Question

A computer system has a cache with a cache access time T_C = 10 ns, a hit ratio of 80 % and an average memory access time T_M = 20 ns. What is the access time for physical memory T_P?
1. 90 ns
2. 80 ns
3. 60 ns
4. 20 ns

Answer 1

Correct Answer - Option 3 : 60 ns

Concept:

T_M = h × T_C + (1 – h) × T_P

T_M is the average memory access time

T_C is the cache access time

T_P is the access time for physical memory

h is the hit ratio

Calculation:

Given that T_C = 10 ns, T_M = 20 ns, h = 80% = 0.8

T_M = h × T_C + (1 – h) × T_P

⇒ 20 = 0.8 × 10 + (1 – 0.8) × T_P

⇒ 20 = 8 + 0.2 T_P

⇒ T_P = 60 ns