a) Assume for a particular code and computer system, the cache hit ratio (fracti

Question

a) Assume for a particular code and computer system, the cache hit ratio (fraction of
memory references satisfied by cache) is .80, time required to access a word in cache is
15 ns, and the time to access a word from main memory into cache is 500 ns. (Words
must go from main memory into cache before it can be sent to the CPU.) What is the
average memory access time for this code on this system?
b) Suppose we have enough money to do one of two things: (1) increase the cache size by a
factor of two that will increase the hit ratio to .90, or (2) increase the clock rate by 50%
that will decrease the time to get a word from cache to the CPU to 10 ns and the time to
get a word from main memory to cache to 333 ns. Which purchase will produce the
lower average memory access time?

Explanation / Answer

1) Find average time that it takes to complete an instruction time= hitratio*timeToAccessCache + missrati*timeToAccessMemory time= .8*15ns + .2*500ns=112 ns 2) find average runtime for 1) .9*15+.1*500=63.5 ns for 2) .8*10+.2*333=74.6 ns therefore, 1st options shows higher increase in performance.

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