unwanted Operand-Size Prefix 0x66 (fasmg)

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Werner Drasche

Joined: 27 Sep 2022
Posts: 2

Werner Drasche 27 Sep 2022, 12:28

main.asm

Code:

include "80386.inc"
include "format/elf32.inc"

public _start

section ".text" executable
_start:
mov eax, 1
xor ebx, ebx
int 0x80

was compiled with

Code:

#!/bin/bash
export INCLUDE=/usr/local/lib/fasmg/x86/include
fasmg main.asm main.o
ld -m elf_i386 main.o -o main
./main

and immediately leads to segfault. This is caused by an unwanted operand size override to 16 bit which transforms 1 instruction into 2 (the rest of the immediate is interpreted separately). I am a total beginner in assembly programming, so help would be greatly appreciated.

Extra info:
nasm doesn't use the prefix and the code runs fine without segfaulting.

27 Sep 2022, 12:28

bitRAKE

Joined: 21 Jul 2003
Posts: 4073
Location: vpcmpistri

bitRAKE 27 Sep 2022, 15:40

Looks like "80386.inc" defaults to use16 - just adding use32 should be sufficient.

Code:

include "80386.inc"
include "format/elf32.inc"
use32

public _start

section ".text" executable
_start:
mov eax, 1
xor ebx, ebx
int 0x80

... untested.

It might be better to use the \format\format.inc, or look at how it uses the other files you are including.

27 Sep 2022, 15:40

Werner Drasche

Joined: 27 Sep 2022
Posts: 2

Werner Drasche 27 Sep 2022, 18:19

Thank you for the quick reply. Your suggested solution works perfectly and from now on I'll be using format.inc as it sets use32 anyway.

27 Sep 2022, 18:19

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