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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 mind:
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 the 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, in order to run this application, there must be 15000 bytes of RAM available; even one byte less, and the application will not be able to run.
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 that was only required once for initialization purposes could be 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.
Virtual memory turns the concept of an application's address space on its head. 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 the Section called 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 that would be required to execute a single machine instruction:
The instruction is read from memory.
The data on which the instruction will operate is read from memory.
After the instruction completes, the results of the instruction are written back to memory.
While the actual number of bytes necessary for each memory access will vary according to the CPU's architecture, the actual instruction, and the data type, even if that one instruction required 100 bytes of memory for each type of memory access, then 300 bytes is still a lot less than the application's 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 using less than its address space.
But that leaves one question:
If only part of the application is in memory, where is the rest of it?
The short answer to this question is that the rest of the application remains on disk. This might at first seem to be a very large performance problem in the making — after all, are not disk drives so much slower than RAM?
While this is true, it is possible to take advantage of the sequential and localized access behavior of applications and to structure the virtual memory subsystem so that it attempts to ensure that those parts of the application that are currently needed — or likely to be needed in the near future — are kept in RAM for as long as they are needed.
In many respects this is similar to the relationship between cache and RAM: a relatively small about of fast storage can be combined with a larger amount of slow storage to look like a larger amount of fast storage.
With this in mind, let us look at the process in more detail.