Operand sizes in USE16/USE32 + Jmp far ?

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ouadji

Joined: 24 Dec 2008
Posts: 1081
Location: Belgium

ouadji 29 Mar 2009, 10:53

hi all,

Code:

format       PE native 4.0 at 10000h
entry        DriverEntry
....
code 32 bits
....
(in this driver, i switch to real mode from protected mode)
USE16
...
JMP FAR AAAA:BBBB (2 words) <----- my question !!!

Jmp far 16 bits :
Opcode : 66 EA BBBB AAAA ( 2 words)

I don't find the correct syntax for to get "66 EA BBBB AAAA" Embarassed

I have to manually encode : db 66h , 0EAh , dw n°1, dw n°2 Rolling Eyes

thank you all

EDIT by DOS386 : enhanced subject

29 Mar 2009, 10:53

revolution
When all else fails, read the source

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

revolution 29 Mar 2009, 12:00

Code:

use32
JMP FAR DWORD AAAA:BBBB ;66 EA BBBB AAAA
use16
JMP FAR AAAA:BBBB ;EA BBBB AAAA

Last edited by revolution on 29 Mar 2009, 12:06; edited 1 time in total

29 Mar 2009, 12:00

edfed

Joined: 20 Feb 2006
Posts: 4354
Location: Now

edfed 29 Mar 2009, 12:02

66 is a size prefix.
your code is first encoded for 32 bits instrctions, then , the processor needs to see the instruction as a 16bit instruction, then, the size prefix is there.

if you encode 32bits instructions in 16 bits code, you have the same result.

29 Mar 2009, 12:02

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8390
Location: Kraków, Poland

Tomasz Grysztar 29 Mar 2009, 12:05

revolution wrote:

Code:

use32
JMP FAR AAAA:BBBB ;66 EA BBBB AAAA
use16
JMP FAR AAAA:BBBB ;EA BBBB AAAA

I'd rather put it this way:

Code:

use32
jmp far 0xAAAA:0xBBBB        ; EA BB BB 00 00 AA AA
jmp far dword 0xAAAA:0xBBBB  ; 66 EA BB BB AA AA

use16
jmp far 0xAAAA:0xBBBB        ; EA BB BB AA AA
jmp far pword 0xAAAA:0xBBBB  ; 66 EA BB BB 00 00 AA AA

ouadji: you're not supposed to put "use16" code into PE drivers (well, unless you really know what you're doing). You only have to put the right size operator into the instruction to generate the 16-bit variant.

29 Mar 2009, 12:05

revolution
When all else fails, read the source

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

revolution 29 Mar 2009, 12:07

Wow, I just fixed my post and already two replies in between. Hehe, that will teach me to post before testing.

29 Mar 2009, 12:07

ouadji

Joined: 24 Dec 2008
Posts: 1081
Location: Belgium

ouadji 29 Mar 2009, 16:17

ok, I understand, thank you very much

PS : to Tomasz Grysztar
"You only have to put the right size operator into the instruction to generate the 16-bit variant" ...
yes indeed, this is the right solution. thank you for your advice.

Wink

29 Mar 2009, 16:17

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