align by 4

Index > Windows > align by 4

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Ali.Z

Joined: 08 Jan 2018
Posts: 233

Ali.Z

Code:

_write.file:
                        mov edx,esi ; esi = buffer that contains data to write
                        push edx
                        mov eax,edi ; edi = number of chars in that buffer
                        call _file.align
                        call _strlen
                        ... ; some code for winapi writefile

Code:

_file.align:
                        xor edx,edx
                        mov ecx,04 ; divisor
                        ; EAX = 96h
                        div ecx
                        add edi,edx ; edx = reminder (2) edi = 96h
                        ret ; file is perfectly aligned the result is 98h

however, when edi and/or eax are 91h .. edx contain 1 after DIV
and when "add edi,edx" the result is 92h which is not aligned by 4

i also tried using:

Code:

@@:
shr eax,3 ; when eax=96 the final result is 2 which is great 96h + 2 = 98h
; but if eax = another number i dont get what i want
cmp eax,04
ja @B

excuse me guys, im poor man and bad in math.

btw i know i could sub by 4, but im looking for faster execution.

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Asm For Wise Humans

22 Jul 2018, 04:54

quirck

Joined: 18 Aug 2013
Posts: 5

quirck

Note that everything aligned by 4 has two least significant bits clear. This leads us to using AND.

Code:

and edi, -4

But clearing the low bits rounds the number down, and we presumably want to round up to the nearest multiple of 4. If the original value was already aligned, there is no problem. But if it wasn't, the result needs to be increased by 4. This can be achieved by adding 3 before AND:

Code:

add edi, 3
and edi, -4

The addition will produce a carry into bit 2 if and only if the original value was not aligned. And with bits 0 and 1 clear, the carry into bit 2 is effectively the desired increase by 4.

It may help to manually study some cases.
If edi = 0, after addition it becomes 3, after AND the result is 0.
If edi = 1, after addition it becomes 4, after AND the result is 4.
If edi = 2, after addition it becomes 5, after AND the result is 4.
If edi = 3, after addition it becomes 6, after AND the result is 4.
If edi = 4, after addition it becomes 7, after AND the result is 4.
If edi = 5, after addition it becomes 8, after AND the result is 8.
And so on.

22 Jul 2018, 07:48

Ali.Z

Joined: 08 Jan 2018
Posts: 233

Ali.Z

perfect, so perfect thanks man.

edit: i misunderstood, but now i do thats why edited.

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Asm For Wise Humans

Last edited by Ali.Z on 22 Jul 2018, 09:18; edited 2 times in total

22 Jul 2018, 09:10

quirck

Joined: 18 Aug 2013
Posts: 5

quirck

What result do you want to get for edi = 96h? 98h? With shr you'll get 4Ch.
Or is your intention not just rounding up but computing something else?

22 Jul 2018, 09:14

Ali.Z

Joined: 08 Jan 2018
Posts: 233

Ali.Z

no its my bad, for not understanding well enough .. now i have tested your code and it works perfectly.

thanks again.

im very happy, whatever value in eax will be aligned by 4 when i call this function.

Code:

_file.align:
                        mov ecx,03
                        add eax,ecx
                        not ecx
                        and eax,ecx
                        ret

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Asm For Wise Humans

22 Jul 2018, 09:19

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 16903
Location: In your JS exploiting you and your system

revolution

You can use constants directly in x86 code.

Code:

add eax,3
and eax,-4

22 Jul 2018, 10:10

Ali.Z

Joined: 08 Jan 2018
Posts: 233

Ali.Z

yeah, but its part of my asm style.
i like to play with registers more than ... yeah

thanks.

22 Jul 2018, 10:47

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