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
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