im confused..

[peterpants]

hi asm masters, whats the right way to call a function with a byte argument, is it?
mov byte ptr [esp], al
or just..
push eax
im confused cause push eax is 4 bytes..

[Vortex]

Hi Peter,

Welcome to the forum.

The stack is always 4 bytes aligned and you should do push eax.

[peterpants]

thank for your help asm master.

[NoCforMe]

Well, you could  do what you asked about:

Code Select Expand



MOV BYTE PTR [ESP],  AL



and it might even work. Two (at least two) problems with this, though:

• The called  routine might expect the rest of the word on the stack to be zero, although if it really does take a BYTE argument it shouldn't.
  
• The stack will be "unbalanced" (i.e., not pointing to where it should  before the call), so you'd have to move the stack pointer by hand:

Code Select Expand



SUB ESP, 2



Which is why we use PUSH instead, as it does all this in one operation.

(Note: I can't remember whether a PUSH increments or decrements the stack pointer. Oh, well.)

[qWord]

Code Select Expand



SUB ESP, 2



this would produce a stack misalignment -> sub esp,4

[NoCforMe]

Arrrrgh! My bad. Shows my background as a DOS programmer. Old habits die hard.

[xandaz]

    so....and what if we push ax? it's still a valid argument right? it does:

Code Select Expand

mov word ptr [esp],ax
sub esp,2~
    you just have to remember to pop a word or a 16b register. But feel free to explain because i don't think that i«m, understanding...

[jj2007]

Try your luck....

Code Select Expand

include \masm32\include\masm32rt.inc

.code
start: push ax
MsgBox 0, "Hello World", "Pushing ax is great:", MB_OK
pop ax
exit

end start


[raymond]

There are many things you simply cannot do in High Level Languages which you can easily do in assembly. It is thus not necessary to duplicate all HLL idiosyncracies in assembly.

For example, ALL arguments MUST be passed on the stack (either directly or as a pointer to memory) in HLL. They CANNOT be passed in a register. However, in assembly, arguments can be passed in registers.

mov al,arg
call function

And you write your function to retrieve the byte argument from the AL register (or whatever other register you decide to use). :clap:

[xandaz]

    ok i see your point jj. But nevertheless it works if you don't call the msgbox func.

[hutch--]

Ray has made the point clearly that you can use registers to pass data to a procedure in assembler. Now it is basically the case that ANYTHING you place in a register (or two) can be passed to a procedure and that includes BIT patterns, BYTE, WORD, DWORD and with 2 registers QUAD sized values.

[jj2007]

    ok i see your point jj. But nevertheless it works if you don't call the msgbox func.

On Win XP SP3 I see a crippled MessageBox, a sign that inside the API the misaligned stack causes trouble. But technically speaking you can use any stack alignment - just balance it after use, and make sure you don't call a Windows API in between. Anyway, I'd call it bad practice, and never had a need for it :bg

[hutch--]

I agree with JJ here, use the native processor data size for the OS version and you get the maximum speed and proper stack alignment. There is nothing to gain by using smaller stack alignments but you can make a mess of it by doing so.

[dedndave]

even in those cases where a misaligned stack may work, it will slow things down terribly   :P
on a 32-bit system, the cpu likes to access data on 4-aligned boundries
this applies to all data, actually - not just the stack

[NoCforMe]

even in those cases where a misaligned stack may work, it will slow things down terribly   :P
on a 32-bit system, the cpu likes to access data on 4-aligned boundries
this applies to all data, actually - not just the stack

So one should put all one's QWORD data first in the data segment (followed by DWORDS and WORDS), correct?

The linker and loader ensure that all program segments are aligned on a [qword? page?] boundary, don't they?