Question

Consider that a level of the memory hierarchy has a hit rate of 80%. Memory requests take 10 ns to complete if they hit in the level, and memory requests that miss in the level take 100 ns to complete. The average access time of the level is
1. 110 ns
2. 100 ns
3. 80 ns
4. 28 ns

Answer 1

Correct Answer - Option 4 : 28 ns

Concept:

First a page is accesses in upper level memory, if it is present there then it is called as hit. But if page is not find in upper level memory, then it is called as miss, for this page is first copied from lower memory to upper level memory.  During miss, it involves first access time for upper level memory then access time for lower level memory.

T1 is the memory access time for memory M1

T2 is the memory access time for memory M2.

H is the hit rate in M1.

So, 1- H is the hit rate in M2.

Calculation:

Average access time will be:

Hit × 10 + min × 100

= 0.8 × 10 + (1 - 0.8) × 100

= (8 + 20) nsec

= 28 nsec