The CPU design team is choosing between two implementations

The CPU design team is choosing between two implementations of the same instruction set each with three classes of instructions. Parameters for the two possibilities are given in the following table.

                             CPU-1       CPU-2

Clock Rate                       2 GHZ       3 GHZ

CPI for ALU Inst       1               2

CPI for Memory Inst             2                2

CPI for Control Inst              4                2

a) For a program with 40% ALU instructions, 50% memory instructions, and 10% control instructions, which design will be faster? Give a quantitative average CPI for each case.

b) For a program with 70% ALU instructions, 20% memory instructions and 10% control instructions, which design will be faster? Give a quantitative average CPI for each case.

Solution

CPI (CPU1) = ((40x1) + (50x2) + (10x4)) / 100 = 1.8

CPI (CPU2) = ((70x2) + (20x2) + (10x2)) / 100 = 2.0

CPU1 execution time = ((1.8)/2000) x 1000 = 0.9ns

CPU2 execution time = ((2.0)/3000) x 1000 = 0.66ns

Design of CPU2 is faster

2. Average CPI for each CPU

CPI (CPU1) = ((70x1) + (20x2) + (10x4)) / 100 = 1.5

CPI (CPU2) = ((70x2) + (20x2) + (10x2)) / 100 = 2.0

CPU1 execution time = ((1.5)/2000) x 1000 = 0.75ns

CPU2 execution time = ((2.0)/3000) x 1000 = 0.66ns

Design of CPU2 is faster