|registers||$$$||32 bytes||in the room||1cycle(afew ns)|
|cache||$$||a few MB||next door||cycles(10's ns)|
|main memory||$||GB||hall||100s of ns|
|disk||c||GB||100 miles||a few ms|
Registers that are built into the CPU are generally accessible within one cycle of the CPU clock. Main memory is accessed via a transaction on the memory bus, and it may take many cycles of CPU clock. Cache is a memory buffer used to accommodate a speed differential.