What makes this kind of arrangement work is one of the most important principle in computer design. The principle of locality. The principle of locality states that programs access a relatively small portion of the address space at any instant of time.
The design goal is to present the user with as much memory as is available in the cheapest technology (points to the disk).
While by taking advantage of the principle of locality, we like to provide the user an average access speed that is very close to the speed that is offered by the fastest technology.
(We will go over this slide in detail in the next lectures on caches).