Problems with GNU as Intel syntax

[SimonZilch]

I'm not sure if it matters, but when I did gas with intel syntax in my OS, I did

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.intel_syntax noprefix

I'm not sure of the difference though

"prefix" means you want to use percent signs('%') before your registers. "noprefix" means you don't. From what I've been able to determine, that's the only difference, and it has nothing to do with dollar signs('$'). Most Intel syntax assemblers (that I've used) don't prefix the registers with anything, but I like the idea of having a prefix to distinguish registers from labels. Unfortunately though, after years of using other assemblers, I'm too accustomed to NOT typing the prefix; so I've just given up trying and dropped the prefix.

Just to remind people of my remaining question:

The second problem I ran into is a warning for this code:
. . .

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	mov dword ptr [0xb8000], %eax

The warning is: "Warning: Treating `dword ptr [0xb8000]' as memory reference"
That doesn't make sense. Of course it's treating it as a memory reference. I very explicitly told it that it's a memory reference. . . .
So how do I get rid of this warning? Is my syntax wrong? If so, how do I do it right?

I've also determined that this gives the same warning:

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	.set VIDEO_MEM, 0xb8000 # same result with .eqv or .equiv instead of .set
	mov dword ptr [VIDEO_MEM], eax

as says: " Warning: Treating `dword ptr [VIDEO_MEM]' as memory reference"
Any thoughts?

[earlz]

This is strange. With my GCC it doesn't give any warnings on your code(with -Wall)

My test file is

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#include <stdlib.h>
#include <stdio.h>

int a;

int main(){

	__asm(".intel_syntax noprefix\n"
		"mov dword ptr [0xb8000], eax\n"
		".att_syntax\n");


	return 0;

}

are your trying to do inline code or standalone assembly files?

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[earlz@EarlzBeta-~] $ gcc --version
gcc (GCC) 3.3.5 (propolice)
Copyright (C) 2003 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

[earlz@EarlzBeta-~] $ as --version
GNU assembler 2.15
Copyright 2002 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License.  This program has absolutely no warranty.
This assembler was configured for a target of `i386-unknown-openbsd4.4'.

[SimonZilch]

This is strange. With my GCC it doesn't give any warnings on your code(with -Wall)
. . .
are your trying to do inline code or standalone assembly files?
. . .

I was doing plain standalone assembly, but it doesn't make any difference:

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simon@zilch1:~$ cat test.c 
int main()
{
        __asm(".intel_syntax noprefix\n"
        "mov dword ptr [0xb8000], eax\n"
        ".att_syntax\n");

        return 0;
}
simon@zilch1:~$ gcc -m32 -Xassembler --32 -c test.c 
/tmp/ccukgSfh.s: Assembler messages:
/tmp/ccukgSfh.s:13: Warning: Treating `dword ptr [0xb8000]' as memory reference

Note that "-m32" and "-Xassembler --32" tell the compiler and assembler (respectively) that I want 32-bit code (since I'm on a 64-bit system). It does the same thing with "-m64 -Xassembler --64".
My assembler version is a little more recent than yours by a few years:

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simon@zilch1:~$ as --version
GNU assembler version 2.17.50.0.18-1 20070731
Copyright 2007 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License version 3 or later.
This program has absolutely no warranty.
This assembler was configured for a target of `x86_64-redhat-linux'.

I guess the warning must have been added to the assembler after your version. I doubt it's anything with the compiler, but my gcc version's a little more recent too.

[earlz]

hmm.. gcc and as must be getting a little rusty with its old age and code bloat lol

[cygx]

There is even more strangeness afoot. Consider the following code:

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void test(void)
{
    __asm__("movl %eax,($0xdeadbeaf)");
    __asm__("movl %eax,(%ecx)");

    __asm__(".intel_syntax prefix");

    __asm__("mov [dword ptr $0xdeadbeaf],%eax");
    __asm__("mov [dword ptr %ecx],%eax");

    __asm__(".intel_syntax noprefix");

    __asm__("mov [dword ptr 0xdeadbeaf],eax");
    __asm__("mov [dword ptr ecx],eax");

    __asm__(".att_syntax prefix");
}

When you look at the objdump, you'll find this

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   3:   a3 00 00 00 00          mov    %eax,0x0
   8:   89 01                   mov    %eax,(%ecx)
   a:   a3 00 00 00 00          mov    %eax,0x0
   f:   89 01                   mov    %eax,(%ecx)
  11:   a3 af be ad de          mov    %eax,0xdeadbeaf
  16:   89 01                   mov    %eax,(%ecx)

Only the line which generates the warning won't silently drop the value!?

----

Now, look at this code

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void test(void)
{
    *(int *)0xdeadbeaf = 0x1234;

    __asm__(".att_syntax prefix");
    __asm__("movl $0x1234,($0xdeadbeaf)");

    /* this won't compile: Error: too many memory references for 'mov'
    __asm__(".intel_syntax prefix");
    __asm__("mov [dword ptr $0xdeadbeaf],$0x1234");
    */

    __asm__(".intel_syntax noprefix");
    __asm__("mov [dword ptr 0xdeadbeaf],0x1234");

    __asm__(".att_syntax prefix");
}

and its objdump

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   3:   c7 05 af be ad de 34    movl   $0x1234,0xdeadbeaf
   a:   12 00 00
   d:   c7 05 00 00 00 00 34    movl   $0x1234,0x0
  14:   12 00 00
  17:   c7 05 af be ad de 34    movl   $0x1234,0xdeadbeaf
  1e:   12 00 00

Anyone here who can explain this?

[cygx]

Anyone here who can explain this?

I actually figured this out myself with help from the nice people at stackoverflow.

Here it the aswer as to How to use address constants in GCC x86 inline assembly.