Writing Functions with Inline Assembly

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If you write a function with inline assembly code, it’s easy to pass arguments to the function and return a value from it. The following examples compare a function first written for a separate assembler and then rewritten for the inline assembler. The function, called power2, receives two parameters, multiplying the first parameter by 2 to the power of the second parameter. Written for a separate assembler, the function might look like this:

; POWER.ASM
; Compute the power of an integer
;
       PUBLIC _power2
_TEXT SEGMENT WORD PUBLIC 'CODE'
_power2 PROC

        push ebp        ; Save EBP
        mov ebp, esp    ; Move ESP into EBP so we can refer
                        ;   to arguments on the stack
        mov eax, [ebp+4] ; Get first argument
        mov ecx, [ebp+6] ; Get second argument
        shl eax, cl     ; EAX = EAX * ( 2 ^ CL )
        pop ebp         ; Restore EBP
        ret             ; Return with sum in EAX

_power2 ENDP
_TEXT   ENDS
        END

Since it’s written for a separate assembler, the function requires a separate source file and assembly and link steps. C and C++ function arguments are usually passed on the stack, so this version of the power2 function accesses its arguments by their positions on the stack. (Note that the MODEL directive, available in MASM and some other assemblers, also allows you to access stack arguments and local stack variables by name.)

The POWER2.C program writes the power2 function with inline assembly code:

/* POWER2.C */
#include <stdio.h>

int power2( int num, int power );

void main( void )
{
   printf( "3 times 2 to the power of 5 is %d\n", \
           power2( 3, 5) );
}
int power2( int num, int power )
{
   __asm
   {
      mov eax, num    ; Get first argument
      mov ecx, power  ; Get second argument
      shl eax, cl     ; EAX = EAX * ( 2 to the power of CL )
   }
   /* Return with result in EAX */
}

The inline version of the power2 function refers to its arguments by name and appears in the same source file as the rest of the program. This version also requires fewer assembly instructions.

Because the inline version of power2 doesn’t execute a C return statement, it causes a harmless warning if you compile at warning level 2 or higher. The function does return a value, but the compiler cannot tell that in the absence of a return statement. You can use #pragma warning to disable the generation of this warning.