A computer has a cache, main memory and disk used for virtual memory. If a refer

Question

A computer has a cache, main memory and disk used for virtual memory. If a reference word is in the cache 4 ns are required to access it. If it is in the main memory but not in the cache , 50 ns are required to load it into the cache and then reference is started again. If the word is not in the main memory , 10 ms are required to fetch from disk , followed by 50 ns to copy it into the cache and then the reference is started again. The cache hit ratio is 0.9 and the main memory his ratio is 0.6. What is the average time in nanoseconds required to access a referenced word on this system?

Explanation / Answer

cache access time: t(c) = 4 ns main memory access time: t(mm) = 50 ns disk access time: t(d) = 10000 ns + 50 ns + 4 ns = 10054 ns system average access time: t t = ? (t(c) + t(mm) + t(d)) *(1/0.9)(1/0.6) t = ? (10108)(1/0.54) = 6239.5 nano sec

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