For a computer to have value as an information processor, people must have ways to supply it with the information it is to process—the input—and to receive the processed information—the output. This usually requires translating the information from one form to another. The input and output devices act as translators. They change the form of the data but not the content, making it possible for a person to communicate with the computer and for the computer to communicate with a person, a machine, or another computer.

The most familiar input device is a keyboard. As a person types words or numbers into the keyboard, an electric circuit beneath the keys turns each keystroke into its particular electrical signal. When that signal reaches the CPU, the electronic decoding circuitry there recognizes the signals as the characters the person typed.

Printers and video display terminals (or monitors) are common output devices. They translate electrical signals sent out from the CPU into a written message or image that is printed out on paper or appears on a screen.

A computer system may have several input and output devices, sometimes called peripherals or I/O devices. Some peripherals, such as keyboard, are strictly for input. Others, such as printers, perform only output functions. There are also a number of devices that translate both ways.

Disk Drives

One of the most useful two-way peripherals is the disk drive. It is used for both retrieving and storing information on magnetic disks.

When the disk drive is used as an input device, the drive “reads” (detects) the information stored on the disk in the form of a magnetic pattern and turns it into a series of electrical pulses. These pulses are sent to the computer’s CPU or to internal memory.

When the disk drive is used as an output device, it stores information sent it from another part of the computer system, for example, internal memory. It “writes” the information as a magnetic pattern on the disk.

Personal computer systems usually use flexible or floppy disks made of plastic with a magnetic coating. The floppy disk in its protective jacket can be slipped in and out of a slot in a relatively inexpensive disk drive. One floppy disk typically holds about a million pieces of information. (These pieces of information are described as bytes.)

All personal computers today have hard disk drives. Hard disks are not removable, but they can store thousands of times as much information as floppies, and they are less likely to be damaged in use.