# Calculating Cpi with Miss Rate

In my assignment I have the following question:

The processor has a clock rate of 1 GHZ. The miss rate in the instruction cache is 1.5%. The miss rate in the data cache is 4%. 30% of the instruction access the data. The miss time for the data and instructions is 70ns.

A. Assuming we don't have any more stalls, what is the CPI of this processor? B. I have to choose between two options: 1. Use a processor with 2 GHZ while the cache stays the same. 2. Use a better cache, which in it the miss time for data and instructions go down to 35 ns, while the clock rate remains 1 GHZ.

Which improvement is better?

What I don't understand here, is how can I calculate the CPI if I don't have the base cpi?

Let's assume the base cpi is X, then the memory stalls as I thought is:

4%*30%*70+1.5%*70=1.89


So the CPI is:

1.89 + X


What am I missing? is it possible to calculate A part as a number?

And for part B, how does the improvement in GHZ affects the performance? What I know is that if we improve the clock rate, the miss penalty goes up

*We don't have to take into account the hit time, for this question.

Your help is much appriciated.

## 1 Answer

I agree with you that you can't figure out effective CPI without knowing the average CPI of the processor. If this is a long-hand assignment, I would use your calculations based on a starting CPI.

Then if you want a numeric CPI, assume an ideal processor that only takes 1 cycle per instruction, yielding 2.89 CPI. State that assumption in your answer.

Disclaimer: I haven't studied processor performance down at this level in over 20 years, and don't have the references at hand to verify your work.

• Thank you. So you have any thoughts on the second part, B? – Alan Sep 21 '14 at 17:49