cache

The idea behind using a cache as the first level of the memory hierarchy is to keep the information expected to be used more frequently by the CPU in the cache (a small high-speed memory that is near the CPU).
The end result is that at any given time some active portion of the main memory is duplicated in the cache. Therefore when the processor makes a request for a memory reference, the request is first sought in the cache. If the request corresponds to an element that is currently residing in the cache, we call that a cache hit. On the other hand, if the request corresponds to an element that is not currently in the cache, we call that a cache miss.

Important Laws:

1) Hit ratio = hit / (hit + miss)

2) access time cache = (hit + miss ) * t cache

3) access time M.M= miss * t M.M

4) Total access time = Access time cache + Access time M.M

5) Average access time :

a- with cache = Total access time / (hit+miss)
b- without cache = t M.M
-----------------------------------------------------------------------------------------
Example:
-----------------------------------------------------------------------------------------
Compute the Average access time for memory system when the time for M.M is 1000 ns, the time for cache is 100 ns and hit ratio is 0.9.