Parameter Passing Conventions in Assembly 

By Lord Lucifer 
September 2, 1998 


Parameter Passing Conventions:  
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C calling convention:
  - Parameters are pushed in reverse order.
  - The function caller adjusts the stack.
 
Stdcall calling convention:
  - Most 32-bit Windows programs use this form.
  - Parameters are pushed in reverse order.
  - The called function takes care of stack adjustment.

Pascal calling convention:
  - The 16-bit Windows API uses this form.
  - Parameters are pushed in foward order.
  - The called function takes care of stack adjustment.


Accessing Parameters and Local Variables:  
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  The stack frame allows parameters and local variables to be
  easily accessed as offsets of register BP (or EBP).
  Take this example function (which uses the stdcall convention):

      void _stdcall Function(long var1, long var2)
      {
        int local1;
        int local2;

        local1 = var1;
        local2 = var2;
      }

  This is the memory map for the function call.
            ____ ____
      ...  |____|____|
      A104 |____|____| - ESP
      A108 |____|____| - local2    
      A10C |____|____| - local1
      A110 |____|____| - EBP
      A114 |____|____| - Function return address
      A118 |____|____| - var1  
      A11C |____|____| - var2
      ...  |____|____| 
      F000 |____|____| - Bottom of stack

  To access in assembly the parameters or the locals, all
  that is needed is the offset from EBP. Therefore:

      [ebp-4] would be the variable local1
      [ebp-8] would be the variable local2
      [ebp+08] would be the parameter var1
      [ebp+0C] would be the parameter var2

  The (unoptimized) disassembly of this simple function is:

      Function    proc
        push    ebp                     ; save ebp
        mov     ebp,esp                 ; set ebp to current esp
        add     esp,-8                  ; adjust esp to point beyond
                                        ; the two local variables
        
        mov     eax,dword ptr [ebp+08]  ; copy var1 into temp eax
        mov     dword ptr [ebp-4], eax  ; copy temp eax into local1

        mov     ebx,dword ptr [ebp+0C]  ; copy var2 into temp ebx
        mov     dword ptr [ebp-8], ebx  ; copy temp ebx into local2

        add     eax,ebx                 ; add local1 and local2 into eax
                                        ; eax is return value

        add     esp,8                   ; adjust esp to boint before the
                                        ; two local variables
                                        ; (point it to saved ebp)
        pop     ebp                     ; restore ebp        
        ret     8                       ; return from function call and
                                        ; adjust stack past the 2 parameters
      Function    endp    


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Copyright (C) 1998 
Lord Lucifer (lord-lucifer@usa.net)