What is the average access speed in a system with layered memory?

Question

Computers

Hello!
I'm very interested in what formula is used to calculate the average access time in a system with multi-level memory. For example, we have a faster memory with a 2ns access time and a slower memory with a 2µs access time. Faster memory acts as a "cache" - before turning to slow memory, we try to find the result there, if it is not there, we climb into slow memory. Let the probability of a successful search for data in the cache be 95%, respectively, the probability of a cache miss is 5%
Will the following formula be correct then: average access time = 0.95 * 2 * 10 ^ (-9) + 0.05 * 2 * 10 ^ (- 6) ?

Answer 1

Looks reasonable, except that we're looking in fast memory anyway, so the 0.95 multiplier isn't needed.