Re: A basic question on asm

From: Grumble (devnull_at_kma.eu.org)
Date: 09/24/04 

Date: Fri, 24 Sep 2004 11:20:40 +0000 (UTC)

C wrote:

> Percival wrote:
>
>> Rong wrote:
>>
>>> I ran into this x86 sentence:
>>> mov %eax,(%esp,1)
>>> I am not sure what (%esp, 1) part mean.
>>> And I could not find the explanation in x86 manual.
>>> Can anyone explain a little?
>>
>> It is AT&T syntax. I am not familiar with it, but possibly the gas
>> manual or some sort should tell you what it means.
>>
>> My *GUESS*, and only a guess, is that it is mov [esp+1], [eax] in
>> intel syntax.
>>
>> But it is only a guess. I need to learn at&T syntax soon.
>
> Close, your example would be "mov %eax, 1(%esp)" in gas.
> "mov %eax, (%esp,1)" is "mov [esp], eax" the 1 being the scale
> so "mov %eax,(%ebx,2)" is "mov [ebx*2], eax".

Nope :-)

mov %eax, (%ebx,2) == mov [2*no_index_register+ebx], eax

In other words, it's probably a syntax error.

$ cat toto.s
         mov %eax,(%ebx,1)
         mov %eax,(%ebx,2)
         mov %eax,(%ebx,4)
         mov %eax,(%ebx,8)
         mov %eax,(,%ebx,1)
         mov %eax,(,%ebx,2)
         mov %eax,(,%ebx,4)
         mov %eax,(,%ebx,8)

$ as -v toto.s
GNU assembler version 2.13.90.0.18 (i386-redhat-linux) using BFD
version 2.13.90.0.18 20030206
toto.s: Assembler messages:
toto.s:2: Warning: scale factor of 2 without an index register
toto.s:3: Warning: scale factor of 4 without an index register
toto.s:4: Warning: scale factor of 8 without an index register

$ objdump -d a.out

a.out: file format elf32-i386

Disassembly of section .text:

00000000 <.text>:
    0: 89 03 mov %eax,(%ebx)
    2: 89 03 mov %eax,(%ebx)
    4: 89 03 mov %eax,(%ebx)
    6: 89 03 mov %eax,(%ebx)
    8: 89 04 1d 00 00 00 00 mov %eax,0x0(,%ebx,1)
    f: 89 04 5d 00 00 00 00 mov %eax,0x0(,%ebx,2)
   16: 89 04 9d 00 00 00 00 mov %eax,0x0(,%ebx,4)
   1d: 89 04 dd 00 00 00 00 mov %eax,0x0(,%ebx,8)

IMO, gas should reject mov %eax,(%ebx,1) as a syntax error on x86. 

-- 
Regards, Grumble

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