Programmer 7500/MAX_PROGRAMMERS.iso/CLIPPER/MISC/FPC225_2.ZIP/F-PCDOC.ZIP/ASSEMBLY.DOC

Text File | 1988-06-01 | 7.7 KB | 181 lines

XI.   PASM, THE F-PC ASSEMBLER




PASM.SEQ is an assembler which is based on an assembler published
in DDJ, February 1982 by Ray Duncan. That assembler was
subsequently modified by Robert L. Smith to repair bugs, and
support the prefix assembler notation.  Bob discovered a very
simple method to let a postfix assembler to assemble prefix code,
by deferring assembly until the next assembler command, when all
the arguments for the previous assembler command are piled on the
top of the data stack.  Tom Zimmer have made additional
modifications to allow switching syntaxes, and to increase
compatibility in postfix mode with the F83 Assembler.

It is not appropriate to
discuss the joy 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 reducing the code to the assembly level.  You will find
numerous examples in which the high level code in F83 is recoded
in assembly, besides most of the F-PC kernel words which were in
assembly already.

The best way to learn 80x8x assembly language is using PASM, armed
with all the examples of code words in F-PC.  Factor your code
carefully so that each word can be conveniently recoded in
assembly.  Take the kernel words as templates to start with, and
modify them so that they will do exactly what you want them to do.



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 closed to MASM.

The assembler supports prefix syntax, in an attempt to provide a
syntax which is more readable to programmers of other languages.
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. Please consider writting any new assembly code
you need in the prefix mode for distribution and assimilation.

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, except the assembly
directives which build control structures in a code word, such as
IF, ELSE, THEN, BEGIN, WHILE, AGAIN, etc.  These directives must
be the first or the only instruction in a line because they act
immediately, not waiting for the next assembly instruction.



2.   PASM GLOSSARY



Here we will only give a small list of 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 "name" as a new code definition. Assembly language follows,
terminated by END-CODE.

A:        Alias of CODE.

END-CODE  ( --- )

Terminates CODE definitions, checks error conditions, and makes
the code definition available for searching and execution.

A;        Alias of END-CODE.



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 following
table.  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.



        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
        LODSW                   AX LODS                 LODS WORD
        LODSB                   AL LODS                 LODS BYTE
        LOOP NAME               NAME #) LOOP            LOOP 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



4.   MACROS IN PASM



Another area of interest is macros, here is the definition of the
NEXT macro:

          : NEXT  >PRE    JMP >NEXT A;    PRE> ;

The macro itself is simply the sequence JMP >NEXT. The surrounding
words are used for support. Since PASM supports both postfix as
well as prefix notation, It is not know on entry to a macro what
mode is selected. 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 assembly of the JMP
instruction before the mode switch.