End of Chapter Summary

A computer with just a CPU and internal memory is of limited value; peripherals allow that computer to communicate with the outside world and store information for later use. Some peripherals are strictly input devices. Others are output devices. Some are external storage devices that accept information from and send information to the CPU.

The most common input devices today are the keyboard and the mouse, but a variety of other input devices can be connected to the computer. Trackballs, touch-sensitive pads, touch screens, and joysticks provide alternatives to the mouse as a pointing device. Bar code readers, optical mark readers, and magnetic ink readers are designed to recognize and translate specially printed patterns and characters. Scanners and digital cameras convert photographs, drawings, and other analog images into digital files that the computer can process. Sound digitizers do the same thing to audio information. All input devices are designed to do one thing: convert information signals from an outside source into a pattern of bits that the computer can process.

Output devices perform the opposite function: They accept strings of bits from the computer and transform them into a form that is useful or meaningful outside the computer. Video displays, including CRT monitors and LCDs, are almost universally used to display information continually as the computer functions. A variety of printers are used for producing paper output. Fax machines and fax modems let you share printed information using standard phone lines. Sound output from the computer, including music and synthesized speech, is delivered through audio speakers. Output devices also allow computers to control other machines.

Unlike most input and output peripherals, storage devices such as disk drives and tape drives are capable of two-way communication with the computer. Because of their highspeed random-access capability, and their large capacity, magnetic hard disks are the most common forms of storage on modern computers. Sequential-access tape devices are generally used to archive only information that doesn’t need to be accessed often. Optical discs are the most common form of removable storage. Most optical drives can read and write data. In the future, solid-state storage technology will probably replace disks and tapes for most applications.

The hardware for a complete computer system generally includes at least one processor, memory, storage devices, and several I/O peripherals for communicating with the outside world. With the hardware components in place, a computer system is ready to receive and follow instructions encoded in software.