A computer has a L1 cache, an L2 cache, and main memory. If a referenced word is

Question

A computer has a L1 cache, an L2 cache, and main memory. If a referenced word is in the L1 cache, 12 ns are required to access it. If it is not in the L1 cache, but is in the L2 cache, 20 ns are required to access it. If the word is not in the L2 cache, it must be fetched from main memory and copied onto the L1 cache taking a total of 60ns; and, then the reference is started again. The L1 cache hit ratio is 0.8, the L2 cache miss ratio is 0.25. What is the average time in nanoseconds required to access a referenced word on this system.

Explanation / Answer

l1=12ns l2=20ns main memory acess=60ns. hit ratio of l1=0.8 0.8=12/x x=15ns hit ratio of l2=0.25 0.25=20/x x=80ns average time taken=80+15=42.5ns.

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