A Note on Byte Operands in ASM

On page 228 of CS:APP we stated that with GCC's inline assembly "...there are no direct ways to specify a program value to use as the destination operand for the setae instruction, since the operand must be a single byte." In fact, you can specify single-byte operands with gcc by declaring variables of type char. This allows us to simplify the inline assembly for ok_smul3 (p. 227) to the following (called ok_smul4):

int ok_smul4(int x, int y, int *dest) 
{
    unsigned char byte_result; 

    *dest = x*y; 

    /* Insert the following assembly code:
       setae byte_result       # Set result
    */
    asm("setae %0"  
	: "=r" (byte_result)  /* Output    */  
	);  

    return (int) byte_result; 
} 

int ok_umul2(unsigned x, unsigned y, unsigned *dest) 
{ 
    unsigned char byte_result; 

    /* Insert the following assembly code:
       movl  x,%eax         # Get x
       mull  y              # Unsigned multiply by y
       movl  %eax, *dest    # Store low-order 4 bytes at dest
       setae byte_result    # Set result
    */
    asm("movl %2,%%eax; mull %3; movl %%eax,%0; setae %1"  
	: "=r" (*dest), "=r" (byte_result) /* Outputs    */  
	: "r"  (x),     "r" (y)            /* Inputs     */  
	: "%eax"                           /* Overwrites */ 
	); 

    return (int) byte_result; 
}