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A cache memory needs an access time of 30 ns and main memory 150 ns, what is the average access time of CPU (assume hit ratio = 80%)?

A cache memory needs an access time of 30 ns and main memory 150 ns, what is the average access time of CPU (assume hit ratio = 80%)? Correct Answer 60 ns

Effective Memory Access Time = Cache hit ratio*Cache access time + Cache miss*( Cache miss penalty + Memory access time)

= 0.8*(30) + (1 - 0.8)*(30 + 150) ns

= 24 + 0.2*(180) ns = 60 ns

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