A personal computer (PC) is a complete microcomputer
that is based on a microprocessor, a small semiconductor chip that performs
the operations of a central processing unit, or CPU. A PC also has other
integrated circuits. It is designed for use by a single user, and usually
includes a keyboard and a monitor, or Video Display Terminal.
Two of the chief measures of computing power are computer memory size and processing speed. The unit of memory is the byte, which can hold one character of text. A kilobyte (Kbyte) is 1,024 bytes, a megabyte (Mbyte) is 1,024 Kbytes, and a gigabyte (Gbyte) is 1,024 Mbytes. These measures have been used to distinguish PCs from larger minicomputers and mainframe computers, but the increasing power of the PC has blurred these distinctions. The memory capacity of early PCs was often as small as 16 Kbytes, but by the mid-1990s typical PCs were equipped with 4 to 16 Mbytes of memory. This can often be expanded to as much as 128 Mbytes or even to several Gbytes in a workstation, which is the most powerful form of PC.
The processing speed of PCs is commonly specified by the speed of the electronic clock that controls internal operations. The latter measure is most commonly used with PCs. Early PCs had clock speeds of one or two megahertz (MHz), but speeds of 100 MHz or more are possible in modern designs.
A computer system consists of three parts: the CPU, input-output devices (I/O devices), and memory. A CPU performs arithmetic and logic operations. PCs generally use processors that can process 16-bit (2-byte) or 32-bit (4-byte) chunks of information.
The most common input devices are keyboards and pointing devices, such as "mice" or "trackballs." The most common output device is the cathode-ray tube (CRT) display, or monitor. Displays provide both graphic and text modes. Graph displays and pointing devices make possible a "point and click" form of control that is easier for the user than typing commands at a keyboard. Other common I/O devices are scanners, microphone and speaker sound interfaces, and modems and network interfaces for communicating with other computers; the mouse, joystick, and light pen, for making tactile input; and printers, for producing "hard," or paper, copy (permanent output).
Primary memory refers to memory that is directly accessible by the CPU. Modern processors can handle from 16 Mbytes to 4 Gbytes. PCs are usually sold with less primary memory than the CPU can handle. Upgrades can be made later on..
Secondary memory refers to external memory required to store data that will not fit into primary memory or that must be kept permanently. (In most PCs, the contents of primary memory are lost when power is removed.) Magnetic disks are the most common form of secondary memory. Hard disks, often called fixed disks because they cannot be removed from the computer, typically can store from 100 million to 500 million characters of text information. Flexible (floppy) disks have much lower capacity but can be removed and stored off-line. Floppy disks are the usual way new software is introduced into a PC.
Other secondary memory devices commonly used are CD-ROM (compact-disc read-only memory) and magnetic tape drives. A CD-ROM can hold about 600 million characters and is ideal as a repository for a large amount of information (such as this encyclopedia) that needs to be readable but does not need to be changed by the computer user. Magnetic tapes have large capacity but are much slower than disks. Tapes are primarily used as backup devices so that valuable information can be restored if a fixed disk drive fails.