memory hierarchy

	cost	size	distance	speed
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.