Question

A two level memory is having 140ns average access time without the level 30ns with level. If the fastest memory access time is 20ns. What is hit ratio?

Answer 1

We can use the formula:
average data access time = Cache access time + (1 - hit ratio) main memory access time;
So if hit ratio will be x, we can write the next:
20 nS + (1 - x) 140 nS = 30 nS
20 nS + 140 nS - 140 nS x = 30 nS
- 140 nS x + 160 nS = 30 nS
- 140 nS x = - 130 nS
x = 0.928
So hit ratio will be 0.928 or 92.8%