While the technology behind the construction of the various
modern-day storage technologies is truly impressive, the average system
administrator does not need to be aware of the details. In fact, there
is really only one fact that system administrators should always keep in
There is never enough RAM.
While this truism might at first seem humorous, many operating
system designers have spent a great deal of time trying to reduce the
impact of this very real shortage. They have done so by implementing
virtual memory — a way of combining RAM
with slower storage to give a system the appearance of having more RAM
than is actually installed.
Let us start with a hypothetical application. The machine code
making up this application is 10000 bytes in size. It also requires
another 5000 bytes for data storage and I/O buffers. This means that,
to run this application, there must be 15000 bytes of RAM available;
even one byte less, and the application would not be able to
This 15000 byte requirement is known as the application's
address space. It is the number of unique
addresses needed to hold both the application and its data. In the
first computers, the amount of available RAM had to be greater than
the address space of the largest application to be run; otherwise, the
application would fail with an "out of memory" error.
A later approach known as overlaying
attempted to alleviate the problem by allowing programmers to dictate
which parts of their application needed to be memory-resident at any
given time. In this way, code only required once for initialization
purposes could be written over (overlayed) with code that would be
used later. While overlays did ease memory shortages, it was a very
complex and error-prone process. Overlays also failed to address the
issue of system-wide memory shortages at runtime. In other words, an
overlayed program may require less memory to run than a program that
is not overlayed, but if the system still does not have sufficient
memory for the overlayed program, the end result is the same —
an out of memory error.
With virtual memory, the concept of an application's address space
takes on a different meaning. Rather than concentrating on how
much memory an application needs to run, a
virtual memory operating system continually attempts to find the
answer to the question, "how little memory does
an application need to run?"
While it at first appears that our hypothetical application
requires the full 15000 bytes to run, think back to our discussion in
Section 4.1 Storage Access Patterns — memory access tends to be
sequential and localized. Because of this, the amount of memory
required to execute the application at any given time is less than
15000 bytes — usually a lot less. Consider the types of memory
accesses required to execute a single machine instruction:
The instruction is read from memory.
The data required by the instruction is read from
After the instruction completes, the results of the
instruction are written back to memory.
The actual number of bytes necessary for each memory access varies
according to the CPU's architecture, the actual instruction, and the
data type. However, even if one instruction required 100 bytes of
memory for each type of memory access, the 300 bytes required is still
much less than the application's entire 15000-byte address space. If
a way could be found to keep track of an application's memory
requirements as the application runs, it would be possible to keep the
application running while using less memory than its address space
would otherwise dictate.
But that leaves one question:
If only part of the application is in memory at any given time,
where is the rest of it?
The short answer to this question is that the rest of the
application remains on disk. In other words, disk acts as the
backing store for RAM; a slower, larger storage
medium acting as a "backup" for a much faster, smaller storage medium.
This might at first seem to be a very large performance problem in the
making — after all, disk drives are so much slower than
While this is true, it is possible to take advantage of the
sequential and localized access behavior of applications and eliminate
most of the performance implications of using disk drives as backing
store for RAM. This is done by structuring the virtual memory
subsystem so that it attempts to ensure that those parts of the
application currently needed — or likely to be needed in the
near future — are kept in RAM only for as long as they are
In many respects this is similar to the relationship between cache
and RAM: making the a small amount of fast storage combined with a
large amount of slow storage act just like a large amount of fast
With this in mind, let us explore the process in more