Chapter 4. Memory Management

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4. Memory Management

Memory is an important resource that must be carefully managed. While the average home computer nowadays has two thousand times as much memory as the IBM 7094 (the largest computer in the world in the early 1960s), programs and the data they are expected to handle have also grown tremendously. To paraphrase Parkinson's law, "Programs and their data expand to fill the memory available to hold them." In this chapter we will study how operating systems manage memory.

Ideally, what every programmer would like is an infinitely large, infinitely fast memory that is also nonvolatile, that is, does not lose its contents when the electric power fails. While we are at it, why not also ask for it to be inexpensive, too? Unfortunately technology cannot turn such dreams into memories. Consequently, most computers have a memory hierarchy, with a small amount of very fast, expensive, volatile cache memory, hundreds of megabytes of medium-speed, medium-price, volatile main memory (RAM), and tens or hundreds of gigabytes of slow, cheap, nonvolatile disk storage. It is the job of the operating system to coordinate how these memories are used.

The part of the operating system that manages the memory hierarchy is usually called the memory manager. Its job is to keep track of which parts of memory are in use and which parts are not in use, to allocate memory to processes when they need it and deallocate it when they are done, and to manage swapping between main memory and disk when main memory is too small to hold all the processes. In most systems (but not MINIX 3), it is in the kernel.

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In this chapter we will investigate a number of different memory management schemes, ranging from very simple to highly sophisticated. We will start at the beginning and look first at the simplest possible memory management system and then gradually progress to more and more elaborate ones.

As we pointed out in Chap. 1, history tends to repeat itself in the computer world: minicomputer software was initially like mainframe software and personal computer software was initially like minicomputer software. The cycle is now repeating itself with palmtops, PDAs, and embedded systems. In these systems, simple memory management schemes are still in use. For this reason, they are still worth studying.