MirBSD manpage: gas-intel-howto(7)

gas-intel-howto(7)       MirOS Tutorial        gas-intel-howto(7)

NAME

     gas-intel-howto - How to write Intel syntax assembly code
     with the GNU assembler

TUTORIAL

     Did you always wonder how this new Intel mode of as(1) could
     be used? Did you wonder why the GNU Texinfo documentation is
     wrong?
     This is the answer.

     First off, how would you start an assembly language program?
     This is a real-life example:

     /* copyright ...
      */

     #include <machine/asm.h>

          .intel_syntax noprefix
          .text

     Now, you can use all the well-known ops and pseudo-ops from
     the gas and intel world. You just need to keep track, which
     one to use.
     Did you know you can concatenate assembly lines with a semi-
     colon (';')? Now, how do you issue comments?
     It's easy. Using gcc -c -o fnord.o fnord.S automatically
     uses the C Preprocessor, cpp(1), on the file first. That's
     why the C-style comment in the above example could be used.

PSEUDO-OPS
     All these pseudo-ops, and more, are supported on ELF:

     .text
        Denotes a code segment.

     .data
        Denotes the data segment.

     .bss
        Denotes reserved space.

     .code32
        Generate code for a 32-bit segment (default).

     .code16
        Generate code for a 16-bit segment.

     .globl symbol
        This defines a global symbol. For ELF you apparently also
        need:

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     .type symbol, @function
        This denotes the ELF symbol type. You'd better use the
        ENTRY(symbol) macro though.

     .byte .word .long
        Define bytes (etc.), akin the db command in TASM.

     .ascii "foo"
        Define a string, also the same as the db command.

     .asciz "foo"
        Define a zero-terminated string. There needs not be extra
        .byte 0x0A commands for control characters; rather use C
        escapes like this:
        .data
        foo: .asciz  "Hello, World!\n"
        .text
        bar: mov     eax,offset foo
             push    eax
             call    printf
             pop     eax
             ret

OPERATIONS

     This example also shows the basic command usage. Some com-
     mands are different from both Intel common and the gas
     intel_mode texinfo documentation.

     lret
        Used exactly as the retf command.

     mov byte ptr [bx],0x80
        The byte ptr is a must.

     jmp 0xF000,0xFFF0
        This is the way long jumps (jmp far) are defined. The
        manual states wrongly that one has to use jmpl or ljmp.

     . = 0x40 + _start
        The org directive refined. Better than in NASM. It also
        is a good way to check code size boundaries.

     rep movsd
        Just write them one after the other.

     fs mov eax,[0]
        This is a segment prefix usage. Note, here is no dword
        ptr needed.

CONCLUSION

     It's not difficult to write intel code in GNU as, once you
     have learned about the differences. In fact, it is even more

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     consistent than for example NASM, which insists on calling
     pushf pushfw, and issues a pushfd when writing pushf in 32-
     bit mode.

ACKNOWLEDGEMENTS

     Copyright (c) 2004 mirabilos. All rights reserved.
     Credits go to Intel, AMD, Microsoft, Borland, the NASM and
     YASM projects and the Free Software Foundation.

BUGS

     Probably some typos and omissions. Also, my nroff isn't the
     best. Should convert to mdoc.
     The discuss list at http://www.mirbsd.org/rss.htm#lists
     takes suggestions, please.

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