instruction Decode and Execution Units into five pipelined stages where each stage when needed operates in parallel with the others on up to five instructions in different stages of execution Each stage can do its work on one instruction in one clock and so the Intel486 processor can execute as rapidly as one instruction per CPU clock An 8 KByte on chip L1 cache was added to the Intel486 processor to greatly increase the percent of instructions that could execute at the scalar rate of one per clock memory access instructions were now included if the operand was in the L1 cache The Intel486 processor also for the first time inte grated the Floating Point math Unit onto the same chip as the CPU see section 2 3 below and added new pins bits and instructions to support more complex and powerful systems L2 cache support and multiprocessor support Late in the Intel486 processor generation Intel incorporated features designed to support energy savings and other system management capabilities into the I
386. y reserved bits when testing the values of registers which contain such bits Mask out the reserved bits before testing Do not depend on the states of any reserved bits when storing to memory or to a register Do not depend on the ability to retain information written into any reserved bits When loading a register always load the reserved bits with the values indicated in the documentation if any or reload them with values previously read from the same register 1 5 ABOUT THIS MANUAL intel NOTE Avoid any software dependence upon the state of reserved bits in Intel Archi tecture registers Depending upon the values of reserved register bits will make software dependent upon the unspecified manner in which the processor handles these bits Programs that depend upon reserved values risk incompatibility with future processors 1 43 Instruction Operands When instructions are represented symbolically a subset of the Intel Architecture assembly language is used In this subset an instruction has the following format label mnemonic argumenti argument2 argument3 where A label is an identifier which is followed by a colon A mnemonic is a reserved name for a class of instruction opcodes which have the same function The operands argumentl argument2 and argument3 are optional There may be from Zero to three operands depending on the opcode When present they take the form of either literals or identifiers

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