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Index > Main > [SOLVED] REPE instruction fails when paired with CMP

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Krecik



Joined: 19 Jul 2012
Posts: 13
Krecik
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
Post 22 Aug 2012, 08:24
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17716
Location: In your JS exploiting you and your system
revolution
REP only works with the string instructions: MOVS, CMPS, LODS, STOS

The Intel and AMD manuals state this quite clearly.
Post 22 Aug 2012, 08:51
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Krecik



Joined: 19 Jul 2012
Posts: 13
Krecik
I tested and REP works also with SCAS and others instructions.

Are the REP operations faster than looping by J# conditions?
Post 22 Aug 2012, 12:06
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JohnFound



Joined: 16 Jun 2003
Posts: 3502
Location: Bulgaria
JohnFound
SCAS is also string instruction. revolution simply forgot about it. REP (of all kinds) does not work with CMP.
Use SCAS instead.
Post 22 Aug 2012, 17:56
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17716
Location: In your JS exploiting you and your system
revolution
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.
Post 23 Aug 2012, 05:32
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Krecik



Joined: 19 Jul 2012
Posts: 13
Krecik
Thank you both.
I learn something new every day!
Post 23 Aug 2012, 08:48
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