Win32 Windows - Iczelion's Win32 Assembly Tutorial

[TBRANSO1]

Hey,

I am going through his tutorials.  I have built, assembled, linked and run my first all Windows App.  I have added a few features beyond.

I was curious about 2 instructions and what they are doing in the WinMain proc:

push hInstance
pop wc.hInstance

I have read of the technique to swap registers with push and pop, is this what this is doing?  if so, how are these swapping, from a global var to a member of a struct?

why not just like:
mov ebx, offset hInstance
mov wc.hInstance, ebx

Thanks

[felipe]

how are these swapping, from a global var to a member of a struct?

If hinstance is a global var then yes (don't remember that example).  :idea:

mov ebx, offset hInstance
mov wc.hInstance, ebx

You don't need the address of the variable hinstance, you need hinstance value, so you should code something like:

Code Select Expand


mov ebx,hInstance
mov wc.hInstance, ebx
  :idea:

why not just like:

Maybe because you don't want to trash that register.  :idea:

[jj2007]

Maybe because you don't want to trash that register.  :idea:

Yes, that's often the reason: push somevar, pop anothervar does not need a register. There are two dedicated macros:
m2m destvar, sourcevar uses push+pop
mrm destvar, sourcevar uses eax to move the values.

[TBRANSO1]

I watched that sequence in the VS debugger and watched that wc.hInstance get filled with the pointer from the global hInstance... it's baffling, but I guess that is what is happening, swapping.

I read about the techniqe, instead of xchg, but this is baffling.  I think that I will have to draw this on paper or get the .lst file and work with it, this is warping my brain.


EDITED LATER:

While reading something else entirely, I stumbled across the answer to this question: http://www.hep.wisc.edu/~pinghc/asm5.html

If it is allocated in the .DATA section, it can be initialised to preset values but if it is allocated on the stack as a LOCAL value within a procedure, the values have to be coded into the structure.
    LOCAL Rct :RECT

    ; code
    mov Rct.left,   1
    mov Rct.top,    2
    mov Rct.right,  3
    mov Rct.bottom, 4

It should be noted that a structure member is a memory operand so you cannot directly move another memory operand into it, you must either use a register to copy it or the stack mnemonics, push / pop.

[hutch--]

If you are writing high level API style code, the push / pop technique is easily fast enough but at the other end with a pure mnemonic algorithm, you design it to use a register as it is a faster technique. Avoid XCHG as its a relic of the DOS days and only exists in micro-code. This variety is normal in assembler where you have multiple ways of doing things, with practice, you just pick the technique that best suits your application.