Discovering computers

[indianj]

Hello everyone, I am taking, Discovering computer class and I was doing my exercise, I came across this exercise and I do not have any idea how to solve it. If anyone know anything about, please post your help here.

Suppose we have a 500 MHz CPU with a memory system that requires 70 ns to do an access (including bus overhead) and a single, on-chip cache. The time to execute an instruction is 5 ns.


a. What is the speed of the CPU if the cache has a hit rate of 95%.

b. What is the speed of the CPU with a 95% hit rate primary cache and a 25 ns secondary cache that hits 90% of the primary cache misses.

c. How will the CPU speed be affected if a 40 ns third cache is added that hits 75% secondary cache misses.

[SeanC]

Is that an introductory level class???

It's worded pretty badly in my opinion.

My complex math skills are pretty much shot but treat this as a math question. Figure out the equation necessary for each and then plug in the values. Then's it's simple math after that.

Sean

[DanU]

I agree, it's badly worded. A 500MHz CPU will run at 500MHz regardless of how slow the memory is, but I don't think that's the answer the instructor is looking for.

A better question would be to ask how long, on average, would it take to execute an instruction. In scenario A, 95% of the time it takes 5ns, and the other 5% of the time you have to add 70ns due to memory access. So, the average time is 5ns * 95% + (70+5)ns * 5% = 8.5ns. Invert that and you get 118 MIPS (million instructions per second). You can extend this logic to solve B and C (Hey, you don't expect me to do all your work do you !)

[indianj]

Thanks DanU for your help.