I am currently new to Assembler, but I've already understood how it generally works.
One question I still have:
why does for example



save the handle into eax ?
Why not into a different one ?

I can't find anything about it in the internet.
Everytime I want to use a WINAPI, I can't unterstand how the values get into various registers...

And why does this

assign the value 0 to eax ?

In other programming language we have to give another variable name like:

so that x becomes false.

Why does

just makes eax 0. May anyone explain that for me ?

Thanks in advance

1) In the Windows API returned values are always in eax.

2) The XOR reg,same_reg function always produces a zero result regardless of the incoming value of the register. It is commonly used in x86 code because the opcode is shorter than for MOV. But the XOR operation also affects the flags so you need to be aware of that.

XOR is a binary operation -- look into Wikipedia to see the logic of it.
But in simple programming terms it would be working like the following:

If bit in SOURCE is 0 -- DESTINATION bit will not change
If bit in SOURCE is 1 -- DESTINATION bit will invert its value

Example:


If SOURCE is the same as DESTINATION, then all '1' bits in DESTINATION will be inverted
to '0' and that makes whole result to be zero:

It's called "calling convention" - being a convention, it's the same across the entire Win32 API.


x86 instructions tend to have hardcoded destination; other achitectures might define the instruction as "XOR destination, operand1, operand2", but for x86 doesn't. With intel assembly syntax, the first operand is usually re-used for destination (but then there's stuff like MUL and DIV with fixed output registers). I suggest you download The Manuals and browse through - they are massive, though. There's also a bit here that's slightly more digestible, and the book project at the asmcommunity (haven't really looked much at it, and it seems page ordering is alphabetized instead of logical progression). I also recall FASM.PDF being a decent read.

Thanks for all the replies!!
Haha. I know what xor means and what it does! I just wanted to know why the result of the operation gets saved in eax too.
Thank you for all the answers! You helped me alot!!

The 1st operand in ASM instruction is USUALLY the one that is a destination,
the one that will be modified as a result of the instruction.