ISRO | ISRO CS 2017 – May | Question 21

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) 60
(B) 30
(C) 150
(D) 70

Answer: (A)
Explanation: Hit ratio of cache = Hcache = 0.8
Tcache = 30 ns
Tmemory = 150 ns
CPU access time = Hcache * Tcache + (1 – Hcache)(Tcache + Tmemory) = 0.8 * 30 + 0.2 * (30 + 150) = 60 ns
So, option (A) is correct.
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