PROTO Directive

The PROTO directive creates a prototype for an existing procedure. A prototype declares a procedure's name and parameter list. It allows you to call a procedure before defining it and to verify that the number and types of arguments match the procedure definition. Create a procedure's prototype by copying the PROC statement and making the two changes:
MASM requires a prototype for each procedure called by INVOKE. PROTO must appear first before INVOKE. The examples show how to find the greatest common divisor (GCD) of two integers, which is the largest integer that will evenly divide both integers.   
    int  GCD( int x, int y ) {
       x = abs( x );
       y = abs( y );
       do {
          int  n = x % y;
          x = y;
          y = n;
       } while y > 0;
       return( x );
    }

 Greatest Common Divisor (GCD) 
 Without PROTO Directive   With PROTO Directive  
 INCLUDE Irvine32.inc
 .data
   x  SDWORD  12
   y  SDWORD  16

 .code
 ;; Find |value|
 abs PROC, ptrValue:PTR SDWORD
      mov   esi, ptrValue
      cmp   SDWORD PTR[esi], 0
      jge   POS
      neg   SDWORD PTR[esi]
 POS: ret
 abs ENDP

 main PROC
   INVOKE  abs, ADDR x
   INVOKE  abs, ADDR y
   .REPEAT
     mov   eax, x
     cdq       ; EDX:EAX = Dividend
     div   y   ; Y       = Divisor
     mov   ebx, y
     mov   x, ebx
     ;; EDX = Remainder

    

     
      
     
     
    
    
    


     
     		
    

   
   .UNTIL (edx == 0)
   mov   eax, x
   call  WriteInt
   exit
 main ENDP
 END main
 
 INCLUDE Irvine32.inc
 abs  PROTO,  ptrValue:PTR SDWORD
 .data
   x  SDWORD  -15
   y  SDWORD   12

 .code
 main PROC
   INVOKE  abs, ADDR x
   INVOKE  abs, ADDR y
   .REPEAT
     mov   eax, x
     cdq       ; EDX:EAX = Dividend
     div   y   ; Y       = Divisor
     mov   ebx, y
     mov   x, ebx
     ;; EDX = Remainder

    

     
      
     
     
    
    
    


     
     		
    

   
   .UNTIL (edx == 0)
   mov   eax, x
   call  WriteInt
   exit
 main ENDP

 ;; Find |value|
 abs PROC, ptrValue:PTR SDWORD
      mov   esi, ptrValue
      cmp   SDWORD PTR[esi], 0
      jge   POS
      neg   SDWORD PTR[esi]
 POS: ret
 abs ENDP
 END main
 Output  Output