Unix PDP-11 Emulator (As11 & Em11) User's Guide
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1. A useful ASCII char newline nl 12 Make space for the stack org 500 stack then declare the startpoint org 1000 start initialise the stack ptr mov stack sp mov greeting sp jsr pc scr_mesg add 2 sp halt greeting byte nl nl hello there everyone byte isn t it a lovely day nl nl byte 0 even For the moment let us not worry about the scr_mesg routine Accept that it simply displays a null termi nated message whose starting address is passed on the stack 2 2 The Format of Assembly Language Programs Most of the lines in the above program contain a single PDP 11 instruction Some lines however declare labels or perform assembler directives known as pseudo ops Any line may be terminated by a comment introduced by a semi colon which acts until the end of the cur rent line A line if so desired can contain nothing except a comment Between the various constituents of a line you may place any number of tabs and blanks which act as sep arators The assembler is not sensitive to upper and lower case 2 3 Basic Concepts 2 3 1 Symbols A symbol is the assembler equivalent of a Modula 2 constant That is it is a name which is used to repre sent a particular numeric value increasing the readability of a program It is an error to redefine a symbol The assembler accepts indefinite length symbols which are sequences of alphanumeric and underscore characters where th2. Autoincrement and autodecrement modes change the register contents by one for byte instructions and two for word instructions with the following exceptions 1 R6 and R7 are always changed by 2 2 In deferred modes a register is always incremented by 2 3 The Emulator Em11 is an interactive source level PDP 11 emulator with built in debugging facilities It is invoked by em11 file This loads the e file you name on the command line into the emulator provides the prompt pdp and waits for a command The emulated PDP 11 has the following characteristics 3 1 Memory 16K 40000 octal bytes of RAM 3 2 Polled I O System Every I O device detailed below has a status register and one or more data buffer registers associated with it These device registers are mapped into memory at a high address beyond the end of the RAM All devices use a common format for their status register Error Unused Busy Unused Start 15 14 8 7 6 1 0 The general way of using an IO device is 1 Set up any relevent data registers 2 Set the start bit bit 0 in the status register This causes the action to start 3 Poll the busy bit bit 7 in the status register until it goes low 4 Check the error bit bit 15 in the status register 5 Read any results out of data registers Version 1 3 3 6th Jan 1989 3 2 1 Delays All the devices when active take a certain amount of time actual
3. or All operators of the same precedence are evaluated left to right The default precedences may be overridden by the use of brackets Note a lt b means shift a b places left Similarly a gt b means shift a b places right 2 4 Instructions The assembler can handle two types of instructions mnemonics and pseudo ops 2 4 1 PDP 11 Mnemonics The mnemonics recognised by this assembler are listed below adc adcb add asl aslb asr asrb bec bes beq bge bgt bhi bic bicb bis bisb bit bitb ble blos blt bmi bne bpl bpt br bvc bvs ccc cle cln clr clrb clv clz cmp cmpb com comb dec decb emt halt inc incb iot jmp jsr mark mov movb neg negb nop reset rol rolb ror rorb rti rts rtt sbc sbcb scc sec sen sev sez sob sub swab sxt trap tst tstb wait xor It is beyond the scope of this manual to explain the operation of these instructions Refer to your lecture notes or to the DEC PDP 11 Processor Handbook for further details Version 1 3 3 6th Jan 1989 2 4 2 Assembler Directives Pseudo Ops Instructions starting with a are all commands to the assembler org expr Sets the Assembler s Location Counter to the value of the given expression The space between the current location counter and the new value is left undefined No check is made by the assembler for overlapping areas but the emulator makes some checks when loading i
4. F 77562 SCR_STAT 771564 SCR_BUF 77566 LPR_STAT 771514 LPR_BUF 771516 DSK1_STAT 77520 DSK1_SEL 77522 DSK1_BAR_ 77524 DSK2_STAT 77530 DSK2_SEL 77532 DSK2_BAR_ 77534 3 2 8 An example of polled I O Here is the rest of the example program we presented a while back It will serve as an illustration of the use of a typical I O device the screen Note that the following routines take their arguments from the stack and do not corrupt any registers This will make them safer and easier to use in different circumstances Version 1 3 3 6th Jan 1989 PROCEDURE scr_mesg mesg ADDRESS Purpose displays message terminated by a zero byte scr_mesg mov r0 sp Save r0 mov 4 sp r0 rO message address sm_ll tstb r0 while r0 0 do beq sm_rts movb r0 sp jsr pce scr_char call scr_char r0 add 2 sp br sm_ll end while sm_tts mov sp r0 restore original rO rts pe PROCEDURE scr_char ch CHAR Purpose Print character on screen scr_stat 77564 scr_buf 77566 scr_char movb 2 sp scr_buf copy char to screen buffer movb 1 scr_stat Start i o operation sc_Ip repeat tstb scr_stat until not busy bmi sc_Ip rts pe 3 3 Emulator Expressions Emulator expressions are almost identical to assembler expressions with only the following differences 1 Two extra unary operators
5. Notice that the original source is listed the emulator does not actively disassemble a portion of memory If your program modifies itself you will have to rely on byte word and ASCII dumps dump byte word ascii expr expr2 The contents of the memory are dumped on the screen as octal bytes octal words or as ASCII char acters Expr if given specifies where the dump is to start from if not given from where the previous dump stopped initially address 0 and expr2 if given specifies how many items the dump should print if omitted 32 items will be printed break Displays all current breakpoints break on off expr Adds or removes a breakpoint at the specified address When the running program hits a breakpoint execution terminates with the program counter pointing to the address of the breakpoint If the first instruction to be executed after a go command is a breakpoint it is ignored Breakpoints are also ignored during the execution of the step command step expr Executes the given number of instructions or a single instruction if no number is given print expr Evaluates the given expression and prints the result in octal hexadecimal decimal and binary lhs expr1 Here the left hand side may either be 1 A register 2 A byte dereferenced expression expr2 ie the byte contents of the given memory location 10 Version 1 3 3 6th Jan 1989 3 A word dereferenced expression
6. Unix PDP 11 Emulator As11 amp Em11 User s Guide Duncan White Jan Simon Pendry Diomidis Spinellis ABSTRACT Asl and emli form an emulated PDP 11 environment which can be used on UNIX systems to design and develop simple PDP 11 assembly language programs The emulated PDP 11 includes 16K bytes of store a screen a keyboard a line printer and two random access disks 1 Filename Conventions Just as Modula 2 uses standard suffixes such as def and mod to identify files as belonging to Modula 2 so the PDP 11 system uses the suffixes a for an assembly language file and e for an emulator input file 2 The Assembler AsI1 is a free format assembler accepting all the standard PDP 11 mnemonics and operand types It is invoked by as11 file The action of the assembler is to translate the single a file named on the command line you may omit the al1 suffix into the corresponding e file Error messages and warnings during assembly are reported on the standard error stream These are intended to be self explanatory The assembler continues after a warning but aborts after a fatal error PDP 11 is a registered trademark of DEC Unix is a registered trademark of AT T Bell Labs Version 1 3 3 6th Jan 1989 2 1 An Example Assembly Language Program To make the following discussion clearer here is a simple example of a PDP 11 Assembly Language pro gram An Example PDP 11 Assembly Language Program
7. ands will be silently truncated from 16 bit values to 8 bit ones Numeric constants may be written in four bases or as character constants 1 By default a number is octal If preceded by an 0 it is decimal If preceded by Ox it is hexadeci mal If preceded by Ob it is binary 2 The value of a single character constant such as A is the ASCII code of the given character 3 The value of a double character constant such as AB is a 16 bit word computed by placing the ASCII code of the first character in the high byte and the ASCII code of the second character in the low byte Version 1 3 3 6th Jan 1989 2 3 6 String constants String constants are allowed in the byte pseudo op and consist of a sequence of characters quoted with single quotes double quotes or slashes Such a string represents a sequence of 8 bit integers 2 3 7 Expressions An expression is a sequence of symbols constants operators and brackets representing a value It is evaluated at assembly time not at run time All arithmetic is performed using 16 bit two s complement There are seven levels of precedence for the operators listed here from highest precedence to the lowest Operators Meaning unary negate 2s complement 1s complement multiplication division modulus addition binary subtraction lt left shift gt right shift i bitwise xor amp bitwise and bitwise
8. are available a the byte dereference operator which returns the byte stored at the given PDP 11 memory address b the word dereference operator which returns the word stored at the given PDP 11 memory address which must be even 2 The assembler location counter the symbol has no meaning for the emulator 3 4 Constants As in the assembler the emulator can accept constants in four bases octal by default or single and double quoted character constants Version 1 3 3 6th Jan 1989 3 5 The Commands The emulator provides a wide variety of commands with which to manipulate the PDP 11 environment help Displays a short list of the commands available In addition the on line manual page em11 L describes the commands available go expr or run expr Runs the program at the given address or the current value of the PC if the address is not given You may interrupt the program should it get into an infinite loop for example by pressing CTRL C quit or exit Quits the emulator register Displays all the registers and the system flags trace Displays the current status of the trace flag trace on off Switches the trace flag on or off When the trace is on each instruction is displayed before it is executed and the contents of the desti nation are printed out in octal after the execution list expr A full page of the original PDP 11 source code starting at address expr is displayed
9. ding or writing an entire sector from memory to the disk or vice versa Both disks may be simultaneously active Each disk has its own complete set of registers we will now describe those for disk 1 but disk 2 has an identical set which start at location 77530 See the table at the end for a summary A status register known as DSK1_STAT located at 77520 A sector select register known as DSK1_SEL located at 77522 This register is laid out as follows R w Unused Cylinder Surface Sector 15 14 9 8 6 5 3 2 0 The r w bit should be set for writing and reset for reading A buffer address register known as DSK1_BAR located at 77524 This register stores the PDP 11 address of the start of the buffer to read write data from to 3 2 6 1 Disk R W Errors The configuration file which governs the delays of the I O operations may also mark specific sectors on one or both disks with a probability of failure This probability governs whether each individual read write operation on that sector succeeds or fails If a read operation fails then the buffer will be filled with garbage currently exclamation marks If a write operation fails then the relevent sector will be filled in a similar fashion Such an error of course sets the error bit in the status register 3 2 7 I O Address summary To summarise the I O devices have the following registers Name Located at KBD_STAT 77560 KBD_BU
10. e first character is not numeric Version 1 3 3 6th Jan 1989 There are two ways of declaring a symbol 1 A symbol assignment of the form symbol expr 2 A label declaration of the form symbol This assigns the current value of the Assembler s location counter to the symbol In the example program there are the lines start mov stack sp The first of these lines declares the label start to represent the starting address of the program All symbols can be used in the rest of the Assembly Language program as the destination of branches and in expressions All the symbols are also available to you in the Emulator 2 3 2 A Recommendation We recommend that labels if given stand alone on the line it makes editing simpler That is we suggest that the above two lines are not laid out as start mov stack sp 2 3 3 Registers The following symbols rOrl r2 r3 r4r5 1617 sp pc are predefined to represent the PDP 11 registers The user may not define symbols to represent registers 2 3 4 Assembler Location Counter All assemblers need to maintain the position in memory at which to generate code This is normally known as the location counter The PDP 11 assembler uses the special symbol dot to represent the location counter You can use freely in expressions 2 3 5 Numeric constants All numeric constants are interpreted as 16 bit two s complement integers although byte sized oper
11. expr2 ie the word contents of the given memory location The appropriate value is stored in the lhs This is like an assignment in Modula 2 move expr expr2 expr3 Moves the section of memory in the address range expr to expr2 to address expr3 gt filename Redirects PDP 11 screen output to the file specified lt filename Redirects PDP 11 keyboard input from the file specified lt gt device Redirects PDP 11 keyboard input and output to the device specified This feature can be used to sim plify debugging when using trace by redirecting all the interaction with the running PDP 11 program to a different window 3 6 Emulator Errors The emulator will on occasion produce an error message and stop the program running Two such errors are 1 A bus error which is caused by an attempt to access a UNIBUS address that does not exist and 2 An odd address error which occurs if a register used in a word autoincrement or autodecrement instruction becomes odd This often occurs because SP or PC are set to an odd value T1
12. ly number of PDP 11 instructions to fin ish Different devices will take different lengths of time to complete an operation Indeed the same I O device may take slightly differing times to complete two identical operations In fact these are set up from a configuration file read in at start up This is the reason why you have to poll the busy bit repetitively 3 2 2 Errors For all the I O devices the error bit will be set by the user performing the above steps in an incorrect way the most typical error is probably to start a new operation while the previous operation is incomplete If this happens both the active operation and the erroneous operation are aborted which will typically cause characters or sectors to be omitted The error bit is cleared when the start bit is next set For disks only there is another error situation discussed in the section on disks The devices are 3 2 3 Keyboard The PDP 11 keyboard is naturally enough simulated by the UNIX keyboard However you can take keyboard input from a file by using the Emulator s lt command The keyboard has a status register known as KBD_STAT which is at location 77560 In addition it has a single byte data register known as KBD_BUPF at location 77562 After a successful keyboard operation has completed this location contains the next character read from the keyboard 3 2 4 Screen The PDP 11 screen is simulated by the UNIX screen You can cap
13. n the executable image symbol expr Sets the given symbol to the given expression even Causes the location counter to be adjusted to the next even address byte expr expr Store the given sequence of 8 bit wide data into successive bytes Remember that strings are also allowed for this pseudo op word expr expr Store the given sequence of 16 bit wide data into successive words Currently the assembler does not support the usual b k and ascii pseudo ops 2 5 Addressing Modes The assembler accepts all the standard PDP 11 addressing modes That is 2 5 1 Basic Addressing Modes 0 7 Name Syntax Effect Register Rn op Rn Register Deferred Rn op men Rn Autoincrement Rn op men Rn inc Rn Autoincrement Deferred Rn addr mem Rn inc Rn op men addr Autodecrement Rn dec Rn op mem Rn Autodecrement deferred Rn dec Rn addr mem Rn op men addr Index x Rn x mem PC inc PC op mem x Rn Index Deferred x Rn x mem PC inc PC addr mem x Rn op men addr Version 1 3 3 6th Jan 1989 2 5 2 PC Addressing Modes Name Syntax Effect Immediate n op mem PC inc PC Absolute n addr mem PC inc PC op mem addr Relative n x mem PC inc PC op mem x PC Relative Deferred n x men PC inc PC addr mem x PC op mem addr 2 5 3 Note
14. ture screen output in a file using the Emulator s gt command The screen has a status register known as SCR_STAT at location 77564 In addition it has a single byte data register known as SCR_BUF at location 77566 You should deposit the character to be displayed in SCR_BUF before starting the I O operation 3 2 5 Line printer The PDP 11 line printer is simulated by the UNIX line printer Each run of a program which writes to the line printer will generate a single job of line printer output The printer has a status register known as LPR_STAT at location 77514 In addition it has a single byte data register known as LPR_BUF at location 77516 You should deposit the character to be printed into LPR_BUF before starting the I O operation 3 2 6 2 disks These disks have 256 byte sectors and have 8 cylinders 8 surfaces and 8 sectors per track When the PDP 11 emulator starts running two Unix files disk dat and disk2 dat are read in by the emula tor and used as initial disk images These files are simply text files which the user should create with vi If the Unix file finishes before the entire PDP 11 disk is full then the rest of that sector has zero bytes writ ten into it and the rest of the disk is left undefined When the emulator terminates the disk images are written out again onto the Unix files disk1 dat and disk2 dat Version 1 3 3 6th Jan 1989 The only I O operation that the disks are capable of is rea
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