Safari Books Online is a digital library providing on-demand subscription access to thousands of learning resources.
You are probably familiar with multitasking in your operating system: the ability to have more than one program working at what seems like the same time. For example, you can print while editing or downloading your email. Nowadays, you are likely to have a computer with more than one CPU, but the number of concurrently executing processes is not limited by the number of CPUs. The operating system assigns CPU time slices to each process, giving the impression of parallel activity.