memory alignment

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t@2

Joined: 05 Sep 2008
Posts: 3

t@2 05 Sep 2008, 14:55

apologies for what might be a dumb post...

- I was wondering - what is the best/standard approach to dynamic
dynamic memory allocation using fasm & win32 - i would like to allocate one or more block aligned on 16byte boundries...

any help would be greatly appreciated...

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o0MyBrainHertz0o

05 Sep 2008, 14:55

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20627
Location: In your JS exploiting you and your system

revolution 05 Sep 2008, 15:04

There are three main Windows allocation functions:

HeapAlloc
LocalAlloc (same as GlobalAlloc)
VirtualAlloc

The first two have no guaranteed alignment so for your case just allocate an extra 15 bytes and zero the last four bits of the return pointer after adding 15. VirtualAlloc is always aligned to a page boundary.

05 Sep 2008, 15:04

t@2

Joined: 05 Sep 2008
Posts: 3

t@2 05 Sep 2008, 15:19

many thanks Smile

- i was looking at these functions - but i didnt see
any mention of alignment...may have missed it - will give this a blast.

05 Sep 2008, 15:19

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20627
Location: In your JS exploiting you and your system

revolution 05 Sep 2008, 15:24

Alignment is simple to implement:

Code:

... ;eax has a memory pointer
add eax,15
and eax,not 15 ;now aligned to 16
...

Only works for power-of-two alignments of course.

05 Sep 2008, 15:24

t@2

Joined: 05 Sep 2008
Posts: 3

t@2 05 Sep 2008, 18:10

yeh thanks man - been a long time since i really worked at this level..
seems its going to take some time to readjust mentally - seeing your
reply makes me feel a little dumb :/ - still i guess its always best to
ask..the last time i really coded at this level was on my amiga..

while im here - is there a way to force stack alignment in a similar
way? - so pushed params are aligned on 16byte boundries? - heh, no doubt this is just as simple... feel free to shoot me Wink

thanks again.. matt/t@2

05 Sep 2008, 18:10

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 05 Sep 2008, 18:51

Code:

and esp, -16

I don't your particular context but in an EBP-based frame you could do this:

Code:

some_proc:
  push ebp
  mov  ebp, esp
  and  esp, -16

; Now you have ESP aligned to a 16-byte boundary

  leave
  ret

[edit]Removed unneded and memory waster "sub esp, 15"[/edit]

Last edited by LocoDelAssembly on 05 Sep 2008, 19:09; edited 1 time in total

05 Sep 2008, 18:51

edfed

Joined: 20 Feb 2006
Posts: 4354
Location: Now

edfed 05 Sep 2008, 18:54

maybe

Code:

add esp,15
and esp,not 15

edit:

yekk!

05 Sep 2008, 18:54

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 05 Sep 2008, 19:08

You know, actually it should be "and esp, -16" alone, better I edit my post.

05 Sep 2008, 19:08

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