Memory cache question


#1

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
memory?
a) 52ns
b) 60ns
c) 70ns
d) 80ns


#2

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
=60ns
so the ans is b) 60ns