Suppose a program has 3000 instructions with the following instruction types: -

Question

Suppose a program has 3000 instructions with the following instruction types:

- R type (70%)

- lw (10%)

- sw (10%)

- beq (8%)

- j (2%)

If we run this program on a 5-stage multi-cycle CPU, and the number of cycles to finish each type of instruction is given as following:

- R type - 4 cycles

- lw - 5 cycles

- sw - 4 cycles

- beq - 3 cycles

- j - 2 cycles

What is the CPI (Clock Cycle per Instruction) for running the program on this CPU ? Suppose the CPU is clocked at 1 MHZ (i.e., 10^6 clock cycles per second). How much time is needed to finish the program?

Explanation / Answer

CPI = 0.7*4 + 0.1*5 + 0.1*4 +0.08*3+ 0.02*2

= 2.8+0.5+0.4+0.24+0.04

= 3.78

now total time needed = CPI* no. of insturctions * time for each clockcycle

= 3.78*3000 * (10)^-6

= 11.54 ms

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