Doubt with TEST instruction

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Cthulhu

Joined: 12 May 2005
Posts: 29

Cthulhu 28 Mar 2008, 11:24

Hello people!
I'm converting a piece of software from assembly to C++ and I found something that is confusing me although it must be a very basic doubt.

Code:

test    edi, 3
jnz     short some_func

EDI points to the following buffer:

Code:

004036D0 >00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................

I think I understood the TEST instruction incorrectly, because I thought the code above would check if the third bit was set, if not it would jump to some_func, but in my case the jnz instruction is not executed and EDI points to an empty buffer.

Can someone explain it to me?
Thanks in advance!
Cthulhu

28 Mar 2008, 11:24

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20430
Location: In your JS exploiting you and your system

revolution 28 Mar 2008, 11:31

TEST is just a binary AND of the two operands. FLAGS = (EDI AND 0x00000003). Check out the Intel/AMD manuals for details on all the instructions. If you haven't yet downloaded the manuals check out my website, all the details are there.

28 Mar 2008, 11:31

DJ Mauretto

Joined: 14 Mar 2007
Posts: 464
Location: Rome,Italy

DJ Mauretto 28 Mar 2008, 11:32

Hello

Code:

test    edi, 3                      ; EDI AND 11B
jnz     short some_func       ; Jump if bit0 or bit1 are set

28 Mar 2008, 11:32

Cthulhu

Joined: 12 May 2005
Posts: 29

Cthulhu 28 Mar 2008, 11:37

DJ Mauretto wrote:

Hello [code]
test edi, 3 ; EDI AND 11B
jnz short some_func ; Jump if bit0 or bit1 are set [\code]

Hi DJ Mauretto!
I understood that from the Intel Manual but what is confusing me is that EDI points to 0x00000000 so the bits 0 and 1 are not set, aren't they? So why ze zero flag was set?

28 Mar 2008, 11:37

DJ Mauretto

Joined: 14 Mar 2007
Posts: 464
Location: Rome,Italy

DJ Mauretto 28 Mar 2008, 11:43

Code:

EDI = 0
TEST EDI,3 = 0 AND 3 = 0 = Zero Flag Set 
JNZ short and so on.... NO JUMP because is Zero

28 Mar 2008, 11:43

Cthulhu

Joined: 12 May 2005
Posts: 29

Cthulhu 28 Mar 2008, 11:46

DJ Mauretto wrote:

Code:

EDI = 0
TEST EDI,3 = 0 AND 3 = 0 = Zero Flag Set 
JNZ short and so on.... NO JUMP because is Zero

Now I understand it! Very Happy

Thanks friend!

28 Mar 2008, 11:46

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20430
Location: In your JS exploiting you and your system

revolution 28 Mar 2008, 11:50

Cthulhu wrote:

... what is confusing me is that EDI points to 0x00000000 so the bits 0 and 1 are not set, aren't they? So why ze zero flag was set?

It does not matter what EDI points to, because it takes the value of EDI directly (in this case EDI=0x004036D0, so you get 0x004036D0 AND 0x00000003).

If you want to test the memory address pointed to by EDI use any of these:

Code:

test dword[edi],3
test word[edi],3
test byte[edi],3

28 Mar 2008, 11:50

Cthulhu

Joined: 12 May 2005
Posts: 29

Cthulhu 28 Mar 2008, 12:10

Thanks a lot revolution and DJ Mauretto!

28 Mar 2008, 12:10

AlexP

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

AlexP 28 Mar 2008, 21:20

Code:

; C-Style

If (edi % 4 == 0)
; code
Else
; here's the jump destination

Is equal to this in ASM:

Code:

test edi,3
jnz .end
; code
.end: ; here's the jump dest.

It's in Intel Optimization Manual, I use it for conditional jumping with power-of-two operand.

28 Mar 2008, 21:20

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