Memory access time - Computer System Architecture

Question 1 UGC NET June-2020
Consider a single-level page table system, with the page table stored in the memory. If the hit rate to TLB is 80%, and it takes 15 nanoseconds to search the TLB, and 150 nanoseconds to access the main memory, then what is the effective memory access time, in nanoseconds?

A – 185

B – 195

C – 205

D – 175

Show Answer With Best Explanation

Answer- B
Explanation:
It is given that,
hit ratio = 0.8 or 80% ,
memory access time = 150 ns,
TLB access time = 15ns,
Miss ratio = 1- hit ratio = 1 – 0.8 = 0.2
Effective memory access time = hit ratio *(memory access time + TLB access time) + miss ratio*
(2*memory access time + TLB access time)
= 0.8*(150 + 15) + 0.2* (2*150 + 15)
= 0.8* 165 + 0.2 * 315
= 132 + 63
=195 ns