Materials used for Computer Structure
Exercise 1
(statement)
A computer has the following features:
- MP block access time: 150 ns.
- Cache access time: 5 ns.
Indicate:
- With a 90% cache hit rate, what is the average memory access time?
- iF the average access time is required to be less than 2 ns, what hit rates are needed?
(solution)
-
𝑇m = 5*0.9 + (150+5)*0.1 = 4.5 + 15.5 = 20 ns
-
2 = 5*h + (150+5)*(1-h) => h = 153/150 => h>1.02
But the hit ratio (h) must be a value between 0 and 1, so 1.02 means that is impossible to archive an average time of 2ns with a cache memory with 5ns of access time.