Question about instructions.

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Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 21 Jan 2011, 19:03

Hello everyone. I'm interesting if there is some instruction that does something if before instruction are compared ? For example:

Code:

cmp eax,9
je somewhere
ret
somewhere:
mov eax,5

Is there any way to do it without jumping ? like move(if equal) and movne(if not equal) instructions like this ? thank you. and sorry for my English.

21 Jan 2011, 19:03

Coty

Joined: 17 May 2010
Posts: 553
Location: ␀

Coty 21 Jan 2011, 19:11

For the ARM processor I believe yes, for x86 no.. at-least I have not seen any yet...

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21 Jan 2011, 19:11

Tyler

Joined: 19 Nov 2009
Posts: 1216
Location: NC, USA

Tyler 21 Jan 2011, 19:12

I don't think there's a universal way, but there are conditional extensions for some instructions. AFAIK, ret doesn't have a native conditional form, but you could do as revolution does for ARM(http://board.flatassembler.net/topic.php?t=11985), and write a macro to emulate conditional ret.

21 Jan 2011, 19:12

Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 21 Jan 2011, 20:09

Well, I dont need only mov, any istructions that are like instructions which are executed after CMP command.. Smile

and on x86 processors.

21 Jan 2011, 20:09

Tyler

Joined: 19 Nov 2009
Posts: 1216
Location: NC, USA

Tyler 21 Jan 2011, 20:16

I wrote:

you could do as revolution does for ARM(http://board.flatassembler.net/topic.php?t=11985), and write a macro to emulate conditional [your instruction here]

Quote:

on x86 processors

If you would have looked, you would have noticed his emulation is conditional jump based. x86 has conditional jumps. Smile

Why do you want to avoid conditional jumps? For code simplicity, or for efficiency? If it's for simplicity, then revolutions method is the easiest way I know of. If it's for efficiency, then the answer is: no, conditional jumps are required for some logic.

21 Jan 2011, 20:16

cod3b453

Joined: 25 Aug 2004
Posts: 618

cod3b453 21 Jan 2011, 22:00

Yes - the cmov instruction with the condition as a suffix:

Code:

mov ebx,5
cmp eax,9
cmove eax,ebx
ret

21 Jan 2011, 22:00

Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 22 Jan 2011, 01:25

Tyler
Sorry I'm just novice about that things. So easiest way is to make macro right ? and of course I need it for simplificy. Smile

cod3b453
Thanks, I'll try that. =)

22 Jan 2011, 01:25

b1528932

Joined: 21 May 2010
Posts: 287

b1528932 22 Jan 2011, 15:30

be sure to test for cmovCC using cpuid.

22 Jan 2011, 15:30

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20450
Location: In your JS exploiting you and your system

revolution 22 Jan 2011, 15:45

Tyler wrote:

... but you could do as revolution does for ARM ...

Slight correction there. I use it for fasmarm. Since ARM already has it as a native form. I use the macro in fasmarm (the x86 asm code) to give compatibility with 386+ CPUs (merely to be compliant with the original principles of fasm as used by Tomasz).

22 Jan 2011, 15:45

Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 27 Jan 2011, 13:49

Okay, thank you guys Smile

27 Jan 2011, 13:49

edfed

Joined: 20 Feb 2006
Posts: 4353
Location: Now

edfed 27 Jan 2011, 16:00

before pentium4, there are no cmov.

27 Jan 2011, 16:00

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20450
Location: In your JS exploiting you and your system

revolution 27 Jan 2011, 16:12

edfed wrote:

before pentium4, there are no cmov.

cmov was introduced with the P6 architecture (Pentium Pro).

27 Jan 2011, 16:12

edfed

Joined: 20 Feb 2006
Posts: 4353
Location: Now

edfed 27 Jan 2011, 16:45

P pro was made the same year than PII, but they waited P4 to introduce it in popular models.

pentium pro is rare to find, more than P4

27 Jan 2011, 16:45

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