It would be nice if we could focus our attention on performing only one action at a time and performing it well, but that is usually difficult to do. The human body performs a great variety of operations in parallel or, as we will say throughout this chapter, concurrently. Respiration, blood circulation, digestion, thinking and walking, for example, can occur concurrently. All the senses—sight, touch, smell, taste and hearing—can be employed
at once. Computers, too, can perform operations concurrently. It is common for personal computers to compile a program, send a file to a printer and receive electronic mail messages over a network concurrently. Only computers that have multiple processors can truly execute multiple instructions concurrently. Operating systems on single-processor computers create the illusion of concurrent execution by rapidly switching between activities, but on such computers only a single instruction can execute at once.
