Problem with 16/32 bit mode code

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faser

Joined: 03 Oct 2010
Posts: 4

faser 03 Oct 2010, 13:18

Hello,

I've downloaded the DLL version of FASM and fired up the ASM demo program.
When I encode: "xor eax, eax" I get "66 31 C0" which is "xor ax, ax"!

Am I missing something or is this a major bug?

Thank you.

03 Oct 2010, 13:18

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20445
Location: In your JS exploiting you and your system

revolution 03 Oct 2010, 13:32

Code:

use32
xor eax,eax ;31 C0
xor ax,ax ;66 31 C0
use16
xor eax,eax ;66 31 C0
xor ax,ax ;31 C0

By default fasm starts in 16 bit mode. Which you will need to change to 32 bit mode if that is what you need.

03 Oct 2010, 13:32

edfed

Joined: 20 Feb 2006
Posts: 4353
Location: Now

edfed 03 Oct 2010, 13:43

revolution wrote:

Code:
use32
xor eax,eax ;31 C0
xor ax,ax ;66 31 C0
use16
xor eax,eax ;66 31 C0
xor ax,ax ;31 C0    
By default fasm starts in 16 bit mode. Which you will need to change to 32 bit mode if that is what you need.

this is due to the PC compatible, it start by default in 16 bit mode, then, fasm too is in 16 bit mode by default.

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03 Oct 2010, 13:43

faser

Joined: 03 Oct 2010
Posts: 4

faser 03 Oct 2010, 13:46

Thanks a lot for your help.

03 Oct 2010, 13:46

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