For a computer to do anything useful, it needs both application software (programs you use) and an operating system (programs the computer uses). The operating system sits between the physical hardware that makes up a computer (the monitor, keyboard, CPU, hard drive, and so forth) and the end-user software that people use to process documents, play games, and all that good stuff.

My brother Tom and I are both into computers. We're computer programmers by trade, but I'm quick to point out that we produce very different kinds of software. The difference, as I like to explain it, is this: I write software for people (application software), and Tom writes software for computers (operating system software).

We commonly think of the CPU as the brain of a computer, but in reality, it can't do much besides crunch numbers and move data around in the computer's memory. The job of the operating system (OS) is twofold:

• To work with computer hardware to process user requests by
  • interpreting keystrokes from the keyboard,
  • displaying text and images on the screen,
  • storing files on the hard disk,
  • sending documents to a printer,
  • communicating over a modem.
• To manage the application software's use of memory (RAM) and processor time.

If you've used a multitasking environment like Windows or a multiuser mainframe system, you've seen the concept of "time-slicing" in action. While your computer has only one CPU, which can do only one thing at a time, the OS can make it seem like several people or programs are using the CPU simultaneously. Similarly, even though the real memory (RAM) is shared by all running applications, the OS can make it seem like you have it all at your disposal, all the time, by sharing it between applications--using a technique called paging.

The OS time-slices by giving one user or application exclusive use of the hardware for a brief instant, and then doing the same for the next user or application. On systems with adequate horsepower, this approach works so that you never even know about that little game of round-robin going on behind the scenes. On a wimpy computer or a mainframe with too many users, it's toe-tappin' time for everybody.

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