Both RAM and ROM chips are linked by circuitry to the CPU.

External storage devices, which may actually be located within the computer housing, are external to the main circuit board. These devices store data as charges on a magnetically sensitive medium such as a magnetic tape or, more commonly, on a disk coated with a fine layer of metallic particles. The most common external storage devices are so-called floppy disks and hard disks, although most large computer systems use banks of magnetic tape storage units. The floppy disks in normal use store about 800 kilobytes (a kilobyte is 1,024 bytes) or about 1.4 megabytes (1 megabyte is slightly more than a million bytes). Hard, or "fixed", disks cannot be removed from their disk-drive cabinets, which contain the electronics to read and write data on to the magnetic disk surfaces. Hard disks currently used with personal computers can store from several hundred megabytes to several gigabytes (1 gigabyte is a billion bytes). CD-ROM technology, which uses the same laser techniques that are used to create audio compact discs (CDs), normally produces storage capacities up to about 800 megabytes.

Output Devices

These devices enable the user to see the results of the computer's calculations or data manipulations. The most common output device is the video display unit (VDU), a monitor that displays characters and graphics on a television-like screen. A VDU usually has a cathode-ray tube like an ordinary television set, but small, portable computers use liquid crystal displays (LCDs) or electroluminescent screens. Other standard output devices include printers and modems. A modem links two or more computers by translating digital signals into analogue signals so that data can be transmitted via analogue telephone lines.

Operating Systems

Different types of peripheral devices-disk drives, printers, communications networks, and so on-handle and store data differently from the way the computer handles and stores it. Internal operating systems, usually stored in ROM memory, were developed primarily to coordinate and translate data flows from dissimilar sources, such as disk drives or coprocessors (processing chips that operate simultaneously with the central unit). An operating system is a master control program, permanently stored in memory, that interprets user commands requesting various kinds of services-commands such as display, print, or copy a data file; list all files in a directory; or execute a particular program.

Programming

A program is a sequence of instructions that tells the hardware of a computer what operations to perform on data.