| For
a computer to have value as an information processor, people must
have ways to supply it with the information it is to process—the
input—and to receive the processed information—the output. This
usually requires translating the information from one form to
another. The input and output devices act as translators. They
change the form of the data but not the content, making it possible
for a person to communicate with the computer and for the computer
to communicate with a person, a machine, or another computer.
The most familiar
input device is a keyboard. As a person types words or numbers
into the keyboard, an electric circuit beneath the keys turns
each keystroke into its particular electrical signal. When that
signal reaches the CPU, the electronic decoding circuitry there
recognizes the signals as the characters the person typed.
Printers and video
display terminals (or monitors) are common output devices. They
translate electrical signals sent out from the CPU into a written
message or image that is printed out on paper or appears on
a screen.
A computer system
may have several input and output devices, sometimes called
peripherals or I/O devices. Some peripherals, such as keyboard,
are strictly for input. Others, such as printers, perform only
output functions. There are also a number of devices that translate
both ways.
Disk Drives
One of the most useful
two-way peripherals is the disk drive. It is used for both retrieving
and storing information on magnetic disks.
When the disk drive
is used as an input device, the drive “reads” (detects) the
information stored on the disk in the form of a magnetic pattern
and turns it into a series of electrical pulses. These pulses
are sent to the computer’s CPU or to internal memory.
When the disk drive
is used as an output device, it stores information sent it from
another part of the computer system, for example, internal memory.
It “writes” the information as a magnetic pattern on the disk.
Personal computer
systems usually use flexible or floppy disks made of plastic
with a magnetic coating. The floppy disk in its protective jacket
can be slipped in and out of a slot in a relatively inexpensive
disk drive. One floppy disk typically holds about a million
pieces of information. (These pieces of information are described
as bytes.)
All personal computers
today have hard disk drives. Hard disks are not removable, but
they can store thousands of times as much information as floppies,
and they are less likely to be damaged in use. |
