test instruction problem..

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Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 28 Jun 2011, 13:02

Hi everyone, I'm bit stuck here.. I got code like this:

Code:

@@:
     mov ecx,-1
     xor eax,eax
     cld
     repne scasb
     test ebx,ebx
     dec ebx
     jnz @b

Here, EBX = 2 but.. This routine loops only 1 times.. why ? Sad

after first one, it changes Z flag.. but I need to do it twice!
Here's result..

Code:

test 2,2 ;Z flag becomes 0, OK JNZ works fine.
test 1,1 ;Z flag becomes 0, OK JNZ should work fine but after DEC EBX Z flag is set to 1.. WHY ?

28 Jun 2011, 13:02

r22

Joined: 27 Dec 2004
Posts: 805

r22 28 Jun 2011, 13:15

Code:

DEC ebx ; changes Z flag

Try using JNS instead of JNZ
*edit* also you really don't need the TEST instruction at all...

Last edited by r22 on 28 Jun 2011, 13:15; edited 1 time in total

28 Jun 2011, 13:15

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20309
Location: In your JS exploiting you and your system

revolution 28 Jun 2011, 13:15

DEC also alters the Z flag. Indeed in your code TEST is superfluous.

28 Jun 2011, 13:15

Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 28 Jun 2011, 13:16

But why ? First time it does not, second time it does.
I'll try JNS..

28 Jun 2011, 13:16

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20309
Location: In your JS exploiting you and your system

revolution 28 Jun 2011, 13:17

JNS will give you two loops, and thus the code will execute three times. Is that what you want?

28 Jun 2011, 13:17

Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 28 Jun 2011, 13:21

Oh yes, I need 3 times with value of 2.. But I don't understand why DEC EBX modifies Z flag.. First time it does not and second time it does.. Can someone explain me please ? Smile

Thanks.

28 Jun 2011, 13:21

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20309
Location: In your JS exploiting you and your system

revolution 28 Jun 2011, 13:24

DEC always changes the Z flag according to the result. The first time through the DEC Z flag result is the same as the TEST result so you don't see the change.

BTW: It is explained in the manuals.

28 Jun 2011, 13:24

Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 28 Jun 2011, 13:27

ahh.. I got it, thank you! Smile

28 Jun 2011, 13:27

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