Consider two different implementations, M1 and M2, of the same instruction set. There are three classes of instructions (A, B, and C) in the instruction set. M1 has a clock rate of 80 MHz and M2 has a clock rate of 100 MHz. The average number of cycles for each instruction class and their frequencies (for a typical program) are as follows: (a) Calculate the average CPI for each machine, M1, and M2.
(b) Calculate the average MIPS ratings for each machine, M1 and M2.
(c)Which machine has a smaller MIPS rating ? Which individual instruction class CPI do you need to change, and by how much, to have this machine have the same or better performance as the machine with the higher MIPS rating (you can only change the CPI for one of the instruction classes on the slower machine)?

Question

Consider two different implementations, M1 and M2, of the same instruction set. There are three classes of instructions (A, B, and C) in the instruction set. M1 has a clock rate of 80 MHz and M2 has a clock rate of 100 MHz. The average number of cycles for each instruction class and their frequencies (for a typical program) are as follows:  (a) Calculate the average CPI for each machine, M1, and M2.
(b) Calculate the average MIPS ratings for each machine, M1 and M2.
(c)Which machine has a smaller MIPS rating ? Which individual instruction class CPI do you need to change, and by how much, to have this machine have the same or better performance as the machine with the higher MIPS rating (you can only change the CPI for one of the instruction classes on the slower machine)?

Quizplus · Accepted Answer

The Answer is A.
For Machine M1:
Clocks per Instruction = (60/100)* 1 + (30/100)*2 + (10/100)*4
= 1.6
For Machine M2:
Clocks per Instruction = (60/100)*2 + (30/100)*3 + (10/100)*4
= 2.5
b.
For Machine M1:
Average MIPS rating = Clock Rate/(CPI * 106)
= (80 * 106) / (1.6*106)
= 50.0
For Machine M2:
Average MIPS rating = Clock Rate/(CPI * 106)
= (100 * 106) / (2.5*106)
= 40.0
c.
Machine M2 has a smaller MIPS rating. If we change the CPI of instruction class A for Machine M2 to 1, we can have a better MIPS rating than M1 as follows:
Clocks per Instruction = (60/100)*1 + (30/100)*3 + (10/100)*4
= 1.9
Average MIPS rating = Clock Rate/(CPI * 106)
= (100 * 106) / (1.9*106)
= 52.6

Consider Two Different Implementations, M1 and M2, of the Same