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.
Basic Structure
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.