Microsoft uses its own calling convention in the 64-bit mode (long mode) using a mixed register/stack paradigm for passing procedure parameters. This means that only the first four parameters may be passed via registers and the rest (if any) should be pushed onto the stack. The following table illustrates which registers are used and in what manner:

All of this looks quite clear, yet there are two things that we need to pay special attention to:

• The stack must be aligned on a 16-bytes boundary
• A 32-bytes shadow space on the stack is required--32 bytes between the last pushed stack parameter (if any) and the return address

The following macro instruction (ms64_call) is simplistic; it is a primitive implementation of this calling convention. This specific macro does not support stack parameters:

macro ms64_call procName, [args]
{
   a = 0
   if ~args eq
      forward
      if a = 0
         push rcx
         mov rcx, args
      else if a = 1
         push rdx
         mov rdx, args
      else if a = 2
         push r8
         mov r8, args
      else if a = 3
         push r9
         mov r9, args
      else
         display "This macro only supports up to 4 parameters!",10,13
         exit
      end if
      a = a + 1
      end if
   common
   sub rsp, 32            ; Allocate shadow space
   call procName          ; Call procedure
   add rsp, 32            ; Free shadow space
   forward
   if ~args eq
      if a = 4
         pop r9
      else if a = 3
         pop r8
      else if a = 2
         pop rdx
      else if a = 1
         pop rcx
      end if
      a = a - 1
   end if
}

Consider an example of calling the procedure labeled my_proc in the 64-bit code, using Microsoft x64 calling convention:

ms64_call my_proc, 128, 32

Such a macro instruction would be expanded to the following:

push rcx         ;Save RCX register on stack
mov  rcx, 128    ;Load it with the first parameter
push rdx         ;Save RDX register on stack
mov  rdx, 32     ;Load it with the second parameter
sub  rsp, 32     ;Create 32 bytes shadow space
call my_proc     ;Call the my_proc procedure
add  rsp, 32     ;Destroy shadow space
pop  rdx         ;Restore RDX register
pop  rcx         ;Restore RCX register