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CHAPTER 8. PASM, THE F-PC ASSEMBLER PASM is an assembler which is based on the F83 8088/86 assembler and an 8086 assembler published in Dr. Dobb's Journal, February 1982, by Ray Duncan. This assembler was subsequently modified by Robert L. Smith to repair bugs, and support the prefix assembler notation. Bob discovered a very simple method to force a postfix assembler to assemble prefix code, by deferring assembly until the next assembler command or the end of line, when all the arguments for the previous assembler command are piled up on the top of the data stack. Tom Zimmer has made additional modifications to allow syntax switching, and to increase compatibility in postfix mode with the F83 Assembler. Writing assembly programs is black magic. It is not appropriate to discuss the joys and frustrations in working at such a low level in this manual. However, F-PC provides the best environment for you to do experiments using assembly language, because you can first verify the algorithm and methodology in high level Forth code and gradually reduce the code to the assembly level. You will find numerous examples in which the high level code in F83 is recoded in assembly, in addition to many of the F83 kernel words which were in assembly already. The best way to learn 8086 assembly language is to use PASM, armed with all the code words in F-PC as templates and examples. Factor your high level words carefully so that words at the bottom level can be conveniently recoded in assembly. Take the F-PC kernel words as templates to start with, and modify them so that they will do exactly what you want them to do. 8.1. PREFIX OR POSTFIX ? PASM supports dual syntaxes. The words PREFIX and POSTFIX switch between the two supported modes. The postfix mode is very similar to F83's CPU8086 Assembler. Prefix mode, which is the default mode, allows a syntax which is much closer to MASM used by Intel and MicroSoft. The assembler supports prefix syntax in an attempt to provide a syntax which is more readable to programmers of other languages. The use of sequential text files for source code encourages the programmer to write programs in the vertical code style with one statement per line. This style is what traditional assembler requires. F-PC works well in this style, if you choose to do so. However, F-PC does not prevent you from writing in the horizontal code style, by which you can squeeze many statements into one line and make you own life miserable. It supports postfix syntax to prevent alienating the established base of F83 users. The prefix notation is close to the original Intel assembly syntax, and certainly will be more familiar to programmers of other languages. All the code words defined in F-PC are coded in the prefix notation. Please consider writing any new assembly code you need in the prefix mode for distribution and assimilation with F-PC. The assembly of a machine instruction is generally deferred to the following three events: when the next assembly mnemonic is encountered, at the end of a line, or when the command END-CODE or A; is executed. Therefore, a good style in writing code words in F-PC is to put one assembly instruction in one line, followed by the parameter specification or the arguments. Multiple assembly instructions are allowed in the same line. It is a good ideal to put the assembly structure words in separate lines with proper indentation so that the nested structures in a code definition can be perceived more readily. 8.2. PASM GLOSSARY Here we will only give a small list of PASM words in this glossary. All assembly mnemonics are identical to those defined in F83 8086 Assembler. All the structure directives and test conditions are also identical to those in F83. Only the most important FORTH words controlling the assembler are listed here. PREFIX ( -- ) Assert prefix mode for the following code definitions. POSTFIX ( -- ) Assert postfix mode for the following code definitions. CODE <name> ( -- ) Define a new code definition using the following string as its name. Assembly commands follow, terminated by END-CODE. END-CODE ( -- ) Terminate a code definition, check error conditions, and make the code definition available for searching and execution. LOCAL_REFS ( -- ) This mode WILL NOT allow local labels to cross CODE word boundaries. The local label mechanism is cleared each time a new CODE word is started. This is the DEFAULT mode. GLOBAL_REFS ( -- ) All local labels will be available across all following code definitions. The label mechanism is NOT reset at the beginning of a CODE definition, so a local label reference can cross CODE word boundaries. The local label mechanism MUST be reset before use in this mode, with the CLEAR_LABELS word. CLEAR_LABELS ( -- ) Clear the local label mechanism, to the unused or clean state, in preparation for using local labels. This word need only be used in the GLOBAL_REFS mode. The LOCAL_REFS mode automatically performs a CLEAR_LABELS each time a CODE definition is started. A; ( -- ) Complete the assembly of the previous machine instruction. BYTE ( -- ) Assemble current and subsequent code using byte arguments, if register size is not explicitly specified in prefix mode. WORD is default in postfix mode. WORD ( -- ) Assemble current and subsequent code using 16 bit arguments, if register size is not explicitly specified in postfix mode. BYTE is default in prefix mode. LABEL ( -- ) Start an assembly subroutine or mark the current code address to be referenced later. Figure 8.1. Comparison of assembly syntax PREFIX POSTFIX MASM AAA AAA AAA ADC AX, SI SI AX ADC ADC AX,SI ADC DX, 0 [SI] 0 [SI] DX ADC ADC DX,0[SI] ADC 2 [BX+SI], DI DI 2 [BX+SI] ADC ADC 2[BX][SI],DI ADC MEM BX BX MEM #) ADC ADC MEM,BX ADC AL, # 5 5 # AL ADC ADC AL,5 AND AX, BX BX AX AND AND AX,BX AND CX, MEM CX MEM #) AND AND CX,MEM AND DL, # 3 3 # DL AND AND DL,3 CALL NAME NAME #) CALL CALL NAME CALL FAR [] NAME FAR [] NAME #) CALL ????? CMP DX, BX BX DX CMP CMP DX,BX CMP 2 [BP], SI SI 2 [BP] CMP CMP [BP+2],SI DEC BP BP DEC DEC BP DEC MEM MEM DEC DEC MEM DEC 3 [SI] 3 [SI] DEC DEC 3[SI] DIV CL CL DIV DIV CL DIV MEM MEM DIV DIV MEM IN PORT# WORD WORD PORT# IN IN AX,PORT# IN PORT# PORT# IN IN AL,PORT# IN AX, DX DX AX IN IN AX,DX INC MEM BYTE MEM INC INC MEM BYTE INC MEM WORD MEM #) INC INC MEM WORD INT 16 16 INT INT 16 JA NAME NAME JA JA NAME JNBE NAME NAME #) JNBE JNBE NAME JMP NAME NAME #) JMP JMP JMP FAR [] NAME NAME [] FAR JMP JMP [NAME] JMP FAR $F000 $E987 JMP F000:E987 LODSW AX LODS LODS WORD LODSB AL LODS LODS BYTE LOOP NAME NAME #) LOOP LOOP NAME MOV DX, NAME NAME #) DX MOV MOV DX,[NAME] MOV AX, BX BX AX MOV MOV AX,BX MOV AH, AL AL AH MOV MOV AH,AL MOV BP, 0 [BX] 0 [BX] BP MOV MOV BP,0[BX] MOV ES: BP, SI ES: BP SI MOV MOV ES:BP,SI MOVSW AX MOVS MOVS WORD POP DX DX POP POP DX POPF POPF POPF PUSH SI SI PUSH PUSH SI REP REP REP RET RET RET ROL AX, # 1 AX ROL ROL AX,1 ROL AX, CL AX CL ROL ROL AX,CL SHL AX, # 1 AX SHL SHL AX,1 XCHG AX, BP BP AX XCHG XCHG AX,BP XOR CX, DX DX, CX XOR XOR CX,DX 8.3. SYNTAX COMPARISON The differences among the F-PC prefix mode, the F83 postfix mode, and the Intel MASM notation are best illustrated by the table in Figure 8.1. Although the table is not exhaustive, it covers most of the cases useful in doing PASM programming. You are welcome to suggest additional cases to be included in this table. 8.4. USAGE OF 8086 MACHINE REGISTERS IN F-PC To write assembly code, you have to know the CPU real well. Most CPU's can be understood and programmed using a CPU model, consisting of the register set and the instructions which manipulate data among the registers, memory, and external devices. In F83, only a 64K bytes segment of memory is used, and all segment registers in 8086 are generally pointing to the same code segment. Since F-PC uses many segments to store code, heads, lists, and other data, you have to know how these segment registers are used and how information in different segments can be accessed conveniently. Following is a list of the 8086 registers and their usage in F-PC: CS Code seg: used for any code definitions (Must be preserved by code word.) DS Data seg: used for data other than ." strings (NOTE: CS=DS and underlying kernel primitives rely on this correspondence! Must be preserved by code word.) ES Extra seg: used as the segment location for the current instruction pointer (IP). (Must be preserved by code word.) SS Stack seg: used as the segment location for the current stack pointer (SP). (Must be preserved by code word.) BP Return Pointer (RP). (Must be preserved by code word.) SP Stack Pointer (SP). (Must be preserved by code word.) SI Instruction Pointer (IP). (Must be preserved by code word.) AX, BX, CX, DX, & DI Scratch registers free to use without restoration. PC Program Counter. Not used by F-PC. DF Direction Flag. Assumed to be 0/increment. Some older FF (or before?) words do an initial CLD (e.g., CMOVE), but this shouldn't be necessary. If you specifically need DF=1, then do: STD ...code... CLD AX is used as a general purpose accumulator. BX is most useful as a base register for indexing into an array. CX is used to hold a count for looping and repeating operations. DX is useful in holding the address of an I/O port. DS:SI pair is used to read memory with auto-indexing, and ES:DI pair is used to write memory with auto-indexing. F-PC uses SP as the data stack pointer and BP as the return stack pointer. SP is convenient in the PUSH and POP instructions, while BP is more convenient in indexing. There are many occasions that you might want to swap SP and BP to use the most effective way to address data on either stack. The F-PC Technical Reference Manual discusses in great details how F-PC itself is constructed based on the 8086 assembly code. If you are interested in squeezing the last drop of blood from PC/XT/AT, be sure to study carefully the Technical Reference Manual and the kernel files in F- PC. 8.5. ADDRESSING MODES The most difficult problem in using 8086 assembler is to figure out the correct addressing mode and code it into an instruction. You can get a good ideal and probably figure out most of the addressing mode syntax from the above table. However, there are cases where the table falls short. Here we will try to summarize the addressing syntax more systematically to show you how F-PC handles addresses in the prefix mode. Register Mode Source or destination is a register in the CPU. The source registers are: AL BL CL DL AH BH CH DH AX BX CX DX SP BP SI DI IP RP CS DS SS ES Destination register specifications are: AL, BL, CL, DL, AH, BH, CH, DH, AX, BX, CX, DX, SP, BP, SI, DI, IP, RP, CS, DS, SS, ES, The register name must be followed by a 'comma', to be recognized by PASM as a destination register. Immediate Mode The argument is assembled as a literal in the instruction. The immediate value must be preceded by the symbol #, which is a word and must be delimited by spaces: MOV AX, # 1234 ADD CL, # 32 ROL AX, # 3 Direct Mode An address is assembled into the instruction. This is used to specify an address to be jumped to or a memory location for data reference. The address is used directly as a 16 bit number. Depending on the instruction, the address may be assembled unmodified or assembled as an eight or 16 bit offset in the branch instructions. To jump or call beyond a 64K byte segment, the address must be preceded by FAR [] . Examples are: CALL FAR [] <label> JMP <dest> MOV BX, <source> INC <dest> WORD JZ <label> The destination address may be taken from the data stack directly: MOV CX, # 16 HERE ( save current code address on stack) ... ... LOOPZ ( loop back to HERE if condition fails) Index Mode One or two registers can be used as index registers to scan through data arrays. The contents of the index register or the sum of the contents of two index registers are used to form a base address. An offset is added to the base address to form the true address for data reference. Examples are: CMP 2 [BP], SI DEC 3 [SI] MOV BP, 0 [BX] The following register index specifications are allowed in F-PC: [SI] [IP] [BP] [RP] [DI] [BX] [BX+SI] [SI+BX] [BX+IP] [IP+BX] [BX+DI] [DI+BX] [BP+SI] [SI+BP] [BP+IP] [IP+BP] [RP+IP] [IP+RP] [BP+DI] [DI+BP] [RP+DI] [DI+RP] There must be an offset number preceding the index register specification, even if the offset is 0. When the index register is used as destination, a comma must be appended immediately: MOV 0 [BX+IP], AX Implied Mode and Segment Override The implied mode is where mistakes are most likely to occur because you will have to be keenly aware of which segment register is used by the instruction at any instance. Since the segment register is implied and not stated explicitly, the bug generally can hide very securely underneath laughing at you. The code works when you test it but fails when the segment register is modified. Branch and jump instructions use CS segment register. Data movement instructions use DS segment register. Stack instructions use SS segment. String instructions use DS:SI as source and ES:DI as destination. If you need to specify an address with a segment register other than the default implied register, use a segment override instruction before the address specification: CS: DS: ES: Examples are: MOV ES: BP, SI CMP CS: 2 [BP], AX ADD AX, ES: 10 [BX+DI] The 8086 addressing modes are so confusing that even experienced programmers need a good Intel 8086 manual to find the right addressing mode and the F-PC assembler syntax table to determine the correct argument list. The best way to write assembly code is still to keep the code short and simple. It is very easy in F- PC to break a long CODE definition into many small fragments which are initially defined as separate CODE definitions. After verifying that each fragment works, you can edit out the CODE, NEXT, and END-CODE lines to combine the fragments into a single CODE definition. Charles Curley kindly contributed an 8086 disassembler. It is helpful to disassemble the code word you defined and see what the computer thinks of what you mean. There is always this 'Do what I mean, not what I say' syndrome. When everything else has failed, you can jump into DOS and call up DEBUG and use the facility there to find what goes wrong. In DOS DEBUG, you can step through a piece of code one instruction at a time. This is absolutely the last thing you want to do. 8.6. ASSEMBLY MACROS IN PASM Another area of interest is the assembly macros. Here is the definition of the NEXT macro: : NEXT >PRE JMP >NEXT A; PRE> ; The macro itself is simply assembling the sequence JMP >NEXT. The surrounding words are used for support. Since PASM supports both postfix as well as prefix notation, it is not known on entry to a macro which mode is in effect. The words >PRE and PRE> select prefix, and restore the previous mode so macros will always be in prefix notation. The A; after >NEXT, forces the assembling of the JMP instruction before switching mode. You can find many other examples of assembly macros in PASM.SEQ, like 1PUSH, 2PUSH, and all the assembly structure building directives. 8.7. LOCAL LABEL To support large code definitions, Bob Smith introduced 'local labels' to F-PC. The local labels are place markers $: preceded by a number. They are used to mark locations in a large code definition for forward and backward jumps and branches which uses $ preceded by the number of the local label as address specification. They can be used quite freely in a range of code words and reused to save head space by replacing LABELs which have global names and cannot be reused. The use of local labels is best demonstrated by an example taken from the software floating point package SFLOAT.SEQ by Bob Smith, shown in Figure 8.2. Up to 32 local labels can be used to mark addresses of assembly code. They can be referred to before or after their placements. They can be referenced across code word boundaries. Local labels in PASM are useful within a CODE definition, using LOCAL_REFS as the default label mode. If you need to use local labels between CODE definitions, you will have to select the GLOBAL_REFS mode, and use CLEAR_LABELS to initialize the local labels manually each time you want to start a new set of local labels. This technique is especially useful where the one-entry-one-exit dogma is very awkward and when a piece of code has multiple entry points and can be shared among many code word definitions. It enables us to construct structured spaghetti code, if there were such thing. Local label allows relative short branches within 127 bytes. To jump beyond this range, the long jump label L$ should be used: ... ... JMP L$ ... ... L$: ... ... The long jump label can only be used to jump forward. After L$: is used, you can do another long forward jump using L$ again. 8.8. INLINE CODE INLINE allows us to include machine code inside a high level colon definition. This is easily done in F-PC because it is built on direct threaded code. Every word is compiled as a code address in the colon definition. The code in the code segment pointed to by the code address is executed directly because it contains genuine 8086 machine code. Whether the code belongs to a colon definition or a code definition does not make any difference. INLINE only has to compile the address pointing to the top of the dictionary in the code segment. The assembler is then invoked to compile machine code. If the code is terminated by NEXT or one of its derivatives, the next word compiled in the colon definition will be executed after the assembly code is done. END-INLINE only has to clean up the assembly environment and return the control back to the colon compiler. Here is an example on how to use INLINE and END-INLINE to add assembly code in the middle of a colon definition: : TEST ( -- ) 5 0 DO I \ Get loop index INLINE pop ax \ pop I add ax, # 23 \ add 23 1push \ push sum END-INLINE . \ print results LOOP ; Figure 8.2. Examples of local labels LABEL DENORM \ CX = count, BX = Hi, DX = LO, AX = Guard, Round, & Sticky. CLEAR_LABELS XOR AX, AX \ Clear GRS bits CMP CX, # 10 \ Check size of shift JL 7 $ \ Branch if shift less than 16 CMP CX, # 18 \ Cnt >= 16. Compare with 24. JGE 1 $ \ Branch if shift >= 24 SUB CX, # 10 \ 16 <= cnt < 24. Subtract 16 from cnt. MOV AX, DX \ Shift by one word. MOV DX, BX XOR BX, BX \ Clear MS word. AND AL, AL \ Don't miss any sticky bits. JZ 8 $ OR AH, # 2 \ Set a sticky bit. JMP 8 $ \ Shift from 0 to 7 bits. 1 $: CMP CX, # 20 \ Cnt >= 24. Compare with 32. JGE 3 $ \ Branch if shift >= 32. MOV AH, BL \ 24 <= cnt < 32, so shift by 3 bytes. MOV AL, DH OR AL, DL \ OR in the sticky bits. JZ 2 $ \ Shall we set a sticky bit? OR AH, # 2 \ Yes. 2 $: MOV DL, BH \ Continue the 3-byte shift. XOR DH, DH \ Clear the high 3 bytes. XOR BX, BX SUB CX, # 18 \ Adjust the shift counter. JMP 8 $ \ Shift remaining 0 to 7 places. 3 $: CMP CX, # 28 \ Cnt >= 32. Check against 40. JGE 5 $ \ Branch if shift count > 40. MOV AX, BX \ 32 <= cnt < 40. Do a 2 word shift. OR AL, DH \ Check the sticky bits. OR AL, DL JZ 4 $ \ If sticky byte not zero, OR AH, # 2 \ then set sticky bit. 4 $: XOR BX, BX \ Clear high and low words. XOR DX, DX SUB CX, # 20 \ Adjust shift counter. JMP 8 $ \ Go to final shift area. 5 $: OR BX, DX \ Shift count >= 40 JZ 0A $ \ In theory, this should never jump. MOV AH, # 2 \ So set a sticky bit. XOR BX, BX \ Clear all the rest. XOR DX, DX RET 7 $: CMP CX, # 8 \ Count < 16. See if less than 8. JL 8 $ MOV AH, DL \ 8 <= cnt < 16. MOV DL, DH \ Move by one byte. MOV DH, BL MOV BL, BH XOR BH, BH \ Clear high byte. SUB CX, # 8 \ Adjust shift count. 8 $: AND CX, CX \ Test for zero count. JZ 0A $ 9 $: SHR BX, # 1 \ Shift right by one bit. RCR DX, # 1 RCR AX, # 1 LOOP 9 $ \ Loop until count is 0. 0A $: RET END-CODE 8.9. ASSEMBLER STYLE To code 80x86 assembly routines successfully, you have to pay attention to many things simultaneously, making it very exciting and interesting occasionally, but frustrating most of the time. To make it easier for yourself, you can follow the following coding style to avoid the most serious pitfalls in writing assembly code: code gizzmo ( input1 input2 input3 ... -- output1 output2 ...) pop ax \ input section pop bx \ pop everything into registers pop cx ... push si \ saving section push ds \ save all registers you will use push es push cs ... \ code body ... \ do your creative coding ... pop cs \ restoring section pop es \ restore all saved registers pop ds pop si push cx \ output section push dx \ 2push push ax \ 1push next end-code \ you can breath now Make a boiler plate code template and delete whatever is not needed. This style sheet will save you lots of grief if you follow it religiously, with a prayer, some incense, and a bottle of aspirins. The 8086 registers have their special characteristics. Counts should be popped into CX, data into AX or DX, and addresses into BX. For string operations, DS:SI is used for source and ES:DI for destination. Because F-PC uses ES:SI as its IP, they must be saved and restored. You should also constantly remind yourself that F-PC requires that CS, DS, and SS all point to the Code Segment, and that ES points to the List Segment. If you do not restore them to this state, NEXT will certainly crash the system to verify the fact. However, you should not fear crashing the system. It is the daily portion which keeps a Forth programmer alert and nourished. It also helps to remind the computer who is the master lest it forgets, which it often does. 8.10. DEBUGGING CODE WORDS Debugging large code words is very difficult. You are encouraged to avoid writing large code words for this reason. The best way to optimize your Forth application is first writing everything in high level colon words. After you have debugged the application and have a working system, examine the colon words carefully to identify the 'critical routines' where the computer spends most of its time. Only these routines need to be converted to code words. If a critical routine is a long colon definition, try to break it into small pieces which can be conveniently converted to code words. If the code words are small, coding and testing them will be easy. If you have to build a large code word, code it in small pieces anyway. It is easy to merge small code words into a large code word. If you take every precaution in writing a code word but it still fails to work, you have a last chance. Because F-PC has the capability to spawn a DOS shell, using the command SYS and ` (back tick), you can invoke the DEBUG program in DOS to step through a code word. For example, if you want to debug the code word GIZZMO, load and assemble it into your dictionary and execute: ' GIZZMO HEX U. to find its address in the dictionary. You can find the code segment of your dictionary by ?CS: U. Write these numbers on a piece of paper so that you can find the code after you are in DOS DEBUG. To invoke DOS DEBUG, type: SYS DEBUG or ` DEBUG You have to make sure that you have COMMAND.COM and DEBUG.COM in your current directory or in the PATH before you execute the SYS word. If these files are accessible, you will see the DEBUG prompt '-'. Now you can use the DEBUG commands to set up registers, disassemble code to see if the code was assembled correctly, and also step through the code one instruction at a time. Go find and dust off your DOS manual. The only other thing you have to be careful about is that DEBUG initializes the segment registers, the stack pointers, and the CPU registers in its own way, very different from the way F-PC sets them up. You must initialize all the registers used by F-PC correctly. If GISSMO needs parameters from the data stack, you must push them on the data stack before stepping through GIZZMO. DEBUG does not provide the most friendly interface to its user. You have to be prepared to pay the price before dipping into assembly coding. Try to minimize the pain by staying in the friendly environment in F-PC as long as possible, where the interpreter, the compiler, the editor, and the high level debugger are ready and eager to render their best services.