The performance of a hierarchical memory is measured by its effective access
time (EAT), or the average time per access. EAT is a weighted average that uses
the hit ratio and the relative access times of the successive levels of the hierarchy.
For example, suppose the cache access time is 10ns, main memory access time is
200ns, and the cache hit rate is 99%. The average time for the processor to access
an item in this two-level memory would then be: