Memory cache question


Consider a high speed 40ns memory cache with a successful hit ratio of 80%. The regular
memory has an access time of 100ns. What is the average effective time for CPU to access
a) 52ns
b) 60ns
c) 70ns
d) 80ns


Effective CPU access time = cache hit ratio* cache access time + cache miss ratio*(cache access time + main memory access time)
cache miss ratio= (1-hit ratio)

=> 0.8 * 40 +[(1-0.80)*(40+100)]
= 32+28
so the ans is b) 60ns