[SOLVED] REPE instruction fails when paired with CMP

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Krecik

Joined: 19 Jul 2012
Posts: 13

Krecik 22 Aug 2012, 08:24

I try to write comparing loop. It should work but it doesn't

Code:

mov ecx,6
mov ebx,my_string
repe cmp byte[ebx+ecx],0
...
my_string db "Hello",0

Is there anything that I must know about?

Last edited by Krecik on 23 Aug 2012, 08:49; edited 1 time in total

22 Aug 2012, 08:24

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20632
Location: In your JS exploiting you and your system

revolution 22 Aug 2012, 08:51

REP only works with the string instructions: MOVS, CMPS, LODS, STOS

The Intel and AMD manuals state this quite clearly.

22 Aug 2012, 08:51

Krecik

Joined: 19 Jul 2012
Posts: 13

Krecik 22 Aug 2012, 12:06

I tested and REP works also with SCAS and others instructions.

Are the REP operations faster than looping by J# conditions?

22 Aug 2012, 12:06

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 22 Aug 2012, 17:56

SCAS is also string instruction. revolution simply forgot about it. REP (of all kinds) does not work with CMP.
Use SCAS instead.

22 Aug 2012, 17:56

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20632
Location: In your JS exploiting you and your system

revolution 23 Aug 2012, 05:32

JohnFound wrote:

SCAS is also string instruction. revolution simply forgot about it.

Yes, I did forget about it. Sorry if it caused some confusion.

Krecik wrote:

Are the REP operations faster than looping by J# conditions?

We don't know. Speed of execution depends upon many many factors. There could be some situations where one is faster than the other, and other situations where the outcome is reversed.

23 Aug 2012, 05:32

Krecik

Joined: 19 Jul 2012
Posts: 13

Krecik 23 Aug 2012, 08:48

Thank you both.
I learn something new every day!

23 Aug 2012, 08:48

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