IES 2018 Memory hit miss ratio

Question

Consider that a level of the memory hierarchy has a hit rate of 80%. Memory requests take 10 ns to complete if they hit in the level, and memory requests that miss in the level take 100 ns to complete. The average access time of the level is

(a) 110 ns

(b) 100 ns
(c) 80 ns

(d) 28 ns

Answer 1

(d) 28 ns

Hit ratio H = 0.8 (i.e. 80% references are hit)

Memory access time on a hit M1 = 10 ns

Memory access time on a miss (or, miss penalty) M2 = 100 ns

Average memory access time = H*M1 + (1-H)*M2

= 0.8 * 10 + 0.2 * 100 ns

= 8 + 20 ns

= 28 ns