CS 2734 Computer Organization II

• In Wednesday's lecture, we wrote a MAC-1 function to do a multiplication. We will count operations on a revised version that passes the two operands on the stack rather than defining static storage for them.

• Development of a MULD M instruction implemented in MIC-1:

         sum = 0;
         for (i = 0; i < 15; i++) {
            if (high bit of a is 1)
                sum += b;
            sum += sum;
            a += a;
         }
         if (high bit of a is 1) 
            sum += b; 
  

• From Lecture 26 we will use the encoding:

           11111110yyyyyyyy                DESP y         
           11111111--------                MULD x
           0000xxxxxxxxxxxx 
  

• Analysis of the MIC-1 implementation:
  • Fetch and decode (16 cycles): 0, 1, 2, 28, 40, 46, 50, 65, 73, 76, 80, 81, 82, 83, 84, 85.
  • Execute (69 cycles):
    • Preloop (2 cycles): 86, 87
    • Loop (16 times 4 cycles): 88 through 92
    • Post loop (3 cycles): 93 through 96

• Analysis of the MAC-1 implementation :
  • Preloop: 7 + 12 + 12 = 31 cycles
  • Loop [MULTLOOP to DONE] 164 times 15 iterations = 2460 cycles
  • Postloop: 10 + 8 + 10 + 10 + 9 + 10 + 11 + 12 = 80 cycles
  Total = 2571 cycles.

Revision Date: 4/2/97