we can't PUSH a byte?

flat assembler
Message board for the users of flat assembler.

Index > Main > we can't PUSH a byte?

Author

Thread

Teehee

Joined: 05 Aug 2009
Posts: 568
Location: Brazil

Teehee

Code:

push teste
push byte[teste]  ; Error: invalid size of operand
push word[teste]
push dword[teste]

teste db 0

_________________
Sorry if bad english.

09 Jul 2010, 11:08

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 17658
Location: In your JS exploiting you and your system

revolution

No. The CPU does not support.

09 Jul 2010, 11:09

Teehee

Joined: 05 Aug 2009
Posts: 568
Location: Brazil

Teehee

oh.. o.o

09 Jul 2010, 11:10

Teehee

Joined: 05 Aug 2009
Posts: 568
Location: Brazil

Teehee

Code:

0040100C   6A 10              push    10h
0040100E   66:68 1000         push    word 10h
00401012   68 10000000        push    dword 10h

why in this case it stores 10h like a byte?

_________________
Sorry if bad english.

09 Jul 2010, 11:17

shoorick

Joined: 25 Feb 2005
Posts: 1608
Location: Ukraine

shoorick

looks more as it stores it as dword Wink

try it under debugger step-by-step Wink

09 Jul 2010, 11:41

edemko

Joined: 18 Jul 2009
Posts: 549

edemko

push dword 10: $68'0000000a

09 Jul 2010, 11:42

edemko

Joined: 18 Jul 2009
Posts: 549

edemko

ie. fasm tries to optimize it to make code a less as possible until you specify explicit

09 Jul 2010, 11:45

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 17658
Location: In your JS exploiting you and your system

revolution

Teehee wrote:

Code:

0040100C   6A 10              push    10h
0040100E   66:68 1000         push    word 10h
00401012   68 10000000        push    dword 10h

why in this case it stores 10h like a byte?

The CPU can push signed values. 0x10 is sign extended to the operand size, in this case a dword.

Values up to 0x7f for positive, and down to 0x80 for negative.

|0x6a,0x80| will push 0xffffff80 to the stack. It is just a short form to save bytes in the code stream.

09 Jul 2010, 12:28

Teehee

Joined: 05 Aug 2009
Posts: 568
Location: Brazil

Teehee

nice.. thank you.

09 Jul 2010, 12:35

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr

Teehee,

If you, for any reason, need to push exactly a byte, lea esp, [esp-1] / mov byte[esp], something may be useful. Wink

11 Jul 2010, 02:15

sinsi

Joined: 10 Aug 2007
Posts: 707
Location: Adelaide

sinsi

Don't push a word onto the stack in windows if you are going to call an API, the stack is supposed to be dword aligned

11 Jul 2010, 02:21

< Last Thread | Next Thread >

Forum Rules:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum