Question

What is TLB for? Why TLB? Given the following number, what is theeffective memory access time?
a. TLB access time: 5 ns

b. Memory access time: 150 ns

c. Hit ratio: 90%

Answer 1

Answer:

A lookaside buffer translation (TLB) is a memory cache that reduces the time it takes to access a user memory place. TLB includes the most lately used page table entries.

TLB is used to overcome the issue of sizes at the time of paging. Page Table Entry (PTE) is used for framing the memory ,but it uses two references one for finding  the frame number and the other for the address specified by frame number.

Formula for finding effective memory access time-

Effective Memory Access Time = (TLB access_time+Memory Access Time)*hit ratio + (TLB access_time+2*Memory Access Time)*(miss ratio)

Given in question,

Hit ratio = 0.90

Memory Access Time = 150ns

TLB access time= 5ns

Effective Memory Access Time = (TLB access_time+Memory Access Time)*hit ratio + (TLB access_time+2*Memory Access Time)*(miss ratio)

                            =(5+150) * 0.90 + (5+2*150)*(1-0.90)

                             =155 * 0.90 + (305*0.1)

                             =139.5 + 30.5

                             = 170ns