Assembly test eax,eax?

What should this do:

Code:

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test eax,eax

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I've seen it in several places.

Am I right test does bitwise AND between the arguments?
But when you use test eax,eax, it's useless, because they're the same numbers?
It's like (var&var) in C++.

What else does test eax,eax do? Why is it used?

IIRC, because mov eax,value doesn't set any of the condition flags, so you need a test of some sort to set the flags if you plan on doing some kind of branch.

Now, when test eax,eax is followed by a jz address, then jz checks the ZF flag?
So it just checks if eax is 0?

Seems like it

>So it just checks if eax is 0?
Yes.

I'm totally confused, here's some code (it's inside of a function):

Code:

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test eax,eax
jz label2
retn
:label
    db 00h
:label2
    mov eax,label
    retn
:label3
    mov edx,[eax]

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If eax is zero, it is set to zero? And :label3 is never reached, because if eax is not zero, then it returns immediately, and otherwise it sets eax to 0 and then returns.

And even if it would reach :label3, eax would be 0 and then it gives edx the value that is on the memory address 0x0? It doesn't make sense.

>If eax is zero, it is set to zero?
No, test doesn't change the value. It's basically an AND instruction that doesn't assign the result to a destination.

>And :label3 is never reached
Not in that snippet, but I doubt that's the whole of the program.

Quote:

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No, test doesn't change the value. It's basically an AND instruction that doesn't assign the result to a destination.

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I don't mean that.

Code:

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test eax,eax
jz label2

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If eax is zero, it jumps to label2.
Label2 sets it to zero:

Code:

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:label
    db 00h
:label2
    mov eax,label
    retn

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>Label2 sets it to zero:
It doesn't seem that way to me, but then again, none of your snippet makes sense out of context. :rolleyes:

It's from a disassembled program...

What does it seem to you?

It sets eax to the address of label, not the value there. Then it returns, and eax is the return value.

Then eax points to 0. :)

Only if it's interpreted as a pointer to a single char.

Otherwise it points to 00B8D94Bh (the hex of the next instructions :), not very meaningful)
It can also be used to jump to that place (which seems useless here).

So I think it is meant to be used as a single char (ah).