Consider a system which implements demand paging to service page faults. It takes 50 ns to serve a page fault, and 150 ns to replace a dirty page. Main memory access time is 90 ns. Page hit ratio is 40 out of 200 and When it is miss the probability of page being dirty is 30 out of 180. What will be the average access time?
(A) 37.33 ns
(B) 65.33 ns
(C) 71.33 ns
(D) 103.33 ns


Answer: (C)

Explanation: Average access time,

= 4/5 * (1/6 * 150 + 5 / 6 * 50 ) + 1 / 5 * 90
= 800 / 15 + 90 / 3 = 1070 / 15
= 71.33 ns 

So, option (C) is correct.


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