Chapter 4. Physical and Virtual Memory

All present-day, general-purpose computers are of the type known as stored program computers. As the name implies, stored program computers load instructions (the building blocks of programs) into some type of internal storage, where they subsequently execute those instructions.

Stored program computers also use the same storage for data. This is in contrast to computers that use their hardware configuration to control their operation (such as older plugboard-based computers).

The place where programs were stored on the first stored program computers went by a variety of names and used a variety of different technologies, from spots on a cathode ray tube, to pressure pulses in columns of mercury. Fortunately, present-day computers use technologies with greater storage capacity and much smaller size than ever before.

4.1. Storage Access Patterns

One thing to keep in mind throughout this chapter is that computers tend to access storage in certain ways. In fact, most storage access tends to exhibit one (or both) of the following attributes:

Sequential access means that, if address N is accessed by the CPU, it is highly likely that address N+1 will be accessed next. This makes sense, as most programs consist of large sections of instructions that execute — in order — one after the other.

Localized access means that, if address X is accessed, it is likely that other addresses surrounding X will also be accessed in the future.

These attributes are crucial, because it allows smaller, faster storage to effectively buffer larger, slower storage. This is the basis for implementing virtual memory. But before we can discuss virtual memory, we must examine the various storage technologies currently in use.