1. Using one level of Cache:
Average Access Time
Using one level of Cache:
Avg. Access Time = CacheTime*CacheHit Rate + (CacheTime+MMTime)* CacheMiss Rate
Note that: Hit Rate = 1 – Miss Rate
Using two levels of Cache:
Avg. AT = L1Time*L1Hit Rate + (L1Time+L2Time)* L2Hit Rate + (L1Time+L2Time+MMTime)* (1- (L1Hit Rate+L2Hit Rate))

2. Average Access Time (Example 1)
A computer system with a cache and a main memory. The access time is: 10 nano seconds for the cache and 100 nano seconds for the main memory. Find the average access time if cache hit rate is 20%?
Solution:
The average access time = 0.2 * * ( ) = 90 nano seconds.

3. Average Access Time (Example 2)
Another computer system uses 2 levels of cache (L1 and L2) and the main memory. The access time is: 5 nano seconds for L1 cache, 10 nano seconds for L2 cache and 100 nano seconds for the main memory. Find the effective access time if L1 cache hit 20%, L2 cache hit is 30%?
Solution:
The average access time = 0.2 * * (5 + 10) * ( ) = 63 nano seconds.