In a system, main memory is 25 times slower than cache memory and it is referred by CPU 20% of the time. How much performance will be gain by the cache?
(A) 5 times
(B) 4.73 times
(C) 4.3 times
(D) 2.73 times


Answer: (C)

Explanation: Main memory is 25 times slower than cache memory, i.e., cache memory is 25 time faster. So,

S = 25 

Where S is speedup of cache memory over main memory and cache is referred 80% of time. So,

F = 0.8 

We know that:
S_overall

= [(1 - F ) + (F / S)]-1
= [1 - 0.8 + (0.8 / 25)]-1
= [0.2 + 0.8 / 25]-1
= 250 / 58 
= 4.310 

So, option (C) is correct.


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