which used more

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l4m2

Joined: 15 Jan 2015
Posts: 667

l4m2 27 Mar 2015, 05:40

Code:

Mov eax,0ffffffffh

(doesn't change flags)

Code:

xor eax,eax
dec eax

(if eax is already 0)

Code:

stc
sbb eax,eax

(if CF is already 1)

Code:

or eax,-1

(only one instruction)

Last edited by l4m2 on 27 Mar 2015, 13:17; edited 1 time in total

27 Mar 2015, 05:40

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 27 Mar 2015, 06:04

Except the 5 bytes "mov", all these are 3 bytes long, but "or eax, -1" is better because is only one instruction.

But sometimes on initialization I use:

Code:

xor  eax, eax
mov  [somewhere1], eax
dec  eax
mov  [somewhere2], eax

The same way, if the CF is already set, SBB can be better.

27 Mar 2015, 06:04

randomdude

Joined: 01 Jun 2012
Posts: 83

randomdude 27 Mar 2015, 11:07

i have always seen or eax,-1 and thats what i use as well

27 Mar 2015, 11:07

l4m2

Joined: 15 Jan 2015
Posts: 667

l4m2 27 Mar 2015, 13:10

And what about

Code:

mov dword[ebp-4],0'ffff'ffffh

Code:

or eax,-1
mov [ebp-4],eax

Code:

or dword [ebp-4],-1

(Mainly the latter two)

27 Mar 2015, 13:10

l4m2

Joined: 15 Jan 2015
Posts: 667

l4m2 27 Mar 2015, 15:13

Code:

mov eax,100

Code:

lea eax,[word 100]

Code:

xor eax,eax
mov al,100

27 Mar 2015, 15:13

randomdude

Joined: 01 Jun 2012
Posts: 83

randomdude 27 Mar 2015, 15:32

Code:

or dword [ebp-4],-1

Code:

mov eax,100

27 Mar 2015, 15:32

HaHaAnonymous

Joined: 02 Dec 2012
Posts: 1178
Location: Unknown

HaHaAnonymous 28 Mar 2015, 00:36

Quote:

which used more

Not sure. I remember I used:

Code:

          xor               eax,eax
          not               eax

But now I just follow the simple way:

Code:

          mov               eax,$FFFFFFFF

Code:

          mov               eax, dword [_value]

_value:
          dd $FFFFFFFF

It works. That is why "mov" exists.

28 Mar 2015, 00:36

l4m2

Joined: 15 Jan 2015
Posts: 667

l4m2 28 Mar 2015, 04:13

HaHaAnonymous wrote:

Quote:

which used more

Not sure. I remember I used:

Code:

          xor               eax,eax
          not               eax

But now I just follow the simple way:

Code:

          mov               eax,$FFFFFFFF

Code:

          mov               eax, dword [_value]

_value:
          dd $FFFFFFFF

It works. That is why "mov" exists.

MOV can be used when needing
a = 1000;

28 Mar 2015, 04:13

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 19069
Location: In your JS exploiting you and your system

revolution 28 Mar 2015, 04:17

This topic would be much more interesting if it was discussing loading of constants in ARM code before the v7 architecture gave us MOVW and MOVT.

For x86 it is trivial.

28 Mar 2015, 04:17

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