CPU
in full central processing unit
Principal component of a digital computer, composed of a control unit, an instruction-decoding unit, and an arithmetic-logic unit.
The CPU is linked to main memory, peripheral equipment (including input/output devices), and storage units. The control unit integrates computer operations. It selects instructions from the main memory in proper sequence and sends them to the instruction-decoding unit, which interprets them so as to activate functions of the system at appropriate moments. Input data are transferred via the main memory to the arithmetic-logic unit for processing (i.e., addition, subtraction, multiplication, division, and certain logic operations). Larger computers may have two or more CPUs, in which case they are simply called “processors” because each is no longer a “central” unit. See also multiprocessing.
Memory
In digital computers, a physical device used to store such information as data or programs on a temporary or permanent basis.
Most digital computer systems have two types of memory, the main memory and one or more auxiliary storage units. In most cases, the main memory is a high-speed RAM. Auxiliary storage units include hard disks, floppy disks, and magnetic tape drives. Besides main and auxiliary memories, other forms of memory include ROM and optical storage media such as videodiscs and compact discs (see CD-ROM).
RAM
in full Random-Access Memory
Computer main memory in which specific contents can be accessed (read or written) directly by the CPU in a very short time regardless of the sequence (and hence location) in which they were recorded.
Two types of memory are possible with random-access circuits, static RAM (SRAM) and dynamic RAM (DRAM). A single memory chip is made up of several million memory cells. In a SRAM chip, each memory cell stores a binary digit (1 or 0) for as long as power is supplied. In a DRAM chip, the charge on individual memory cells must be refreshed periodically in order to retain data. Because it has fewer components, DRAM requires less chip area than SRAM; hence a DRAM chip can hold more memory, though its access time is slower.
ROM
in full Read-Only Memory
Form of computer memory that does not lose its contents when the power supply is cut off and that is not rewritable once it is manufactured or written.
It is generally employed for programs designed for repeated use without modification, such as the start-up procedures of a personal computer; the ROM is used for storing the program used in the control unit of the computer. See also CD-ROM, compact disc.
CD-ROM
in full Compact Disc Read-Only Memory
Type of computer storage medium that is read optically (e.g., by a laser).
A CD-ROM drive uses a low-power laser beam to read digitized (binary) data that have been encoded onto an optical disc in the form of tiny pits, then feeds the data to a computer for processing. Because it uses digital data, a CD-ROM can store images and sound in addition to text and is thus used in video and audio devices to store music, graphics, and movies (see compact disc). Unlike conventional magnetic-storage technologies (e.g., hard disks), CD-ROM drives cannot write information (that is, accept the input of new data), hence the tag “read-only.” Recordable compact discs (called CD-R) must be written on a CD-R recorder and can be played on any CD-ROM drive.
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Hard Disk
Magnetic storage medium for a microcomputer.
Hard disks are flat, circular plates made of aluminum or glass and coated with a magnetic material. Hard disks for personal computers can store up to several gigabytes (billions of bytes) of information. Data are stored on their surfaces in concentric tracks. A small electromagnet, called a magnetic head, writes a binary digit (1 or 0) by magnetizing tiny spots on the spinning disk in different directions and reads digits by detecting the magnetization direction of the spots. A computer's hard drive is a device consisting of several hard disks, read/write heads, a drive motor to spin the disks, and a small amount of circuitry, all sealed in a metal case to protect the disks from dust. In addition to referring to the disks themselves, the term hard disk is also used to refer to the whole hard drive.
Floppy Disk
or diskette
Magnetic storage medium used with computers.
Floppy disks are made of flexible plastic coated with a magnetic material, and are enclosed in a hard plastic case. They are typically 3.5 in. (9 cm) in diameter. Data are arranged on their surfaces in concentric tracks. A disk is inserted in the computer's floppy disk drive, an assembly of magnetic heads and a mechanical device for rotating the disk for reading or writing purposes. A small electromagnet, called a magnetic head, writes a binary digit (1 or 0) onto the disk by magnetizing a tiny spot on the disk in different directions, and reads digits by detecting the magnetization direction of the spots. With the increasing use of e-mail attachments and other means to transfer files from computer to computer, the use of floppy disks has waned, though they are still widely used to keep second (backup) copies of valuable files.
