CS 2734 Computer Organization II

Lecture 27: Multiplication in MAC-1 using Shift and Add - 3/26/97

Optional Reading: Paul pages 109-118

Method 1: Unsigned binary multiplication - starting from right:

        /* sum = a*b   - no overflow */
        bs = b
        as = a
        sum = 0
        if (low bit as is 1)
            sum = sum + bs
        shift as right
        shift bs left
            ....

Method 2: Unsigned binary multiplication- starting from left:

        sum = 0
        if (high bit a = 1)
           sum = sum + b
        shift sum left
        shift a left
           .....

Advantages of Method 2:
- Only shift left (this is an add)
- Only test high bit (this is the sign bit)
Implement Method 2 in MAC-1

SKILL: You should understand how to perform a multiplication using shift and add.

Revision Date: 4/2/97