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Hardware /Part 2/

Most digital computers store data both internally, in what is called main memory, and externally, on auxiliary storage units. As a computer processes data and instructions, it temporarily stores information in main memory, which consists of random-access memory (RAM). Random access means that each byte can be stored and retrieved directly, as opposed to sequentially as on magnetic tape.

Memory chips are soldered onto the printed circuit boards, or RAM modules, that plug into special sockets on a computer's motherboard. A set of 16 chips on a RAM module can store up to 1 GB of data. This kind of internal memory is also called read/write memory.

Another type of internal memory consists of a series of read-only memory (ROM) chips. Unlike in RAM, what is stored in ROM persists when power is removed. The programs stored in these chips correspond to commands and programs that the computer needs in order to boot up, or ready itself for operation, and to carry out basic operations. Because ROM is actually a combination of hardware (microchips) and software (programs), it is often referred to as firmware.

Auxiliary storage units supplement the main memory by holding programs and data that are too large to fit into main memory at one time. They also offer a more permanent and secure method for storing programs and data.

Floppy-disk drives can store from 1.4 to 2.8 MB of data on one disk and have been used primarily in PCs. Hard-disk drives, or hard drives, contain nonremovable magnetic media and are used with all types of computers. They access data very quickly and can store hundreds of GB of data.

Magnetic-tape storage devices are usually used together with hard drives on large computer systems that handle high volumes of constantly changing data. The tape drives, which access data sequentially and relatively slowly, regularly back up, or duplicate, the data in the hard drives to protect the system against loss of data during power failures or computer malfunctions.

Flash memory is a solid-state electronic storage medium that combines the recordability of RAM with the persistence of ROM. Since its invention in two basic forms in the late 1980s (by Intel and Toshiba), it has become standard for portable devices such as digital cameras, cellular telephones, PDAs (personal digital assistant), MP3 players, and video-game machines. In the early 21st century, flash memory devices that could fit on a key ring and had storage capacities of up to 1 GB (and later more) began to serve as portable hard drives.

Optical discs are nonmagnetic auxiliary storage devices that developed from audio compact disc (CD) technology. Read-only CDs, whose data can be read but not changed, are called CD-ROMs (compact disc–read-only memory). Recordable CDs—called CD-R for write once/read many (WORM) discs and CD-RW for rewritable discs—have been used by many businesses and universities to periodically back up changing databases and by individuals to create their own music CDs.

Digital video disc (DVD) is a newer optical format that uses a higher-power laser to read smaller data-storage regions. Although DVDs are the same size as CDs, single-sided discs (the most common) hold up to 4.7 GB. There exist several types of recordable, as well as rewritable, DVDs.

Components that let the user see or hear the results of the computer's data processing are known as output devices. The most common one is the video display terminal (VDT), or monitor, which uses a cathode-ray tube (CRT) or liquid-crystal display (LCD) to show characters and graphics on a television-like screen.

Modems (modulator-demodulators) are input/output (I/O) devices that allow computers to transfer data between each other. A basic modem on one computer translates digital pulses into analog signals (sound) and then transmits the signals through a telephone line or a communication network to another computer. A modem on the computer at the other end of the line reverses the process. Different types of modems are used to transmit information over digital telephone networks (digital subscriber line, or DSL, modems), television cable lines (cable modems), and wireless networks (high-frequency radio modems).

Printers generate hard copy—a printed version of information stored in one of the computer's memory systems. Color ink-jet and black-and-white laser printers are most common, though the declining cost of color laser printers has increased their presence outside of the publishing industry.

Most PCs also have audio speakers. These allow the user to hear sounds, such as music or spoken words, that the computer generates.

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