Modern computers can do several things at the same time. Windows users are not fully aware of this, so let's give an example that will illustrate things more closely. When the PC system is booted, many processes that are hidden from the user are started. For example, a process that manages incoming e-mails, a process in charge of obtaining latest antivirus definitions, and so on. Explicit user processes may also be running: printing files, or burning CDs, all while the user is performing some other tasks such as surfing the Web. Activities such as these must be managed, and a multitasking system, which supports multiple processes, can be very handy here. In such a multitasking system, the CPU switches between processes quickly, running each process for a few milliseconds. Strictly speaking, at any instant of time, the CPU is running only one process by switching quickly among processes—giving the illusion of parallelism.

Operating systems over the years have evolved to a sequential...