Average memory access time with a cache: formula and a worked example

Average memory access time (AMAT) tells you how fast a memory system is on average, once you account for cache hits and misses.

For one cache level plus main memory, with hit ratio h, cache access time Tc and main memory time Tm:

• If memory is accessed only after a miss is detected: AMAT = h x Tc + (1 - h) x (Tc + Tm)
• If cache and memory are accessed in parallel: AMAT = h x Tc + (1 - h) x Tm

Example: h = 0.9, Tc = 10 ns, Tm = 100 ns. With the sequential formula, AMAT = 0.9 x 10 + 0.1 x 110 = 20 ns. Parallel access gives 0.9 x 10 + 0.1 x 100 = 19 ns.

The exam trap is picking the wrong formula, so always check whether the question says the memory is accessed after a miss or simultaneously.

I wrote up 6 more solved numericals (including multi-level cache and effective CPI) on my blog TechVani: https://techvaniai.blogspot.com/2026/10/cache-memory-numericals-6-solved.html