You may have heard it said that UNIX has built-in multitasking . This means that UNIX computers can work on more than one process simultaneously. You don’t have to wait for one program to complete its business before you can start another one.

A PC or Macintosh computer today can also run several programs alongside one another, but it requires applications to be “good citizens” and permit other programs to have a share of the processing pie. UNIX does it all at the operating system level. This doesn’t mean much to users any more, but it’s important to programmers and computer scientists.

A single-processor UNIX system actually does work on only one process at a time, but it works in very small time slices. It gives a slice of time to each process, and then moves to the next one. If you give a command, and there are ten processes running, UNIX will give a little time to your command, a little to the second process, a little to the third, until it comes around to your command again and does a little more work on yours. UNIX works very quickly between tasks, so if the load of processes is normal, it will seem as if the computer is dedicated to fulfilling your commands.

However, systems can be slow when there are too many processes. Then it becomes painfully obvious that you are in a multiuser environment. You issue a simple command like date and there’s a definite pause before you see any output. Or, a print command that normally takes a minute might take ten minutes or more to complete. If you use an X terminal, you may notice that your windows aren’t redrawing very quickly when you move them around.

A slowdown of the system can result from a large number of people each doing a few things, or a smaller number of people doing many things.