User`s Manual - DMCS Pages for Students
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1. 68000 ASSEMBLER BY PAUL MCKEE User s Manual Contents 1 Introduction 2 Source Code Format 2 1 source Line Format aa es pals sd paos de EY doe de A we ES 2l A A A 2 1 2 Operation Field 6 sre dig dare ea Aig me a Eg Zar Operands Field irigis prado fei eee We Ses RS eo 2 1 4 Comment Field si aa a ba a A Be eS 22 FSV OLS sy Got A oh alk ae A a a A Be a Re RED oe 23 sPIXPPOSSIOUS a SiGe a Bon A A aa Toa E dd 2 3 1 Operands in Expressions lt 2 oi hg ies A REA Decimal Numbers ida AV A AA e Hexadecimal Numbers ga go eee e ae Binary Numbers eie apa saen d CrN A ASNE ath ta ee ans Octal Umberto oa aa a a eA ASCI T O nst ntS a ia e aa id a So des ede ira dl 2 3 2 Operators in Expressions adios opa le a 2 4 Addressing Mode Specifications o a 3 Assembly Details Sel Branch Instructions sai rio a a pees oe O 3 2 MOVEM Instruction bs a e wee E ER a 3 3 Quick Instructions MOVEQ ADDQ SUBQ 4 Assembler Directives AA ORG et Ongin o esa ee it aaa edt ei Maret BEA a 4 2 Symbol Definition Directives lt 4 4 5 464 40 e644 4 BAA ee A 4 2 1 EQU Equate Symbol ugt e ra ee A 422 SE LS Seb Symbol pd e RA als 4 2 3 REG Register Range 24ers el ee a e 4 3 Data Storage Directives ma aan ieee ae ee ae eM 4 3 1 DC Define Constant ad feng Se e a ee da 4 3 2 DCB Define Constant Block 4 3 3 DS Define Storage sn ak Sl ya BE Se ee 4 4 END End of Sou2. C Program Counter Addressing Mode Specifications Mode Assembler Format Data Register Direct Address Register Direct Address Register Indirect Address Register Indirect with Predecrement Address Register Indirect with Postincrement Address Register Indirect with Displacement Address Register Indirect with Index Absolute Short or Long chosen by assembler Program Counter with Displacement Program Counter with Index Immediate Dn An An An An lt ex16 gt An lt ex8 gt An Xn s lt ex gt lt ex16 gt PC lt ex8 gt PC Xn s H lt ex gt In addition to the general addressing modes the following register names may be used as operands in certain instructions e g MOVEC or EORI to CCR SR Status Register CCR Condition Code Register USP User Stack Pointer VBR Vector Base Register 68010 SFC Source Function Code Register 68010 DFC Destination Function Code Register 68010 2 Assembly Details 3 1 Branch Instructions The branch instructions Bcc BRA and BSR are unique in that they can take a S size code This suffix directs the assembler to assemble these as short branch instructions i e one word instructions with a range to 128 to 127 bytes If the S size code is used and the destination is actually outside this range then the assembler will print an error message If the L size code is used the assembler will use a long branch which is a two word instruction with
3. M when it begins work If any errors occur the program will print ERROR in line X or WARNING in line X this information is also placed in the listing file Upon conclusion it will print the number of errors encountered and warnings generated during the assembly If there is an error in the command line e g if no file name is specified then the assembler will print a brief usage summary and abort 9 5 2 Listing File Format The assembler produces a listing file which shows the source code alongside the the object code produced by the assembler A typical listing file line might appear as follows not to scale 0000102E 22D8 200 LOOP MOVE L A0 A1 Sample The eight digit hexadecimal number at the left is the assembler s location counter the generated instruction word 22D8 is placed at that address The next number is the source file line number and the remainder of the line simply repeats the source line Remember that if the source lines are no longer than 80 columns then the listing file lines will not exceed 132 columns If an error is encountered or a warning is generated for a given source line then in the listing that line will be followed by a line that describes the error At the end of the listing the program prints the total number of errors and warnings There is only limited space to list the object code in this format There is sufficient space for the longest possible instruction but the DC directive p
4. a range of 32768 to 32767 bytes If neither size code is specified then the assembler will use a short branch if possible the branch destination must be known on the first pass to be within the short branch range otherwise it will use long branch 3 2 MOVEM Instruction The MOVEM instruction which is used for saving and restoring sets of registers has one the following two forms MOVEM lt register_list gt lt effective_address gt MOVEM lt effective_address gt lt register_list gt The register list may be an explicit register list of the form described in Section 4 2 3 On the other hand if a particular set of registers is to be saved and restored repeatedly the REG directive Section 4 2 3 can be used to define a register list symbol that specifies the registers For example if the register list symbol WORKSET is defined as follows WORKSET REG AOQ A4 D1 D2 then the following instructions will perform the same function MOVEM L WORKSET SP MOVEM L AO A4 D1 D2 SP If a register list symbol is used it must be defined before it appears in any MOVEM instructions 3 3 Quick Instructions MOVEQ ADDQ SUBQ The MOVE ADD and SUB instructions have one word quick variations which can be used certain addressing modes and operand values The assembler will use these faster variations automatically when possible or they may be specified explicitly by writing the mnemonic as MOVEQ ADDQ or SUBQ The MOVEQ instructi
5. age will be printed if the value will not fit as a signed or unsigned number in the data size selected If word or longword size is selected then the start of the block will be placed on a word boundary by increasing the location counter by one if necessary If an instruction follows a DCB B directive the assembler will automatically adjust the location counter if necessary to place the instruction on a word boundary 4 3 3 DS Define Storage The define storage directive generates an uninitialized block of bytes words or longwords The format of the directive is lt label gt DS lt size gt lt length gt The label will be defined to equal the address of the start of the block The size code specifies that a block of bytes B words W or longwords L is being set up if omitted word size is used The length argument is an expression that tells the number of bytes words or long words that are to be in the block This value must be known on the first pass at the point where the DCB directive appears and it must be non negative The effect of the DS directive is basically to increase the value of the location counter by lt length gt times one if DS B is used two if DS W is used or four if DS L is used If word or longword size is selected then the start of the block will be placed on a word boundary by increasing the loca tion counter by one if necessary Thus DS W O can be used to force the location counter to b
6. and quoted strings there must be no spaces or tabs within a field 2 1 1 Label Field Legal labels follow the rules for forming symbol names described in section 2 2 Labels may be distinguished in one of two ways 1 They may begin in column 1 or 2 they may end in a colon which does not become part of the label but simply serves to mark its end A line may consist of a label alone When a label is encountered in the source code it is defined to have a value equal to the current location counter This symbol may be used elsewhere is the program to refer to that location 2 1 2 Operation Field The operation field specifies the instruction that is to be assembled or the assembler directive that is to be performed A size code B W L or S may be appended to the operation code if allowed to specify Byte Word Long or Short operations respectively The operation field must not begin in the column 1 because the operation would be confused with a label 2 1 3 Operand Field The operand field may or may not be required depending on the instruction or directive being used If present the field consists of one or more comma separated items with no intervening spaces or tabs There may be spaces or tabs within an item but only within quoted strings 2 1 4 Comment Field The comment field usually consists of everything on a source line after the operand field No special character is needed to introduce the comment and it may
7. contain any char acters desired A comment may also be inserted in the source file in another way An asterisk at the beginning of the line or after the label field will cause the rest of the line to be ignored i e treated as a comment 2 2 Symbols Symbols appear in the source code as labels constants and operands The first character of a symbol must be either a letter A Z or a period The remaining characters may be letters dollar signs periods or underscores _ A symbol may be of any length but only the first 8 characters are significant Remember that capitalization is ignored so symbols which are capitalized differently are really the same 2 3 Expressions An expression may be used in the source program anywhere a number is called for An expression consists of one or more operands numbers or symbols combined with unary or binary operators These components are described below The value of the expression and intermediate values are always computed to 32 bits with no account being made of any overflow that may occur Division by zero however will cause an error 2 3 1 Operands in Expressions An operand in an expression is either a symbol or one of the following sorts of constants Decimal Numbers A decimal number consists of a sequence of decimal digits 0 9 of any length A warning will be generated if the value of the number cannot be represented in 32 bits Hexadecimal Numbers A h
8. cord requires 2 3 or 4 bytes to be represented No record is longer than 80 characters 10
9. e aligned on a word boundary without allocating any space However if an instruction follows a DS B directive the assembler will automatically adjust the location counter if necessary to align the instruction on a word boundary 4 4 END End of Source File The end directive is used to mark the end of the source file It is purely optional The format is simply END The assembler will ignore anything in the source file after the END directive 4 Usage 5 1 Command Line The 68000 Assembler is run by typing a command line of the following form ASM lt options gt lt filename gt The options are a string of letters preceded by a dash which alter the behavior of the assembler The following option letters are allowed C Show all the Constants produced by DC directives see Section 5 2 L Produce a Listing file N Produce No object code file If these options are not specified the defaults are to show only one line of data from a DC directive to produce no listing and to produce an object file The filename is the name including directory specifica tions of the file to be assembled No default file extension is applied The names of the listing and object code files if generated are constructed by using the source file name with an extension of LIS for the listing or H68 for the object file the output files are always placed in the user s default directory The program will print 68000 Assembler by PG
10. exadecimal number consists of a dollar sign followed by a sequence of hexadecimal digits 0 9 and A F of any length A warning will be generated if the value of the number cannot be represented in 32 bits Binary Numbers A binary number consists of a percent sign followed by a sequence of binary digits 0 and 1 of any length A warning will be generated if the number consists of more that 32 digits Octal Numbers An octal number consists of a commercial at sign 0 followed by a sequence of octal digits 0 7 of any length A warning will be generated if the value of the number cannot be represented in 32 bits ASCII Constants An ASCII constant consists of one to four ASCII characters enclosed in single quote marks If it is desired to put a single quote mark inside an ASCII constant then two consecutive single quotes may be used to represent one such character If the ASCII constant consists of one character then it will be placed in the bottom byte of the 32 bit value two characters will be placed in the bottom word with the first character in the higher order position If four characters are used then all four bytes will contain characters with the first in the highest order location However if three characters are used then they will be placed in the three highest order bytes of the 32 bit value with 0 in the low byte this is to accommodate the high byte first addressing used on the 68000 Note that ASCII constants
11. h the MOVEM instruction directly In order to avoid confusion it is best to avoid specifying a range that includes both an address register and a data register although the assembler will not treat this as an error 4 3 Data Storage Directives 4 3 1 DC Define Constant The define constant directive is used to store strings and lists of constants in memory The format of a DC directive is lt label gt DC lt size gt lt item gt lt item gt The label will be defined to equal the address of the start of the list of data The size code specifies that a list of bytes B words W or longwords L is being defined if omitted word size is used A list of items follows the directive each item may be an expression or a string If an item is an expression the expres sion is evaluated and stored as the size indicated i e T a byte a word or a longword An error is generated if the value will not fit as either a signed or unsigned value in the specified size If an item is a string delimited by single quotes then the string will be stored in successive entities of the size specified if words or longwords are being generated and the string does not fit into an whole number of words or longwords then the string will be padded with zeros at the end to make a whole number of words or longwords Strings and expressions may intermixed in a single DC directive If words DC W or longwords DC L are being generated then the sta
12. in expressions are different from strings in DC directives as the latter may be of any length 2 3 2 Operators in Expressions The operators allowed in expressions are shown in the following table in order of de creasing precedence Within each group the operators are evaluated in left to right order except for group 2 which is evaluated right to left Operators in Expressions 1 Parenthesized subexpressions Unary minus two s complement Bitwise not one s complement 3 lt lt Shift left x y produces x shifted left by y bits and zero filled gt gt Shift right Bitwise and Bitwise or Multiplication Integer division Modulus x y produces the remainder when x is divided by y Addition Subtraction a Er xn x 2 4 Addressing Mode Specifications The 68000 and 68010 provide 14 general addressing modes The formats used to specify these modes in assembly language programs are listed in the table below The following symbols are used to describe the operand formats Operand symbols Dn Data Register An Address Register SP may used instead of A7 Xn Data or Address register S Index register size code either W or L W will be assumed if omitted lt ex8 gt Expression that evaluates to an 8 bit value may be empty in which case 0 will be used lt exl6 gt Expression that evaluates to a 16 bit value may be empty in which case 0 will be used lt ex gt Any expression P
13. on may be used for moving an immediate value in the range 128 to 127 into a data register The assembler will assemble a MOVE L lt value gt Dn as a MOVEQ if the value is known on the first pass The ADDQ SUBQ instruction adds subtracts an immediate value from 1 to 8 to from any alterable destination The assembler will use the quick form if the value is known on the first pass to be in the range 1 to 8 3 Assembler Directives 4 1 ORG Set Origin The assembler maintains a 32 bit location counter whose value is initially zero and which is incremented by some amount whenever an instruction is assembled or a data storage directive is carried out The value of this location counter may be set with the ORG directive This is typically done at the start of a program and at appropriate places within it The format of the ORG directive is lt label gt ORG lt expression gt where lt expression gt is an expression containing no forward references 1 e its value must be known on the first pass at the point where the ORG directive appears An error will result if an attempt is made to set the location counter to an odd value in this case the location counter will be set to the specified value plus one The lt label gt is optional and if present the specified symbol will be set to the new value of the location counter 6 4 2 Symbol Definition Directives 4 2 1 EQU Equate Symbol The equate directive is used to define symbols wh
14. ose value will not change within the program The format of this directive is lt label gt EQU lt expression gt where lt expression gt is an expression containing no forward refer ences i e its value must be known on the first pass at the point where the EQU directive appears The lt label gt must be specified since it tells what symbol is being defined If lt label gt is omitted an error will result If an attempt is made to redefine a symbol that was defined with EQU either as a label or using any symbol definition directive an error message will be printed 4 2 2 SET Set Symbol The SET directive is similar in function and format to the equate directive with one important difference symbols defined using SET may be redefined later using another SET directive but not using a EQU or REG directive The format of this directive is lt label gt SET lt expression gt 4 2 3 REG Register Range Register ranges consist of lists of registers separated by slashes Each register range may be either a single register An or Dn or a range of registers An Am or Dn Dm which denotes all the registers between the two registers listed they may be given in either order For example the following register list specifies that DO D1 D2 D3 D7 Al A2 and A3 are to be saved or restored D3 D0 D7 A1 A3 The registers and ranges may be specified in any order The same format for register lists may be used wit
15. oses a problem since it may generate e g dozens of longwords from a single source line The assembler s C command line option controls the assembler s actions when the object code exceeds the space available on one line If C is not specified then the assembler will print only one listing line with an ellipsis at the end of the object code field indicating that some of the data produced by the directive was omitted from the listing If C is included on the command line then the assembler will use as many source lines as are needed to print all the data produced by the directive Each line after the first will contain only the location counter and the object code field the source line is not repeated 5 3 Object Code File Format The 68000 Assembler produces an object code output file in S record format The object file name is the source file name with the extension changed to H68 The object file and the listing file are always placed in the user s default directory The S record format is designed to allow files containing any data to be interchanged in text file format with checksum error detection The format of these files will not be described here but the following technical information will be provided The first line of the object file is an SO header record and the last line is an S9 termination record The lines in between are S1 S2 or 3 records depending on the whether the address of the first byte in a re
16. rce File 0 00002 eee ee 5 Usage SL Command Line 40 ude woh re eee dos teen hos HRS a 5 2 Listing File Format da cd mig A aie Bg Ge ee GE ls ee el EO 5 3 Object Code File Format ent a A A et 1 Introduction The program described here 68000 Assembler is a basic two pass assembler for the 68000 and 68010 microprocessors It supports the complete instruction set of both processors as well as a modest but capable set of assembler directives The program produces formatted listing files as well as object code files in S record format The program was written in VAX 11 C by Paul McKee during the fall semester 1986 The program should be portable with reasonable changes to any C language implementation that supports 32 bit integers 1 Source Code Format 2 1 Source Line Format The input to the assembler is a file containing instructions assembler directives and comments Each line of the file may be up to 256 characters long It is recommended however that the source lines be no longer that 80 characters as this will guarantee that the lines of the listing file do not exceed 132 characters in length The assembler treats uppercase and lowercase identically Each line of the source code consists of the following fields LABEL OPERATION OPERAND OPERAND COMMENT For example LOOP MOVE L AO A1 Sample source line The fields may be separated by any combination of spaces and tabs Except for the comment field
17. rt of the list of constants will be aligned on a word boundary by increasing the location counter by one if necessary This is not performed for DC B directives so that strings of bytes may be contiguous in memory If an instruction follows a DC B directive the assembler will automatically adjust the location counter if necessary to place the instruction on a word boundary An example of a DC directive that defines a null terminated string TEXT DC B DC Example 0D 0A 0 This directive results in the following data at location TEXT 44 43 20 45 78 61 6D 70 6C 65 OD OA 00 hexadecimal 4 3 2 DCB Define Constant Block The define constant block directive generates a block of bytes words or longwords that are all initialized to the same value by the assembler The format of the directive is lt label gt DCB lt size gt lt length gt lt value gt The label will be defined to equal the address of the start of the block The size code specifies that a block of bytes B words W or longwords L is being set up if omitted word size is used The length argument is an expression that tells the number of bytes words or long words that are to be in the block This value must be known on the first pass at the point where the DCB directive appears and it must be non negative The value argument is an expression whose value is to be placed in each data item in the block it needn t be known on the first pass An warning mess
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