The average memory access time is 130 ns and average time of cache memory is 100 ns. With 75% hit ratio, the average time to access a word is?

Explanation

Using the standard average memory access time (AMAT) formula, AMAT = Cache access time + (Miss rate × Main memory access time) — the cache is always checked first (taking the cache time), and on a miss the main memory is additionally accessed. With cache time Tc = 100 ns, main memory time Tm = 130 ns and a hit ratio of 75% (miss rate = 25%), AMAT = 100 + (0.25 × 130) = 100 + 32.5 = 132.5 ns. Hence the average time to access a word is 132.5 ns.