A little about Model Flat,Stdcall
Win32 Asm Basics
by Lucifer
Jan 26, 1999


Model Flat, Stdcall:
-----------------------------------------------------------------------------
At the top of nearly every win32asm source file you will find the line:
.model flat,Stdcall
This tells the assembler what memory model and calling convention to use.

Flat:
Windows uses a flat memory model, which basically is a 32-bit protected mode
architecture, with segmentation effectively disabled.  The reason that
windows uses the flat memory model is for portability; The segmented
architecture of the intel processor is not found on other processors.  

For more information on the Flat Memory Model see the Intel Documentation at:
ftp://download.intel.com/design/intarch/papers/esc_ia_p.pdf

Stdcall:
The Windows API uses the stdcall calling convention exclusively (excepty for
one function - wsprintf).  The proprties of the stdcall convention are that
the function parameters are pushed onto the stack in reverse order and the
function is responsible for adjusting the stack.  The stdcall option in the
.model directive tells the assembler that every procedure uses the stcall
convention. It basically changes the code inserted by the assembler for the
PROC and ENDP directives:

  ;Written code
  Procedure PROC p1:DWORD, p2:DWORD
        xor eax,eax
        ret
  Procedure ENDP

  ;Assembled code
  Procedure PROC p1:DWORD, p2:DWORD
        ENTERD  00000h,0                ; inserted by assembler
        xor eax,eax
        LEAVED                          ; inserted by assembler
        RET     00008h                  ; changed by assembler        
  Procedure ENDP

As can be seen, using stdcall adds some overhead to the program (7 bytes in
this example). However, it allows the ability to pass function parameters on
the stack easily.  One advantage of using assembly language, though, is the
ability to pass parameters using the registers, which both increases speed
and reduces code size.

Generally, for type checking, readability, and programming ease it is
worthwile to pass parameters via the stack.  But if maximum speed and minimum
code size are required, parameters should be passed through the registers.


Copyright (C) 1999
lord-lucifer@usa.net