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1. Given the simplicity of the PDP 8 architecture machine code programming is possible although tedious since operand address calculation must be done by hand Numeric code can be entered directly into the PDP 8 memory from the Debug screen display by positioning the memory cursor at the address where you want to enter the code typing the octal value and hitting enter The value will be deposited to that location and the memory cursor will advance to the next For example the following machine code program computes c la bl We display the code using the address contents convention nnnn mmmm 0200 7300 clear accumulator and link 0201 1300 load B from address 0300 0202 7041 negate complement and add 1 0203 1301 add A from address 0301 0204 7500 skip if negative 0205 5207 jump to address 0207 0206 7041 negate A B to make positive 0207 3302 deposit A B to C address 0302 0210 7402 halt 0300 0017 A 0301 0022 B 0302 0000 C Fig 21 PDP 8 machine code to compute c la bl 28 e B Shelburne To execute the above machine code do the following 1 From the Debug screen display use the PC command p 0200 to position the memory cursor at address 02000 and display page of memory 2 Beginning at address 02000 enter the code as given above 7300 1300 7041 etc After entering the last instruction 7402 at address 02100 position the memory cursor at address 03000 and enter the data 0017 0022 3 Reposition the memo2. Check if z jmp Reset If z reset sequence CH Loop A CH i CRLF Loop d Handler XHold cll XHold i XIntH e a Se OS a J MN ei a ag i A PDP 8 Emulator Program Else increment ASCII code Get a Do CR LF minus ASCII z Interrupt Handler Save AC Get character Is it a CR No continue Yes halt Get Bell Sound It Get Line Feed Display It Restore AC Return Halt Program Restart Basic I O Routines Page 30 isz jmp Reset tad dca jms jmp A rat CH 0 MZ 123 Interrupt 5000 XIntH 0 dca krb tad sna jmp cla tad USE jmp tls cla tad tsf jmp trs cla tad jmp Xhalt hlt jmp XHold 0 XCR 13d Beep 07 LF 10d 7400 see BI SMain OS code Fig 25 PDP 8 interrupt demo program 35 Note that the interrupt handler does itsown I O It does NOT make any calls to the simple character I O routines on page 30 To do so would cause problems since if an interrupt occurred during the execution of one of these I O routines the interrupt handler would overwrite the original return address In the current implementation of the PDP 8 Emulator Program an interrupt can only be generated by hitting a ke y 36 e B Shelburne 11 ADDITIONAL RESOURCES Because of the commercial success of the PDP 8 it being the first widely sold mini computer references to the PDP 8 often occur in textbooks on computer organization
3. Recall that direct addressing subroutine calls forces subroutine code to be placed on the zero or current page Placing subroutine code on every page where it is called causes unnecessary duplication and space limitations restrict the use of page zero for subroutine code The sample hello world program below uses three of the four I O subroutines Type PrtStr which prints a null terminated string of characters and CRLF which prints a Carriage Return Line Feed 26 e B Shelburne combination a newline character While not elaborate all four subroutines allow the writing of PDP 8 programs that do limited I O Basic I O Routine Vectors Page 0 0050 GetChar XGetChar Type XType CRLF XCRLF PtrStr XPrtstr Code Segment Page 1 0200 Main cla cll clear AC and Link kee clear keyboard flag tls wake up printer cla cll tad Str jms i PrtStr jms i CRLF hlt jmp Main Data Segment 0300 SEL lt a Str stores its own address Bello World 0 Null Terminates String Basic I O Subroutines Page 30 7400 XGetChar 0 store return address here ksf Is keyboard flag raised me no loop krb yes read character to AC jmp i XGetChar return XType 0 store return address here tsf is printer ready jmp 1 no loop tls yes print character cla cll clear AC and Link jmp i XType return XCRLF 0 store return address here eta cll tad 5 get CR
4. test if A Ptr gt 0 else A Ptr lt 0 add to PosSum store PosSum jump to increment Ptr add to NegSum store NegSum increment Ptr increment Count go again done allow easy restart Data starts at address 0300 address of array A pointer to item in array Count of items in array Sum of negatives Sum of positives Array A starts at address 0400 Page 2 a 5p 23 Ap a 0 Fig 14 A PAL program to separately sum the positive and negative integers in an array The third program a variant of the previous uses auto index addressing with auto index register 00100 to access elements in the array Since auto index addressing first increments the contents of the auto index register before applying indirection auto index register 00100 must be loaded 20 B Shelburne with the address of the array minus 1 This value the address of the array minus 1 is stored on page zero making the array accessible from any page Code Section 0200 code starts at address 0200 main cla cll clear AC and Link dca Count clear Count AC is zero dca PosSum clear PosSum AC is zero dca NegSum clear NegSum AC is zero tad Addr get address of array 1 dca 10 and store in auto index register 10 Loop cla cll clear AC and Link tad i 10 get A i sna if Ali 0 jmp Done then done spa test if A i gt 0 jmp 4 else A i lt 0 tad PosSum add to PosSum dca PosSum store PosSum jmp 43 ju
5. 11 with bit O being the most significant bit msb gt lt lsb 4 4 4 4 4 4 4 0 1 2 3 4 5 6 7 8 9 TOs lt T Fig 5 Bit Numbering for a PDP 8 Word Memory is partitioned into 32 pages of 128 words each A 12 bit address is divided into a five bit page number bits 0 4 and a seven bit offset bits 5 11 For example address 02000 is page 01o offset 0000 a postfix o denotes octal The reason for this page offset scheme will be clearly seen when the format of memory reference instructions is examined There is a single 12 bit accumulator AC with a one bit link register L that captures any carry out of the accumulator Many PDP 8 instructions make implicit use of the link accumulator pair Link bit Accumulator 0 I 2 3 4 9 6 7 8 9 1O TI Fig 6 The Link Accumulator Pair A 12 bit multiply quotient register MQ is used by multiplication and division operations in PDP 8 models augmented with the Extended Arithmetic Element EAE instructions are not implemented by the PDP 8 Emulator Program Special purpose registers include a 12 bit Switch Register SR used to enter values from the console of the PDP 8 a 12 bit Program Counter PC which holds the address of the next instruction to be executed a three bit Instruction Register IR a 12 bit Central Processor Memory Address register CPMA and a 12 bit Memory Buffer registe
6. 1999 Structured Computer Organization 4 ed Prentice Hall Upper Saddle River NJ Received November 2001 accepted February 2002
7. device number 03 for input and a printer actually a display screen device number 04 for output The list of 22 B Shelburne opcode 6 I O instructions implemented on the PDP 8 Emulator Program for these two devices is given below Opcode 6 Instruction Format 4 4 4 4 4 4 4 4 4 4 4 4 i i 1 QO device number function 4 4 4 4 4 4 4 4 4 4 4 0 1 2 3 4 5 6 7 8 9 10 11 Mnemonic Opcode Full Name Description KFC 6030 Clear Keyboard Flag KSF 6031 Skip on Keyboard Flag Set KCC 6032 Clear Keyboard Flag and Accumulator KRS 6034 Read Keyboard buffer Static AC4 AC11 AC4 AC11 or Keyboard Buffer KRB 6036 Read Keyboard Buffer dynamic C AC 0 Keyboard Flag 0 AC4 AC11 AC4 AC11 AC4 AC11 or Keyboard Buffer TFL 6040 Set Printer Flag TSF 6041 Skip on Printer Flag Set TCF 6042 Clear Printer Flag TPG 6044 Load Printer Buffer with Accumulator and Print Printer Buffer AC4 AC11 TES 6046 Load Printer Sequence Printer Flag 0 Printer Buffer AC4 AC11 Table 6 Opcode 6 T O Instructions T O on the PDP 8 is done by transferring eight bit ASCII characters between the right most eight bits of the Accumulator AC4 AC11 and an eight bit I O device buffer keyboard or printer display Device Flag 10 1 0 busy 1 ready 4 lt gt 4 AC4 AC11 I O Buffer Fi
8. see Kraft and Toy Stallings or Tanenbaum An excellent source for information on the PDP 8 is Bell and Newell s book Computer Structures Reading and Examples which has a very nice article on the PDP 8 including a very nice ISP instruction set processor description Another source of information is Bell Mudge and McNamara s Computer Engineering A DEC View of Hardware Systems Design which deals more with the hardware aspect of the PDP 8 within the context of other DEC computers A more current text is Blaauw and Brook s Computer Architecture Concepts and Evolution which also contains a detailed architectural description of the PDP 8 set in the context of other computer architectures And I would be negligent if I failed to mention the availability of numerous web based resources for the PDP 8 a simple search on the word PDP 8 will bring up numerous sites with links to color photos of PDP 8 s A good web site to start at is Doug Jones DEC PDP 8 Index http www cs uiowa edu jones pdp8 which contains among other things a very nice link to a Programmer s Reference Manual for the PDP 8 http www cs uiowa edu jones pdp8 man PDP 8 hardware and software manuals when available are a major source for understanding the inner workings of the PDP 8 In particular the two texts from the PDP 8 Handbook series listed under References provided a wealth of programming information and techniques for the PDP 8 Most of the p
9. about commands The Write command dumps the contents of PDP 8 memory to an ASCII text file The Load option can be used to load this file back to PDP 8 memory Using the format mmmm nnnn where mmmn is the address and nnnn is the contents the Write command outputs to a file each non zero word in PDP 8 memory The final line output is the value of the Program Counter written as nnnn Any PDP 8 object code file in this format whether generated by the Write command or created by an ASCII text editor can be loaded using the Load command 6 e B Shelburne 2 3 The Editor Assembler Pea EE Editer Fig 3 Editor Assembler Screen Display The Editor Assembler screen display allows the user to create edit debug and assemble PDP 8 Assembler Language pal programs Selecting Edit Text puts the user in Edit mode the cursor appears in the text area where the user can enter and edit assembler language programs The built in editor is simple and obvious to use but does not have cut and paste capabilities Use Esc or F10 to exit Edit mode Assemble uses a two pass assembler to assemble the current contents of the text window directly to PDP 8 memory linkers and loaders are not needed A successful assemble will allow the user the option of creating a list file which cross lists source code with object code Successfully assembled code in memory can be executed from the Debug screen display or the Run PDP 8 screen display A syntax
10. groups are neither permitted nor necessary Like Group One some Group Two combinations have their own mnemonics LAS Load Accumulator with Switch Register is the mnemonic for CLA OSR Opcode 7 Group Three Microinstructions bit 3 1 and bit 11 1 1 J 1 1 cla mga mql Le ill 0 1 2 3 4 5 6 7 8 9 10 11 Mnemonic Opcode Full Name Description priority CLA 7601 Clear Accumulator 1 MQA 7501 Or Accumulator with MQ register 2 C AC C AC 1 C MQ MOL 7420 Load MQ register from Accumulator and Clear Accumulator 2 C MQ C AC C AC 0 SWP 7524 Swap AC and MQ registers 3 CAM 7621 Clear AC and MQ registers 3 Table 4 Opcode 7 Group Three Microinstructions Note Extended Arithmetic Element operations which are not supported by the PDP 8 Emulator Program would appear here as Opcode 7 Group Three microinstructions A PDP 8 Emulator Program 13 4 PDP 8 MEMORY REFERENCE INSTRUCTION MRI ADDRESSING MODES 4 1 Zero Current Page Direct Addressing Because the format of a PDP 8 memory reference instruction only has room for a seven bit operand field zero page and current page addressing are used to obtain a 12 bit effective address The addressing mode is determined by bit 4 of the instruction With zero page addressing bit 4 0 five zeros are concatenated to the front of the seven bit offset to obtain a 12 bit address
11. of the array which for this program is kept on a different page A PDP 8 Emulator Program 19 Indirect addressing requires that the address of the array be stored in a variable Addr then loaded into a second variable Ptr which is incremented using ISZ at the end of each pass through a loop As each element in the array is accessed it tested for being greater than zero using the SPA SNA Skip on Positive Accumulator and Skip on Non Zero Accumulator combination If true the jump instruction following the test is skipped and the value is added to PosSum otherwise the jump statement transfers control to a segment of code which adds the value to NegSum The program loops until a zero entry in the array is found Code Section 0200 main cla cll dca Count dca PosSum dca NegSum tad Addr dca Ptr Loop cla GLI t d i Ptr sna jmp Done spa sna jmp 4 tad PosSum dca PosSum jmp 3 tad NegSum dca NegSum isz Ptr isz Count jmp Loop Done hlt jmp Main Data Section 0300 Addr A Ptt Q Count 0 NegSum 0 PosSum 0 Array 0400 Aye E2 derbi 139 SMain De a r a oO aie ora a r a a aa OEE OE EO EO EO EOE JS ao a i anan a 937 code starts at address 0200 clear AC and Link clear Count AC is zero clear PosSum AC is zero clear NegSum AC is zero get address of array and store in Ptr clear AC and Link get A Ptr if A Ptr 0 then continue else done
12. that continues until a null character is detected Finally to make the code more readable we assign the value 00100 to the symbol IR10 Index Register 100 using IR10 10 A PDP 8 Emulator Program 25 Subroutine Vectors 0050 subroutine entry point stored at address 00500 Type XType Code Segment 0200 Code Segment starts at 02000 Start cla eti Clear AC and Link tls Wake Up Printer tad STR Get address of output string dca IR10 and store it in Index Register 10 Loop tad i IR10 Get next character sna Skip if not a Null character jmp End else end jms i Type Type it jmp Loop Loop End hilt Done jmp Start Data Segment 0250 Place String at 02500 STR STR stores its own address string data follows Hello World 13d 10d 0 IR10 10 Index Registerl0 is address 10 Subroutine Segment Page 30 7400 XType 0 Store Return Address Here tsf Is Printer Ready jmp 1 No Loop Wait Loop tls Yes Print the character la yell Clear AC and Link jmp i XType Return Start Fig 19 A PDP 8 Hello World Program 8 A PDP 8 BIOS BASIC INPUT OUTPUT SUBROUTINES Given below is a set of four basic I O subroutines for the PDP 8 This code fits on a single page page 30 and is made accessible to any page in memory by storing the starting addresses for the subroutines on page zero The subroutines are called using indirect subroutines calls e g jms i Type not jms Type
13. the memory cursor which automatically advances to the next location Write and Load commands to save and restore the current contents of the PDP 8 memory require path names that conform to MS DOS file naming conventions That means all file and directory folder names are restricted to letters digits and or underscore characters and are 8 characters or less For example at the gt prompt gt w c pgm01 obj will save the current contents of memory to a text file I use obj for object code The same code may be loaded using gt l c pgm01 obj The Write command scans PDP 8 memory and saves every non zero location using the address contents format nnnn mmmm For example the program assembled using the a command above when saved to a text file using the w command would appear as 30 B Shelburne 0200 7300 0201 1300 0202 1301 0203 3302 0204 7402 0205 5200 0300 0004 0301 0005 0302 0011 0205 Fig 22 Text file from Debug screen w command The current value of the memory cursor is saved as nnnn The Load command expects a file in the format created by the Write command Load does no syntax checking and since it ignores all characters past the ninth comments may be attached to each line PDP 8 programs that do I O using opcode 6 instructions can be run from the Debug screen display Output generated by a PDP 8 program will appear in yellow to distinguish it from user command input in the Command window However when using t
14. 0 The directive nnnn where nnnn is any octal value sets the value of the location counter used by the assembler It is used to specify where assembled code is placed in memory the PDP 8 Emulator Program generates non relocatable code The directive abel indicates the end of the program and specifies label as the entry point address of the first instruction for the program Symbolic addresses identifiers and labels must begin with a letter and contain only letters and digits and should not exceed eight characters Special characters and embedded blanks are not permitted The letter A or A is reserved for indirect addressing Symbolic addresses are not case sensitive The default radix is 8 and all numbers are considered octal unless they contain an 8 or 9 or have a suffixed d decimal A suffixed o octal is also permitted but not necessary The directives decimal and octal can be used to change the default radix beginning at the line containing the directive A number of characters have special meaning or use 16 e B Shelburne Symbol Use Meaning Ii reserved for indirect addressing slash denotes that a comment follows comma terminates a symbolic address but is not part of the symbolic address dot denotes the current address of the instruction asterisk sets the value of the location counter dollar sign indicates the end of the program semi colon terminates a statement used for multi
15. 1 6699 used by other ASCII editors a tab character appears as an o in the Edit text window so tabs should be avoided The built in editor does not generate tabs it inserts spaces for tabs 10 3 The Run PDP 8 Pgm Screen Paper Tape Option A teletype was the standard I O device for the PDP 8 A teletype could have an 8 hole paper tape reader punch attached to it Eight hole paper tape had 8 tracks spanning the width of the tape where the presence or absence of a hole encoded a 1 or 0 Octal values between 000 and 377 could be written to and read from paper tape which gave the PDP 8 a limited file handling capability files being paper tape track 87654321 LIIIIILI 377 lt octal values LILIIIL 377 LLI Won LLI 102 Nnnn 000 aa a a a 073 NNNUNN 000 nyunungyo 102 nnn Wl 001 wuunpaye 012 nn LL nnn 050 Fig 23 Eight Track Paper Tape A paper tape reader punch is simulated on the PDP 8 Emulator Program by the Punch and Reader options on the Run PDP 8 Screen display These options allow a PDP 8 program to read from or write to a text file instead of paper tape When either or both options are selected the user is prompted for the path name of the text file MS DOS file naming conventions required and for whether the file is ASCII characters or inary Binary format encodes rows of 8 bits as lines of three digit octal integers between 000 and 377 Binary format was included so that the PDP 8 Emulator Progra
16. 2 and 64 which is 32 32 1 1056 or 20400 This code demonstrates PDP 8 conditional branching which is done by pairing a test and skip instruction with a jump instruction It also uses dot notation to assemble the target addresses for jump instructions Code Section 0200 code starts at address 0200 main cla cll clear AC and Link tad N load N cia negate it dca Count deposit it to Count and clear AC dca X clear X AC is zero dca Even clear Even AC is zero dca Odd clear Odd AC is zero Loop isz X add 1 to X cla cll clear AC and Link tad X load X rar rotate least significant bit into Link snl skip on non zero link X is odd jmp 5 i else X is even ral restore X tad Odd and add to dca Odd Odd jmp 4 jump to end of loop ral restore X tad Even and add to dca Even Even isz Count increment Count and skip on zero jmp Loop otherwise Loop hlt done jmp Main allow easy restart Data Section 0300 Data starts at address 0300 N 100 N equals 64 Count 0 Loop Counter xX 0 added to Odd or Even Odd 0 Sum of Odds Even 0 Sum of Evens SMain Fig 13 A PAL program to separately sum even and odd integers Note The cla c11 instruction at Loop 1 is not necessary since the AC will always be cleared at this point The next program separately sums the negative and positive values in an array Indirect addressing is required to access the elements
17. 7 To execute this program do the following 1 Start the PDP 8 Emulator Program and go to the Editor Assembler screen Go into Edit mode Edit Text and enter the above code as is When done use Esc or F10 to exit Edit mode 2 Select Assemble to assemble the code to memory If there are errors fix them and try again until you get a clean compile Creating a list file is optional You may want to save your source code to a file 3 Quit the Editor Assembler and go to the Debug screen display Since this PDP 8 program does no I O you can only see the results of the execution from the Debug screen When you enter the Debug screen display you should see the memory cursor positioned at address 02000 of page 1 of memory If not use the arrow keys to move it so the PC contains the correct starting address 4 To execute the code at the gt prompt in the Command window type g Enter If the program executes correctly you should see the result 670 55d at address 03030 The memory cursor should be highlighting the 5200o instruction jmp Main at address 02150 5 To re execute the program either use the arrow keys to reposition the memory cursor at address 02000 or start where the previous execution left off at address 02150 with the jmp Main instruction If you single step through the program using the space bar observe how the accumulator changes You may want to reset Sum address 03030 to zero and or enter a different value f
18. A PDP 8 Emulator Program BRIAN J SHELBURNE Wittenberg University The clean simple and elegant architecture of the classic PDP 8 makes it an ideal candidate for studying concepts in computer organization The PDP 8 Emulator Program allows a user to write edit assemble debug trace and execute PDP 8 machine code and PDP 8 assembler language programs With it the user can obtain a feel for the PDP 8 The PDP 8 Emulator Program includes a simple built in text editor which is used to write and edit PDP 8 Assembler Language programs an assembler to translate these programs into PDP 8 machine code and a virtual PDP 8 engine upon which to execute the code PDP 8 code can be executed from a debug screen display that allows the user to observe the contents of registers and memory as the code executes or code can be executed using an I O interface that requires user written PDP 8 T O routines This paper provides an introduction to both the PDP 8 architecture and PDP 8 Assembler Language and discusses how to use the PDP 8 Emulator Program The PDP 8 Emulator Program was written using Borland s Turbo Pascal v 6 0 and runs under MS DOS in a Command window Categories and Subject Descriptors C 0 Computer Systems Organization General Modeling of Computer Architecture B 0 Hardware General PDP 8 1 6 5 Simulation and Modeling Model Development K 2 History of Computing K 3 1 Computers and Education Computer Uses in Education General
19. Terms Design Additional Keywords and Phrases computer architecture simulator education 1 INTRODUCTION The PDP 8 Emulator Program pdp8main exe simulates the PDP 8 architecture on an Intel 80x86 computer With it a user can edit and execute PDP 8 machine code or PDP 8 assembler language programs and thereby develop a feel for the PDP 8 The design philosophy behind the PDP 8 Emulator Program was to incorporate as much as possible the underlying architecture of an actual PDP 8 This philosophy accounts for some of the PDP 8 Emulator Program s unusual features Much of the structure terminology and notation used in the PDP 8 Emulator Program reflects that used by a real PDP 8 The PDP 8 Emulator Program makes use of a number of display screens One display the debug screen allows the user to observe the contents of memory and all registers From this display screen a user can enter PDP 8 machine code directly into memory and observe its execution Another display is a small built in text editor that allows the user to create and edit PDP 8 Assembler Language PAL programs An assembler can be invoked to translate the PAL code Author s address Department of Mathematics and Computer Science Wittenberg University Springfield Ohio 45501 USA e mail bshelburne wittenberg edu Permission to make digital hard copy of part of this work for personal or classroom use is granted without fee provided that the copies are not made or distri
20. With current page addressing bit 4 1 the leading five bits page number of the address of the MRI instruction is concatenated to the seven bit offset Thus an MRI instruction can only directly reference an operand that is either on page zero or on the same page as the instruction itself This means that most operands have to be fairly close to the MRI instruction that references them Example zero page addressing Address 64240 contains the instruction 11670 written 64240 11670 Since bit 4 of the instruction is zero the effective address of this TAD instruction is obtained by concatenating five zeros to the seven bit offset for an effective address of 01670 1 1 6 7 op IM offset 001 001 110 111 gt 001 O 0 1 110 111 0 pi 6 i7 effective address gt 00000 1 110 111 000 001 110 111 Fig 9 Zero Page Addressing Example Example current page addressing Given 64240 13670 where bit 4 of the instruction is 1 the effective address of this TAD instruction is obtained by concatenating the leading five bits of the address of the instruction 110 10 from 110 100 010 100 64240 to the seven bit offset for an effective address of 65670 6 4 2 4 1 1 6 7 110 100 010 100 lt address contents gt 001 001 110 111 110 01 lt page number 1 110 111 lt offset effective address gt 110 10 1 110 111 65670 Fig 10 Current Page Addressing Example Since page zero is accessible from any address in memory global variables a
21. al programs A successfully assembled program can be executed in the Debug Screen display or through the Run PDP 8 Pgm Screen display Help On line help is provided for the PDP 8 Assembler Language and for the PDP 8 Emulator Program F1 also invokes help Run PDP 8 Program This provides a simple I O interface for executing PDP 8 programs 4 e B Shelburne 2 2 The Debug Screen PITAM CoE HHH Se Heel IR An iH I I aw H Fig 2 Debug Screen Display All CPU Registers appear on the upper left The first row displays the Link bit used for a carry out and the 12 bit Accumulator On the second row are octal displays for the same Link Accumulator pair plus the Multiplier Quotient register Next is the Program Counter and the three bit Instruction Register followed by the Central Processor Memory Address register and Memory Buffer register followed by the Switch Register and Run bit on the fifth row One 128 word page of memory is displayed in octal on the upper right 16 rows of 8 words with row and column headers for addressing PgUp and PgDn can be used to display previous and successive pages of memory The highlighted position is the memory cursor which can be moved using the arrow keys Typing an octal number will insert it at the memory cursor whose address always appears in the Program Counter Figure 2 displays page 1 of memory words 0200 octal through 0377 Unlike the other display screens commands for
22. anches Group three functions use the MQ register and include extended opcodes for the optional Extended Arithmetic Element The EAE implements in hardware an extended set of operations including multiplication and division These operations are not implemented by the PDP 8 Emulator Program Opcode 7 Group One Microinstructions bit 3 0 1 1 1 O Jcla ecll cma cml rar ral 0 l1l iac 0 al 2 3 4 5 6 7 8 9 10 11 Mnemonic Opcode Full Name Description priority NOP 7000 No Operation CLA 7200 Clear Accumulator 1 CLL 7100 Clear Link 1 CMA 7040 Complement Accumulator 2 CML 7020 Complement Link 2 IAC 7001 Increment Accumulator 3 RAR 7010 Rotate Link Accumulator pair Right 4 RTR 7012 Rotate Link Accumulator pair Right Twice 4 RAL 7004 Rotate Link Accumulator pair Left 4 RTL 7006 Rotate Link Accumulator pair Left Twice 4 Table 2 Opcode 7 Group One Microinstructions 12 e B Shelburne Note Priority numbers 1 4 determine the order in which combined instructions are executed Priority 1 instructions e g CLA are executed before priority 2 e g CMA etc The combination CLA CMA would clear the accumulator then complement it thereby setting all bits to 1 Some combinations have their own mnemonics For example CMA IAC Complement Accumulator and Increment Accumulator equivalent to negating the accumulator is
23. ator could use the SwitchReg AddrLoad and Deposit switches to enter a small program like a loader directly into PDP 8 memory Using SwitchReg AddrLoad and Examine an operator could also examine verify the contents of memory Both of these actions can be simulated by the PDP 8 Emulator Program Another action performed from the PDP 8 console is to set the Program Counter to the starting address of a program The Switch Register can be loaded with the starting address the AddrLoad switch can be used to copy the contents of the Switch Register to the Program Counter and the Go option below can be used to start execution Go This starts execution of the program in PDP 8 memory beginning with the instruction whose address is contained in the PC register Execution continues until either a halt instruction is encountered or the user hits Ctrl C abort Clear This clears the screen and the contents of all registers Memory is untouched 3 A QUICK OVERVIEW OF PDP 8 ARCHITECTURE The following sections provide a quick introduction to the architecture of the PDP 8 and to PDP 8 Assembler Language PAL programming The sections are brief and do not cover all details but knowledgeable readers should be able to learn enough to write and execute simple PDP 8 programs using the PDP 8 Emulator Program A PDP 8 Emulator Program 9 The memory of the PDP 8 consists of 4096 2 3 twelve bit words Bits in a word are numbered left to right 0
24. buted for profit or commercial advantage the copyright notice the title of the publication and its date of appear and notice is given that copying is by permission of the ACM Inc To copy otherwise to republish to post on servers or to redistribute to lists requires prior specific permission and or a fee 2001 ACM 1531 4278 01 0600 0001 5 00 2 B Shelburne into machine code that can be executed by the virtual PDP 8 PDP 8 programs can be executed from the debug screen or executed under a simple I O interface called the Run PDP 8 screen The latter requires writing PDP 8 I O routines Use of the PDP 8 Emulator Program requires some understanding of PDP 8 architecture The PDP 8 is a 12 bit word addressable machine with a single 12 bit accumulator and 4096 words of memory partitioned into 32 pages of 128 words each Instructions are fixed length and are 12 bits wide A three bit opcode field permits only eight instructions although two of the eight instructions opcodes 6 and 7 use the other nine bits as extended opcodes In particular opcode 7 is a family of orthogonal micro instructions which can be combined to generate a surprisingly rich set of operations There are four addressing modes zero page addressing current page addressing both forms of direct addressing indirect and auto index indirect The first two reflect the partition of memory into 32 pages of 128 words each The last one is used to handle data structures l
25. ddress contents pairs of octal numbers with the address and corresponding contents each split over two rows of paper tape Only six tracks of the paper tape were needed for each half integer To indicate an address bit 7 was set to one for the upper half of the address Thus address contents pair 0200 7300 would be encoded as the four octal values 102 lt bit 7 is 1 address 000 07 3 lt bit 7 is 0 contents 000 The RIM loader combined pairs of numbers by reading the first six bits shifting them to the left then literally r ing n the second six bits An address was deposited to location 77760 bit 7 of the first number read was a 1 an instruction it was deposited to the location pointed to by location 77760 the skip on non zero link at address 77720 determined which This continued until there was no more input to read The RIM loader was synchronized by prefixing a number of 3770 s to the tape before the code the skip on positive accumulator at 77640 synchronized the program with the tape To use the RIM loader to load a program first enter the RIM code into PDP 8 memory To make this more interesting use the Switch Register Load Addr Deposit and Examine switches to enter the RIM loader from the console Next create a text file in Asinary format containing the code you want to load lines of three digit octal values between 000 and 377 making sure to insert lines of 377 s at the beginning and end of the file for synchron
26. error will halt the assembly process putting the user in edit mode and placing the cursor on the line but not at the point in the line where the error occurred This happens for each syntax error the process being similar to the way Borland Turbo Pascal compilers found and reported syntax errors Edit Text puts the user in edit mode Include File can be used to insert a text file at the cursor position in the text area New Text clears the text area and puts the user in edit mode Read File reads a text file into the text area after first clearing it Save Text will save the contents of the text area Path names used by the Include Read and Save commands must conform to MS DOS file naming conventions A PDP 8 Emulator Program 7 2 4 The Run PDP 8 Program Screen FFFA FET i Waa aeua gin paia giti L oa i f chan Fig 4 Run PDP 8 Program Screen Display The Run PDP 8 screen display provides an I O interface in which to execute PDP 8 programs with the upper portion functioning as a 20 line display Only an executing PDP 8 program can generate output to this display The contents in octal of seven registers are displayed These are some of the register displays that appeared on the PDP 8 console Of the nine selections the first five SwitchReg through Go correspond to switches found on a PDP 8 console Their use is discussed below The sixth Clear clears the screen display and sets all registers to 0 leaving the conte
27. ffer for display Both subroutines begin by entering a wait loop that repeatedly checks if the device flag is set ready When the device flag is set control breaks out of the loop allowing both to execute a transfer instruction to move the byte between the device buffer and the accumulator The program terminates when the user hits a carriage return ASCII code 13 Main Code Segment 0200 Main cla cll clear AC and link kee clear keyboard flag tis wake up printer display unprintable null Again jms i GetChar read a character dca Ch store it tad Ch get the character and tad MCR check if its a CR sna if not skip next instruction jmp Done else done cla cll clear AC tad Ch and load character jms i Type echo to screen jmp Again go again Done hlt jmp Main I O routines 0250 GetChar 0 return address here ksf is keyboard flag raised jmp k no loop krb yes read character to AC jmp i GetChar return Type 0 return address here tsk is printer flag raised jmp 1 no loop tls yes print character cla cll clear AC and link jmp i Type return Data Segment 0300 Ch 0 character MCR 13d minus ASCII code for CR SMain Fig 18 A PAL program to read and echo characters 24 B Shelburne Use of the KCC instruction at address 0201o to initially clear the keyboard flag seems to be a standard trick The same seems to be true for the TLS instructi
28. g 17 Accumulator Device Flag Device Buffer Schematic A device flag bit handles synchronization between the CPU and I O device When the I O device is busy the flag is cleared to 0 when the device is ready the flag is set to 1 To perform I O the CPU executes a wait loop that checks the device flag When the I O device is ready to send receive a character it sets the device flag to 1 The CPU detects that the device flag is set using a Akip on flag set nstruction and breaks out of the wait loop Once out of the wait loop the CPU transfers the byte in the 8 bit device buffer to from the right most 8 bits of the accumulator and clears the device flag using a Aransfer and clear flag nstruction A PDP 8 Emulator Program 23 While ten opcode 6 I O instructions are supported by the PDP 8 Emulator Program only five seem to be used two for output to the printer display TSF Skip on Printer Flag Set and TLS Load Printer Sequence and three for input from the keyboard KSF Skip on Keyboard Set KCC Clear Keyboard Flag and KRB Read Keyboard Buffer I O is best done with code consolidated into subroutines The following program demonstrates the standard way to read and write characters by reading and echoing a character typed from the keyboard It uses two I O subroutines GetChar and Type GetChar reads a character from the keyboard returning it in the accumulator Type transfers an eight bit character in the accumulator to the output bu
29. given the mnemonic CIA 7041 Complement and Increment Accumulator STL Set Link to One is the mnemonic for CLL CML GTK Get Link put the link in bit 11 of the accumulator is the mnemonic for CLA RAL Opcode 7 Group Two Microinstructions bit 3 1 and bit 11 0 4 4 4 4 4 4 4 4 4 4 4 4 1 1 1 1 cla sma sza snl 0 1 osr hlt 0 4 4 4 4 4 4 4 4 4 4 4 4 0 1 2 3 4 5 6 7 8 9 10 11 Mnemonic Opcode Full Name Description priority SMA 7500 Skip on Minus Accumulator 1 SZA 7440 Skip on Zero Accumulator 1 SNL 7420 Skip on Non zero Link 1 SPA 7510 Skip on Positive Accumulator 1 SNA 7450 Skip on Non Zero Accumulator 1 SZL 7430 Skip on Zero Link 1 SKP 7410 Skip Always 1 CLA 7600 Clear Accumulator 2 OSR 7404 Or Switch Register with Accumulator 3 C AC C SW 1 C AC HLT 7402 Halt 4 Table 3 Opcode 7 Group Two Microinstructions Note The first three instructions SMA SZA and SNL are the conditional skip instructions The second three instructions SPA SNA and SZL are obtained by inverting the logic of the first three indicated by bit 8 1 Combinations of SMA SZA and or SNL will skip if at least one condition is true For example SMA SZA will skip if AC is less than or equal to 0 Combinations of SPA SNA and or SZL will skip if all conditions are true For example SPA SNA will skip if AC is greater than 0 Combinations between the two sub
30. he space bar to single stepping through a program that is trying to read input the interface has difficulty in distinguishing between input meant for the PDPB8 program and user command input To get around this problem any string of input preceded by a single quote is ignored by the command interface and used as input to the PDP 8 program Ctrl C will halt any executing program Illegal Debug screen commands are ignored there is no error message 10 2 The Editor The text editor is simple and obvious to use but it does not have block copy and paste capabilities and lines are restricted to 77 characters in length Inserting characters into the middle of a line of 77 characters will delete characters at the end adding characters at the end will start a new line Two lines can be joined by positioning the cursor on the first character of the second line and hitting backspace The Ins key will switch between insert mode a flat cursor and overwrite mode a block cursor Ctrl Y will delete an entire line The Read Include and Write commands prompt the user for file and path names that must conform to MS DOS file naming conventions Assembler programs can be created and edited using any ASCII text editor e g Notepad then Read or Included into the Editor text area to be assembled Assemble will only assemble the current contents of the Edit text area The assembler does not recognize tab characters that may be A PDP 8 Emulator Program 3
31. his will execute a memory read whereby the contents of memory whose address is in the CPMA register will appear in the MB register where it can be checked The CPMA and PC registers are subsequently incremented 10 5 The Run PDP 8 Pgm Screen The RIM Loader The following sixteen word program called the RIM Read Input Mode loader was used to load PDP 8 programs from paper tape see Introduction to Programming PDP 8 Handbook Series p 5 5 It is one of a number of such loaders used by the PDP 8 The RIM loader is first loaded into memory by hand since it was small then paper tapes of other programs were prepared and read in by the RIM loader 7756 6032 clear keyboard flag and accumulator 7757 6031 skip on keyboard flag set 7760 5357 jump 1 7761 6036 read keyboard 7762 7106 clear link and rotate accumulator link twice left 7763 7006 rotate accumulator link twice left 7764 7510 skip on positive accumulator bit 0 0 7765 5357 else jump to address 7757 7766 7006 rotate accumulator link twice left 7767 6031 skip on keyboard flag set 7770 5367 jump 1 7771 6034 or keyboard buffer with accumulator 7772 7420 skip on non zero link 7773 3776 deposit AC to effective address contained at 7776 clear AC 7774 3376 deposit AC to 7776 7775 5356 jump to 7756 Fig 24 RIM Loader see Introduction to Programming PDP 8 Handbook Series p 5 5 A PDP 8 Emulator Program 33 The format for RIM loadable code consisted of a
32. ike strings and arrays The PDP 8 Emulator Program only supports the features found in a minimally configured PDP 8 For example it only supports integer addition other operations like multiplication and division must be done in software This design decision was made to keep the PDP 8 Emulator Program simple and easy to use yet despite its simplicity the PDP 8 is elegant in its ability to implement complex operations with a limited instruction set A real PDP 8 had the option of including an Extended Arithmetic Element unit that did multiplication and division in hardware The PDP 8 Emulator Program can accept and execute PDP 8 machine code or it can assemble and execute PDP 8 Assembler Language PAL programs Since assembler is the primary way to write programs using the PDP 8 Emulator Program much of the following paper is a quick tutorial in PDP 8 Assembler Language Why the PDP 8 The PDP 8 was introduced in 1965 and by today s standards its architecture and capabilities are archaic truly a dinosaur except the PDP 8 was small However its simple design and low cost made the PDP 8 a very successful computer 50 000 units were produced Because of its simple design PDP 8 assembler is easy to understand write and use Even PDP 8 machine coding can be done without too much difficulty Assembler languages for today s machines have a steep learning curve and machine code is out of the question A toy computer simulator could provide an eas
33. ined For example the following PAL code performs the operation c a b code section 200 code begins at 02000 Main cla cll clear AC and Link tad B load B cia negate it tad A add A AC A B dca C store at C hlt halt jmp Main to continue go to Main data section 0300 data begins at 03000 A 17 17 octal 15 decimal B 22 22 octal 18 decimal c 0 SMain Fig 11 A PAL program to compute c a b A PDP 8 Emulator Program 15 Note To load the accumulator first clear it CLA CLL then add in TAD the value to be loaded The CIA Complement and Increment Accumulator instruction negates the accumulator Recall that CIA is a mnemonic for CMA IAC 66 99 An offset can be a number a symbolic address a dot used to denote the current address of the instruction or a simple expression using the above For example jmp 6 could be used in place of jmp Main and since the address of B is one more than the address of A tad A 1 could be used in place of tad B To allocate storage for variables and constants use a symbolic address comma followed by the value use 0 if you don care about the initial value By default all values are octal To specify a decimal use a suffixed d or use an 8 or 9 as a digit see example following Data should be located on the same page as code or on page zero for global variables A 15d decimal 15 B 18 decimal 18 since 8 is not an octal digit Cy
34. ization From the Run PDP 8 Program screen activate the paper tape reader by selecting Reader When prompted enter the file name and select binary mode To start the RIM loader enter starting address 77560 into the Program Counter register using the Switch Register and Load Addr buttons and select the Go button The loader program should read the binary file into memory When you think the file is read use Ctrt C to stop the program equivalent to hitting the Stop button 10 6 PDP 8 Interrupt System Device 0 for opcode 6 was the PDP 8 interrupt system When the interrupt system is enabled an external interrupt immediately disables the interrupt system and executes an automatic JMS 0 instruction There are a number of opcode 6 interrupt instructions but only three are implemented by the PDP 8 Emulator Program Mnemonic Opcode Full Name Description SKON 6000 Skip if interrupts enabled and disable interrupts ION 6001 Execute the next instruction then enable interrupts IOF 6002 Disable interrupts Table 7 PDP 8 Interrupt Instructions 34 B Shelburne T O can be done more efficiently using interrupts If the interrupt system is enabled whenever an T O device needed to be serviced for example a key is press on the keyboard or the printer is ready to print another character an interrupt will be generated This causes an automatic subroutine jump to location 0 JMS 0 where an interrupt handler can Apoll he different devices to dete
35. jms XType Ps and type it tad 4 get LF jms XType and type it jmp i XCRLF return 13d 10 carriage return line feed XPrtStr 0 dca 10 store address at index reg 10 tad i 10 get character sna is it null jmp i XPrtStr yes return jms XType no type it jmp 4 get next character SMain Fig 20 A PDP 8 Hello World Program using Basic I O Subroutines A PDP 8 Emulator Program 27 GetChar reads a character from the keyboard returning it in the accumulator Type displays the ASCII character stored in the eight right most bits of the accumulator and clears the accumulator CRLF outputs a carriage return line feed combination a newline character PrtStr print string displays a null terminated string whose address is passed to the subroutine using the accumulator The first word of the string must contain its own address which is why the code sequence cla cll tad Str precedes the jms i PrtStr instruction PrtStr immediately deposits the address to auto index register 10 then uses auto index addressing to load each character and checks that it not null before calling the Type command to display it Note that the accumulator is used to pass parameters to three of the subroutines Code to read and write integers is left to the reader Reading and writing unsigned 4 digit octal values is fairly easy to do but code to read and write decimal integers requires multiplication and division by decimal 10 9 PDP 8 MACHINE CODE
36. m could simulate using a loader program to load programs from paper tape discussed below When either option is selected it is highlighted in yellow The Reader option disables PDP 8 program input from the keyboard and the Punch option disables PDP 8 program output to the screen 10 4 The Run PDP 8 Pgm Screen Program Loading The PDP 8 Emulator Program allows users to enter code directly into memory from the Debug screen display or to assemble programs to memory from the Editor Assembler display Machine 32 B Shelburne code can also be loaded into memory from the Run PDP 8 Program screen with the Switch Register Load Addr Deposit and Examine switches using a technique similar to one used to enter a program from the console of a real PDP 8 To load code by hand into PDP 8 memory do the following 1 Use Switch Reg to enter the address of the first instruction into the Switch Register 2 Use Load Addr to copy the contents of the Switch Register to the CPMA register and the Program Counter PC register Use Switch Reg to enter the first instruction into the Switch Register 4 Use Deposit to deposit the contents of the Switch Register to the MB register and execute a memory write which will copy the contents of the MB register to the address given in the CPMA register The CPMA and PC registers are subsequently incremented 5 Repeat steps 3 and 4 for all instructions To verify the code use Examine in place of Deposit T
37. mp to increment Count tad NegSum add to NegSum dca NegSum store NegSum isz Count increment Count jmp Loop go again Done hlt done jmp Main allow easy restart Data Section 0100 global variables stored on page 0 Addr A 1 address of array A 1 0300 local variables stored starting at address 0300 Count 0 Count of items in array NegSum 0 Sum of negatives PosSum 0 Sum of positives Array 0400 Array starts at address 0400 Page 2 By S27 De Di Sie SB p lly HO De Ag SF 0 SMain Fig 15 A PAL program using auto index addressing to access array elements Since none of these programs do I O they must be run in the Debug screen display to see results PDP 8 I O will be discussed below following the section on PDP 8 subroutines 6 CALLING PDP 8 SUBROUTINES Since we have not discussed how to do I O on the PDP 8 all of the sample programs presented thus far were run from the Debug screen display to see results Simple PDP 8 I O will be discussed in the section following this one on subroutines since we will consolidate I O code into subroutines A PDP 8 Emulator Program 21 The JMS Jump to Subroutine instruction works by first storing the return address at the first address of the subroutine and then branching control to the second location A return is made by an indirect jump using the return address stored at the first location In the sample program below the subroutine Abs which return
38. nts of PDP 8 memory intact Since a standard PDP 8 had a paper tape reader punch attached to it Punch and Reader are used simulate paper tape I O using text files This gives the PDP 8 Emulator Program a limited file I O capability SwitchReg The PDP 8 Switch Register was a set of 12 toggle switches on the PDP 8 console that could be used to manually enter numbers directly into the PDP 8 When this option is selected the user is prompted for a four digit octal integer Deposit This deposits the contents of the Switch Register to the Memory Buffer MB register and then performs a memory write to write the contents of the MB register to the memory address given by the Central Processor Memory Address CPMA register The CPMA and Program Counter PC registers are both incremented This is used to deposit values to memory from the PDP 8 console 8 B Shelburne AddrLoad This deposits the contents of the Switch Register to the CPMA register and the Program Counter PC This is used to set up the CPMA in preparation for a Deposit or Examine operation Examine This fetches the contents of memory at the address contained in the CPMA register placing it in the MB register The CPMA and PC registers are both incremented This is used to examine the contents of memory from the PDP 8 console The SwitchReg Deposit AddrLoad and Examine switches were included to make the PDP 8 Emulator Program more realistic On a physical PDP 8 an oper
39. on at address 02020 that causes the output device to display an unprintable null character Storing the negative value of an ASCII code to be tested for in this case a minus 13 decimal and adding the value to check for equality is also standard Finally GetChar only reads a character to echo it to the printer display the program must explicitly call the Type subroutine Since this program does PDP 8 I O it can be executed from the Run PDP 8 Pgm screen display 7 2 A PDP 8 Hello World Program The following program displays the string Hello World It also illustrates a number of PDP 8 T O techniques First the subroutine Type is located on page 30 and its starting address is stored on page zero The indirect subroutine call jms I Type is used to invoke it This allows the routine to be called from any page in memory Next the string Hello World s implemented as a null terminated array of characters The data structure for the string stores its own address using the character followed by the string to be displayed including the carriage return line feed character combination 13d 10d ending with a null character Storing the address of the string as its first word fits the increment then indirect effective address calculation of auto index addressing Auto index addressing using auto index register 00100 is used to load each character into the accumulator and Type is called to display the character All of this is embedded in a loop
40. or N address 03000 before re executing Since the purpose of this program is to demonstrate various PDP 8 instructions it is not very efficient in summing the integers from 1 to N 5 2 More Examples of PDP 8 Assembler Programs Since a good way to learn PDP 8 assembler is to study examples of PDP 8 programs three additional programs are presented that demonstrate a variety of programming and addressing techniques The first program separately sums the odd and even integers between and 1000 64 Its similar to the previous program except that it tests if X is odd or even and uses separate variables Odd Even to hold each sum After it loads an integer X into the accumulator the RAR Rotate Accumulator and Link Right instruction rotates the least significant bit into the Link which is then tested by the SNL Skip on Non Zero Link skip instruction If the Link bit is 1 an odd integer the JMP jump instruction following is skipped and the following RAL Rotate 18 B Shelburne Accumulator and Link Left instruction restores X which is then added to the variable Odd Otherwise the JMP instruction is not skipped and control branches to a different code segment that also uses the RAL instruction to restore X but adds it to Even At the end the value of Odd address 03030 should be the sum of the odd integers between 1 and 63 which is 32 1024 or 20000 and the value of Even address 03040 should be the sum of the even integers between
41. r MB The last two registers are used by the CPU to access memory with the former containing the address and the latter containing the value to be read or written The PDP 8 has eight opcodes and three instruction formats Opcodes 0 5 are memory reference instructions MRI opcode 6 is a family of I O instructions and opcode 7 is a set of orthogonal microinstructions that operate on the Link Accumulator pair 10 e B Shelburne 4 4 4 4 4 4 4 4 4 opcode IA MP Offset Address 4 4 4 4 4 4 4 4 4 4 0 1 2 3 4 5 6 y 8 9 TO LE Bits 0 2 operation code Bit 3 Indirect Addressing Bit 0 direct 1l indirect Bit 4 Memory Page 0 Zero Page 1 Current Page Bits 5 11 Offset Address Fig 7 Memory Reference Instruction Format The MRI instruction format only has room for a seven bit offset address hence the page offset addressing scheme To obtain a 12 bit address either five zeros are prefixed to the offset zero page addressing is indicated by clearing bit 4 to 0 or the five leading bits of the address of the instruction is prefixed to the offset current page addressing is indicated by setting bit 4 to 1 This means that any MRI instruction can directly access 256 out of 4096 words Indirect addressing is used bit 3 equals 1 to access any other word in memory In the descriptions below EAddr is short for Effective Address and C means con
42. re stored on page zero Local variables should be stored on the current page 14 B Shelburne 4 2 Indirect Addressing Since zero current page direct addressing can only access two pages out of 32 the PDP 8 uses indirect addressing to access the other 30 pages bit three equals 1 Here the zero current page address calculation yields not the effective address but the address of the effective address 4 3 Auto Index Addressing Memory locations 00100 00170 on page zero function as auto index registers Whenever addresses 00100 00170 are accessed using indirection their contents are FIRST incremented then used as the effective address Example auto index addressing Let address 00100 contain the value 34070 The instruction 14100 references auto index register 00100 indirectly bit 3 1 bit 4 0 offset 0100 Therefore we increment the contents of index register 00100 and apply indirection the effective address is 34100 Examples using each of these addressing modes are given in the following sections 5 PDP 8 ASSEMBLER LANGUAGE PAL PROGRAMMING 5 1 The Basics The format of a PDP 8 Assembler Language instruction has up to five fields symbolic address opcode s i offset comment wee 1 Commas terminate a symbolic address label an denotes indirect addressing and a indicates that a comment follows All fields are optional except for opcodes note the plural since opcode 7 microinstructions can be comb
43. rmine which device generated the interrupt The following PDP 8 program demonstrates how the interrupt system works The main program starting at location 0200 endlessly prints out the alphabet in lower case Whenever the user presses a key an interrupt is generated The automatic jms 0 instruction branches control to location 0001 where a jms i IntH instruction branches control to the interrupt handler at location 5000 The handler saves the contents of the accumulator checks to see if the character read from the keyboard is a carriage return and if so halts the program sounds the bell outputs a line feed character restores the accumulator then returns control to address 0002 where the ion instruction enables the interrupt system following the execution of the return from interrupt instruction jmp i 0 at address 0003 Note that only the ion instruction is used Interrupt Routine 0000 0 Return Address for Interrupt jms i IntH Jump to Handler ion Turn on Interrupt System jmp i 0 Return from Interrupt IntH XIntH Address of Interrupt Handler Basic I O Routine Vectors Page 0 0050 GetChar XGetChar Type XType CRLF XCRLF PrtStr XPrtstr Code Segment Page 1 0200 Main cla cl Clear AC and Link ksf Wake up keyboard tls Wake up printer ion Turn on Interrupt System cla cll tad A Get a dca CH Loop Cla CLL tad CH jms i Type Display letter cla cll tad CH iac tad MZ
44. rogramming examples presented in this paper were derived from examples in these manuals I also have a custom published text The PDP 8 Emulator User s Manual which I use in teaching computer organization electronic copies of which are available from me 12 SUMMARY AND OBSERVATIONS Because the PDP 8 Emulator Program captures the simple and elegant architecture of the PDP 8 it makes an ideal introduction to computer organization With a not very steep learning curve and an easily understood architecture students can quickly learn to write PDP 8 programs In my own computer organization course I have three labs and four programming assignments of increasing difficulty which make use of the PDP 8 Emulator Program including one exercise where students have to write machine code just to give them the experience of having done it at least once However I do not use the PDP 8 as the main architecture to study currently the Intel 80x86 architecture holds that role But using the PDP 8 I am able to compare and contrast the two architectures demonstrating design trade offs and showing that there more than one way to do things architecturally A PDP 8 Emulator Program e 37 The PDP 8 is a good example of how a simple instruction set can execute complex programs and I point out that there is something RISC y about the PDP 8 I also like the fact that the simple opcode 6 instructions permit the user to write all of his her own I O routines
45. ry cursor at address 02000 the starting address for the code At the Command Window prompt A gt Ghit g enter to execute the program or space to single step through the code The answer will appear in location 03020 10 ADDITIONAL FEATURES In the interest of completeness the following section reviews a number of PDP 8 Emulator Program features in greater detail and introduces some additional features that give it a greater capacity for simulation There is a more complete description of the Debug screen commands the capabilities of the Editor and the various options supported under the Run PDP 8 Program screen Using the Run PDP 8 Program screen an example is given that simulates using the PDP 8 console to load code directly into memory The PDP 8 Emulator Program also features a simulated paper tape reader punch which gives it a limited file handling capability Programs punched on paper tape could be loaded into PDP 8 memory using a loader program One such loader program is presented and it is shown how this program can be used to load other programs from paper tape Finally the PDP 8 could handle interrupts so there is a short discussion on how interrupts are implemented on the PDP 8 Emulator Program along with an example program that makes use of them 10 1 The Debug Screen The Assemble command can be used to assemble single lines of PAL code at the memory cursor Small programs can be entered and assembled as shown by the following e
46. s The opcode 6 instruction format contains two perand fields a device number field in bits 3 8 e g device 03 is a keyboard device 04 is a display screen and an extended function field in bits 9 11 which specifies an operation for the device A PDP 8 Emulator Program 11 e g function 001 is Akip if device flag set I O on the PDP 8 is somewhat complicated and will be covered in detail in a later section ee 1 0 device number function 0 1 2 3 4 5 6 7 8 9 10 F4 Bits 0 2 Opcode 6 Bits 3 8 Device Number Bits 9 11 extended function operation specification Fig 8 Opcode 6 Instruction Format Opcode 7 like opcode 6 is a family of micro instructions that include instructions to test increment complement and rotate the link accumulator pair Each bit in the extended opcode field bits 3 11 independently controls a different function These functions can be combined to create a class of fairly powerful instructions Opcode 7 instructions fall into three groups The first group contains functions to clear complement increment and or rotate the link accumulator pair The execution of functions within a group is prioritized to prevent ambiguities For example a clear operation will occur before a complement operation Group two contains two sub groups of conditional skip operations that are used for conditional br
47. s the absolute value of the accumulator is used to compute c la bl Abs is called by the jms instruction at Main 4 Code Segment 0200 code starts at address 0200 Main cla cll clear AC and link tad B Load B cia Negate B tad A A B jms Abs take absolute value d ag store results at C hlt halt jmp Main to continue goto entry point Subroutine 0250 subroutine starts at address 0250 Abs 0 store return address here sma skip on minus accumulator jmp 2 otherwise jump current address plus 2 cia negate accumulator jmp i Abs return via indirect jump Data Segment 0300 data starts at address 0300 A 17 B 22 Cr 0 Main entry point is Main Fig 16 A PAL program to compute c la bl The Abs subroutine is on the same page as the subroutine call it cannot be called from a different page unless indirection is used To execute an indirect subroutine call store the address of the subroutine at a location AdrAbs on page zero and use the indirect subroutine call jms i AdrAbs Examples of indirect subroutine calls are given in the next section 7 DOING SIMPLE PDP 8 I O 7 1 Programmed I O with busy waiting Programmed I O using opcode 6 instructions is the simplest method of doing I O on the PDP 8 The format of an opcode 6 instruction has a six bit device number field and a three bit function field The PDP 8 Emulator Program only supports two I O devices a keyboard
48. something that cannot be easily or realistically done with today s architectures Finally the PDP 8 is a real architecture warts and all not a toy architecture Why create a toy architecture to teach computer organization when there is a real one readily available ACKNOWLEDGEMENTS Thanks to Ken Irwin at Wittenberg for converting my files to pdf format and Bill Yurcik at Illinois Wesleyan for his helpful and greatly appreciated editorial suggestions REFERENCES 1973 PDP 8 e PDP 8 m amp PDP amp f Small Computer Handbook PDP 8 Handbook Series Digital Equipment Corporation Maynard MA 1975 Introduction to Programming PDP 8 Handbook Series Digital Equipment Corporation Maynard MA BELL C GORDON AND NEWELL ALLEN 1971 Computer Structures Reading and Examples McGraw Hill New York NY BELL C GORDON MUDGE J CRAIG AND McNAMARA JOHN E 1978 Computer Engineering A DEC View of Hardware Systems Design Digital Press Bedford MA BLAAUW G A AND BROOKS F P 1997 Computer Architecture Concepts and Evolution Addison Wesley Longman Reading MA KRAFT G D AND TOY W N 1979 Mini Microcomputer Hardware Design Prentice Hall Englewood Cliffs NJ SHELBURNE B 2001 A PDP amp Emulator User Manual custom published Wittenberg University Springfield OH TALLINGS WILLIAM 2000 Computer Organization and Architecture 5 ed Prentice Hall pper Saddle River NJ an TANENBAUM ANDREW S
49. statement lines equal sign equates a symbol to a value e g x 1 sets the symbol x equal to 1 This does not allocate storage gt single quote used to indicate a character e g Aor a string Hello They should occur in pairs Table 5 Reserved Words and Symbols In the code below a counting loop is used to sum the integers from to N It first initializes X to 0 then uses a loop to increment and add it to Sum The counting loop is controlled by the variable Count which is initialized to minus N then incremented on each pass through the loop until it equals 0 ISZ instructions are used for loop control and to increment X By convention most PAL programs begin on page one at location 02000 Code Section 0200 Code starts at address 0200 Main cla cll clear AC and Link tad N load N cia negate it dca Count deposit it to Count and clear AC dca X clear X AC is zero dca Sum clear Sum AC is zero Loop isz X add 1 to X tad Sum load Sum assumes AC is 0 tad X add in X dca Sum store result in Sum AC cleared isz Count increment Count and skip on zero jmp Loop otherwise loop hlt done jmp Main allows easy restart Data Section 0300 Data starts at address 0300 N 12 N equals 10 decimal Count 0 Loop Counter xX 0 Added to Sum Sum 0 Running Sum SMain end of pgm entry point F E g 12 A PAL program to sum the integers from 1 to N A PDP 8 Emulator Program 1
50. tents of Branch instructions are implemented by changing the contents of the PC register Opcodes 0 5 Memory Reference Instructions 4 4 4 4 4 4 4 4 4 4 opcode IA MP Offset Address 4 4 4 4 4 4 4 4 4 4 0 1 2 3 4 5 6 7 8 9 TO Ti Mnemonic Opcode Full Name Description AND 0 Bit wise And C AC C AC C EAddr TAD 1 Twos complement Add C AC C AC C EAddr on carry out complement Link ISZ 2 Increment and Skip on Zero C EAddr C EAddr 1 if 0 then skip next instruction C PC C PC 1 DCA 3 Deposit and Clear Accumulator C EAddr C AC C AC 0 JMS 4 Jump to Subroutine C EAddr C PC C PC EAddr 1 JMP 5 Jump always C PC EAddr Table 1 Six Memory Reference Instructions Note A load is done by the TAD instruction after first zeroing out the accumulator using CLA Clear Accumulator see opcode 7 instructions below DCA Deposit and Clear Accumulator is a destructive store By itself ISZ Increment and Skip on Zero can be used to increment a value in memory or when followed by a JMP Jump Always instruction it can be used to implement a counting loop by incrementing a negative count to zero The JMS Jump to Subroutine instruction saves the return address at the first location in the subroutine and branches to the second location A return is accomplished by an indirect jump Opcode 6 is a family of I O instruction
51. the Debug screen display are typed at the A prompt in the command area at the bottom of the display Debug commands are single letters followed by octal values where appropriate or an assembler instruction for the assemble command All integers are assumed to be octal The bottom line lists thirteen debug commands a followed by an assembler instruction to Assemble a line of code at the memory cursor b to set a Break Point at the current position of the memory cursor or to clear the current breakpoint d followed by an octal value to Deposit a value to memory the d is optional g to Go execute a program starting with the instruction whose address is contained in the Program Counter memory cursor l to Load a program into memory A PDP 8 Emulator Program 5 see Write option below m followed by an octal address to display the Memory Page for that address p followed by an octal address to set the Program Counter which also moves the memory cursor and displays the corresponding memory page r to Reset memory and registers to zero s followed by an octal value to enter a value into the Switch Register q to Quit the Debug screen Esc also works u to Unassemble the value at the memory cursor useful for translating machine instructions to assembler w to Write the contents of memory to a file which can be re loaded see Load above and space to single step trace through a program On line help using F1 provides more details
52. xample which computes z x y Position the memory cursor at address 03000 and at the gt prompt enter the commands gt a x 4 gt a Yr 5 gt a z 0 A PDP 8 Emulator Program 29 to define three variables x y and z Then reposition the memory cursor at address 02000 and at the gt prompt enter the commands gt a main cla cll gt a tad x gt a tad y gt a dca z gt a hlt gt a jmp main The Unassemble command to un assemble machine code to assembler is useful for debugging code Position the memory cursor at the instruction you want unassembled and at the gt prompt hit u enter Unassemble is incorporated into the Single Step space command which unassembles both the instruction just executed and the instruction to be executed next The Break point command BkPt sets a single program breakpoint at the memory cursor or it clears the current breakpoint wherever it is Consequently to set a different breakpoint move the memory cursor to where you want to set the new one and at the gt prompt enter b to clear the current break point then enter b again to set it two commands A breakpoint may also be set using the Go command if an octal address is included after the g at the gt prompt e g gt g 210 The Deposit command is used to enter octal values at the memory cursor At the gt prompt enter any octal value preceded by a d the d is optional followed by enter The value will be deposited at
53. y assembler and the capability of easy machine coding but toy architectures are not real The PDP 8 Emulator Program simulates a real architecture warts and all that is simple A PDP 8 Emulator Program 3 and easy to understood making assembly language and machine code programming relatively easy to do if you want to go this way Like the PDP 8 the PDP 8 Emulator Program is old written almost ten years ago using MS DOS based Borland Turbo Pascal version 6 0 It has been used since then in my computer organization course as a gentle introduction to von Neumann architectures and assembler language programming Despite its age it runs well in an MS DOS Command window 2 AN OVERVIEW OF THE PDP 8 EMULATOR PROGRAM 2 1 The Main Menu Screen At the Main opening screen the user is presented with four options plus Quit To make a selection the user can use arrow keys to highlight the option and hit Enter or he she can type the first letter D E H R Q Esc can be used to return to the Main screen or from the Main screen Esc can be used to quit the program PEFEA FET 1 Feu laiar E i i ni Fig 1 Main Screen PDP 8 Emulator Program Debug Screen This allows the user to directly view the contents of memory and all registers The user can enter execute and trace single step simple PDP 8 programs Editor Assembler A simple text editor allows the user to create edit debug and assemble PPD 8 Assembler Language p
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