Section or structure bug?

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AlexP

Joined: 14 Nov 2007
Posts: 561
Location: Out the window. Yes, that one.

AlexP 01 Apr 2008, 03:52

If I compile with this (short snippet of my code):

Code:

 section '.udata' data readable writeable

 aes_keys   aes_keys_
 aes_tables aes_tables_

It compiles to ~7k, but if I have this:

Code:

 section '.udata' data readable writeable

 mmx_num_16 dd 16
 aes_keys   aes_keys_
 aes_tables aes_tables_

It also compiles to ~7k. If I put it after the structures:

Code:

 section '.udata' data readable writeable

 aes_keys   aes_keys_
 aes_tables aes_tables_
 mmx_num_16 dd 16

It compiles to around 21k. Finally, if I simply place it inside of the aes_keys structure, it compiles normally to ~7k. But if I place it inside of the aes_tables structure, it compiles to 21k. Why is it that if I place a dword declaration AFTER the structures it compiles to more than 6 times as large?, and if I place it BEFORE it compiles normally?

01 Apr 2008, 03:52

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20621
Location: In your JS exploiting you and your system

revolution 01 Apr 2008, 03:54

Not a bug. You have told fasm to put initialised data (mmx_num_16 dd 16) into the output file. So it has to extend the file to the point where you have that one dword of initialised data and then place the value 16 at that position.

01 Apr 2008, 03:54

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20621
Location: In your JS exploiting you and your system

revolution 01 Apr 2008, 03:56

This compiles to 1 meg

Code:

rb 1 shl 20-1
db 0

This compiles to 1 byte

Code:

db 0
rb 1 shl 20-1

01 Apr 2008, 03:56

AlexP

Joined: 14 Nov 2007
Posts: 561
Location: Out the window. Yes, that one.

AlexP 01 Apr 2008, 12:49

Ha! Wow... Well, thanks, just a little odditiy that took me awhile to spot. Turns out I didn't need that dword anyways Smile

01 Apr 2008, 12:49

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