vices are usually used together with hard disk drives on large computer systems that handle high volumes of constantly changing data. The tape drives, which access data very slowly, regularly back up, or duplicate, the data in the hard disk drives to protect the system against loss of data during power failures or computer malfunctions. magnetic-drum memories store data in the form of magnetized spots in adjacent circular tracks on the surface of a rotating metal cylinder. They are relatively slow and are rarely used today. Optical discs are nonmagnetic auxiliary storage devices that developed from compact-audio-disc technology. Data is encoded on a disc as a series of pits and flat spaces, called lands, the lengths ofwhich correspond to different patterns of 0s and 1s. One removable 43/4-inch (12-centimeter) disc contains a spiral track more than 3 miles (4.8 kilometers) long, on which can be stored nearly a billion bytes (gigabyte) of information. All of the text in this encyclopedia, for example, would fill only one fifth of one disc. Read-only optical discs, whose data can be read but not changed, are called CD- ROMs(Compact disc-read-only memory). Recordable CD-ROM drives, called WORM (write-once/read-many) drives, are used by many businesses and universities to periodically back up changing databases and to conveniently distribute massive amounts of information to customers or users. Video display terminal Output devices let the user see the results of the computer's data processing. The most common output device is the video display terminal (VDT), or monitor, which uses a cathode-ray tube (CRT) to display characters and graphics on a television-like screen. Modems (modulator-demodulators) are input-output devices that allow computers to transfer data between each other. A modem on one computer translates digital pulses into analog signals (sound) and the...
