JANUARY 2017 - PAPER II

JANUARY 2017 – PAPER II

OPERATING SYSTEMS

37. In a paging system, it takes 30 ns to search translation Look-a-side Buffer (TLB) and 90 ns to access the main memory. If the TLB hit ratio is 70%, the effective memory access time is 

(1) 48ns

(2) 147ns

(3) 120ns

(4) 84ns

Ans :- 2

Explanation:-

This is a paragraph taken from “Operating systems” book by Silberschatz.

In order to calculate the effective memory access time, we are going to apply the explanation given in the above mentioned book.

A 70 percent ratio means that we find the desired page number in the TLB 70 percent of the time. If it takes 30 ns to search the TLB and 90 ns to access memory, then a mapped-memory access takes 30+90 ns when the page number is in the TLB. If we fail to find the page number in the TLB(30), then we must first access memory for the page table and frame number(90ns) and then access the desired byte in the memory(90ns), for a total of 210ns. To find the effective memory access time, we weight each case by its probability,

Effective access time = 0.70 X 120 + 0.30 X 210

                                  = 84 + 63 = 147ns

So, the correct answer is 147ns. The option is 2.