The work of a computer can be characterized by an input-process-output model in which a program receives input from an input device, performs some processing on the input, and produces output to an output device. Users employ a variety of input devices to interact with the computer, but most user interfaces today are based upon a keyboard and a mouse pointing input device.

A keyboard consists of a number of switches and a keyboard controller. The keyboard controller is built into the keyboard itself. When a key is pushed, a signal called a scan code is sent to the controller. A different scan code is sent when the key is released. The use of two scan codes allows keys to be used in combination. The controller is able to tell whether a key is being held down while another key is struck, or to...