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Effective access time

Let the page fault service time be 20 ms in a computer with average memory access time being 30 ns. If one page fault is generated for every 106 memory accesses, What is the effective access time for the memory (in ns).

Answer

Effective Memory Access Time = page fault rate * (page fault service time+memory access time) + (1- page fault rate) * (memory access time )
EMAT=P*(PFS + MA)+(1-P)*MA
EMAT=P*PFS + P*MA + MA - P*MA
EMAT=P*PFS + MA
EMAT=(1/106 )*20msec + 30nsec
EMAT=20nsec+30nsec
EMAT=50nsec

1Comment
Suman @suman08513
23 Jan 2017 10:14 pm

@sumit sir,

 

EMAT = P* PFS + (1-P)*MA only(According to Galvin text book). Why to add MA to PFS when there is a page fault in P*(PFS + MA)+(1-P)*MA