AMAT (average memory access time): definition

Beginner

On average, how long the processor waits each time it asks memory for something.

Novice

$\text{Average memory access time} = \text{hit time} + \text{miss rate} \times \text{miss penalty}$ . Example: a 1-cycle cache that misses 10% of the time, with 20 cycles to fetch a missed line, averages $1 + 0.1 \times 20 = 3$ cycles. With several cache levels, the miss penalty is itself the average access time of the next level.

Expert

A first-order latency model. Processors that keep working past a miss (out-of-order cores), or that have several misses outstanding at once, hide part of the penalty, so the real slowdown is smaller than AMAT suggests for those cores and close to it for simple in-order ones.