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Only one user at a time uses a desktop computer; however, multiple users simultaneously use server computers. As a result, a server operating system must provide support for multiple users who access the server remotely via the network.

At the heart of multiuser support is multitasking, which is the capability of an operating system to execute more than one program (a task or a process) at a time. Multitasking operating systems are like the guy who used to spin plates balanced on sticks on the old Ed Sullivan Show back in the 1950s. He’d run from plate to plate, trying to keep them all spinning so they wouldn’t fall off the sticks — and just for grins, he was blindfolded or rode on a unicycle.

Although multitasking creates the appearance that two or more programs are executing on the computer at one time, in reality, a computer with a single processor can execute only one program at a time. The operating system switches the CPU from one program to another to create the appearance that several programs are executing simultaneously, but at any given moment, only one of the programs is actually executing. The others are patiently waiting for their turns. (However, if the computer has more than one CPU, the CPUs can execute programs simultaneously, which is multiprocessing.)

For multitasking to work reliably, the server operating system must completely isolate the executing programs from each other. Otherwise, one program may perform an operation that adversely affects another program. Multitasking operating systems do this by providing each task with its own unique address space that makes it almost impossible for one task to affect memory that belongs to another task.