we can't PUSH a byte?

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Teehee

Joined: 05 Aug 2009
Posts: 570
Location: Brazil

Teehee

Code:

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

teste db 0

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Sorry if bad english.

09 Jul 2010, 11:08

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 17934
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: 570
Location: Brazil

Teehee

oh.. o.o

09 Jul 2010, 11:10

Teehee

Joined: 05 Aug 2009
Posts: 570
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: 17934
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: 570
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: 723
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

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