Need size example.

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Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 24 Apr 2011, 00:28

Hello everyone. I need little example how to take 2 dwords and do something with it. and of course in 32 bit mode.
I have question, can CDQ instruction do that ?
for example:

Code:

mov eax,0xFFFFFFFF
mov edx,0xEEEEEEEE
cdq
mov qword[eax],0xAAAAAAAAAAAAAAAA

something like this.. thank you.

24 Apr 2011, 00:28

bitRAKE

Joined: 21 Jul 2003
Posts: 4060
Location: vpcmpistri

bitRAKE 24 Apr 2011, 04:25

CMPXCHG8B can store 2 DWORDS. PUSHAD can store more. Very Happy

Not clear what you are after. Otherwise I'd give an example.

After CDQ, EDX is either 0 or -1.
(Based on high-bit of EAX.)

Is MMX or SSE an option?

24 Apr 2011, 04:25

ouadji

Joined: 24 Dec 2008
Posts: 1081
Location: Belgium

ouadji 24 Apr 2011, 09:47

but "pushad" can't be used with "lock" Crying or Very sad

but ...

push ss
pop ss
pushad

A pop_ss instruction inhibits all interrupts,
including the NMI interrupt, until after execution of the next instruction.

_________________
I am not young enough to know everything (Oscar Wilde)-

24 Apr 2011, 09:47

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20418
Location: In your JS exploiting you and your system

revolution 24 Apr 2011, 09:52

Hehe:

Code:

macro lock instr {
  push ss
  pop ss
  instr
}

24 Apr 2011, 09:52

Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 24 Apr 2011, 10:30

I really don't know what to do so.. All examples are welcome for me.. Smile

I don't know what's MMX and how to ise that.. and what that does "push ss/pop ss/pushad" ? I know only that crashes olly (like exploit)
I need to push 8 byte argument in API call.. for example:

Code:

movq mm0,0xFFFFFFFFFFFFFFFF
invoke MessageBox,0,mm0,mm0,MB_OK

This would not work right ?.. I need something like this but with QWORD..

24 Apr 2011, 10:30

SPTH

Joined: 24 Jul 2004
Posts: 91

SPTH 24 Apr 2011, 16:18

Code:

invoke MessageBox,0,mm0,mm0,MB_OK

Code:

push MB_OK
push mm0
push mm0
push 0
stdcall [MessageBox]

So you can simple push two 32bit values to get one 64bit value on stack, for the API to work with it:

Code:

mov eax, dword[HighDWORD]
push eax
mov eax, dword[LowDWORD]
push eax
stdcall [API_THAT_NEEDS_ONE_64BIT_VALUE_ON_STACK]

If you explain what you need more, somebody could help you i guess.

- - -
Something else

Code:

push ss
pop ss

isnt that just a NOP??? :o

24 Apr 2011, 16:18

ouadji

Joined: 24 Dec 2008
Posts: 1081
Location: Belgium

ouadji 24 Apr 2011, 18:08

No, this is not like a "nop"
the combining of these two instructions offer, for the third, a very interesting property.

Quote:

INTEL 2B 4-190 (253667-038US April 2011)

Pop a Value from the Stack - pop ss

A POP SS instruction inhibits all interrupts, including the NMI interrupt, until after execution of the next instruction. This action allows sequential execution of POP SS and MOV ESP, EBP instructions without the danger of having an invalid stack during an interrupt.

MOV ESP, EBP ...
or any other instruction !
the third instruction will be executed atomically.

_________________
I am not young enough to know everything (Oscar Wilde)-

24 Apr 2011, 18:08

Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 24 Apr 2011, 23:47

I'm trying to do following but I fail.
movq mm0,0xFFFFFFFFFFFFFFFF
says mm0 is invalid address..

24 Apr 2011, 23:47

SPTH

Joined: 24 Jul 2004
Posts: 91

SPTH 25 Apr 2011, 00:47

See page 47 in "flat assembler 1.69 Programmer's Manual" by Tomasz Grysztar (in the root dir of your FASM compiler, called FASM.PDF):

Quote:

movq copies a quad word from the source operand to the destination
operand. At least one of the operands must be a MMX register, the second
one can be also a MMX register or 64{bit memory location.
Code:
movq mm0,mm1 ; move quad word from register to register
movq mm2,[ebx] ; move quad word from memory to register
    

You can eighter use a different MMX register or a memory offset to fill the mm0 register. You can not use a constant.

See the manual for more infos. (you can use ctrl+F there, too :D )

25 Apr 2011, 00:47

Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 25 Apr 2011, 02:20

Oh, I got it! Thank you Smile

and also, last little question.
how much bytes is TBYTE ? Is that larger than QWORD ?

25 Apr 2011, 02:20

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20418
Location: In your JS exploiting you and your system

revolution 25 Apr 2011, 02:33

Overflowz wrote:

... and also, last little question.
how much bytes is TBYTE ? Is that larger than QWORD ?

Have you seen this link?

http://flatassembler.net/docs.php

25 Apr 2011, 02:33

Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 25 Apr 2011, 14:33

revolution
ohh.. sorry I'm always forgetting that Sad

thanks anyway Smile

Also, I'm interested why I can't do something like this ?

Code:

qval db 0xFFFFFFFFFFFFFFFF
..
mov qword[ebp],qval

says, illegal unstruction at "mov qword[ebp],qval"..
but mov tbyte[ebp],qval works fine.
can someone tell me what the point of using those registers ? and for what ? (of course from your experience)

25 Apr 2011, 14:33

SPTH

Joined: 24 Jul 2004
Posts: 91

SPTH 25 Apr 2011, 16:04

1) http://flatassembler.net/docs.php?article=manual -> ctrl+F -> "tbyte" ->

Quote:

qword 64bit=8byte
tbyte 80bit=10byte

Quote:

but mov tbyte[ebp],qval works fine.

Code:

include '...\FASM\INCLUDE\win32ax.inc'

.data
qval db 0xFFFFFFFFFFFFFFFF

.code
start:
        mov tbyte[ebp],qval
.end start

Gives "invalid size of operand". mov can be used for byte, word, dword - NOT qword (use movq) or tbyte (see http://flatassembler.net/docs.php?article=manual#2.1.13) or something else than 8,16,32bit.

Reading the manual and searching the answeres to these questions will help you alot understanding assembler more, I promise you Wink

25 Apr 2011, 16:04

Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 25 Apr 2011, 17:17

SPTH
I got it.. Thank you Smile

25 Apr 2011, 17:17

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