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I.
INTRODUCTION
Computer,
electronic device that can receive a set of instructions, or
program, and then carry out this program by performing
calculations on numerical data or by manipulating other forms
of information.
The
modern world of high technology could not have come about
except for the development of the computer. Different types
and sizes of computers find uses throughout society in the
storage and handling of data, from secret governmental files
to banking transactions to private household accounts.
Computers have opened up a new era in manufacturing through
the techniques of automation, and they have enhanced modern
communication systems. They are essential tools in almost
every field of research and applied technology, from
constructing models of the universe to producing tomorrow’s
weather reports, and their use has in itself opened up new
areas of conjecture. Database services and computer networks
make available a great variety of information sources. The
same advanced techniques also make possible invasions of
personal and business privacy. Computer crime has become one
of the many risks that are part of the price of modern
technology.
II.
TYPES OF COMPUTERS
Two
main types of computers are in use today, analogue and digital.
Analogue computers exploit the mathematical similarity between
physical interrelationships in certain problems, and employ
electronic or hydraulic circuits (see Fluidics) to
simulate the physical problem. Digital computers solve
problems by performing calculations and by dealing with each
number digit by digit.
Installations
that contain elements of both digital and analogue computers
are called hybrid computers. They are usually used for
problems in which large numbers of complex equations, known as
time integrals, are to be computed. Data in analogue form can
also be fed into a digital computer by means of an analogue-to-digital
converter, and the same is true of the reverse situation (see
Digital-to-Analogue Converter).
A)
Analogue Computers
The
simplest analogue calculating device is the slide rule, which
employs specially calibrated scales to facilitate
multiplication, division, and other functions. The analogue
computer is a more sophisticated electronic or hydraulic
device that is designed to handle input in terms of, for
example, voltage levels or hydraulic pressures, rather than
numerical data. In a typical electronic analogue computer, the
inputs are converted into voltages that may be added or
multiplied using specially designed circuit elements. The
answers are continuously generated for display or for
conversion to another desired form.
B)
Digital Computers
Everything
that a digital computer does is based on one operation: the
ability to determine whether a switch, or "gate", is
open or closed. That is, the computer can recognize only two
states in any of its microscopic circuits: on or off, high
voltage or low voltage, or—in the case of numbers—0 or 1.
The speed at which the computer performs this simple act,
however, is what makes it a marvel of modern technology.
Computer speeds are measured in megahertz, or millions of
cycles per second. A computer with a "clock speed"
of 100 MHz—a fairly representative speed for a microcomputer—is
capable of executing 100 million discrete operations each
second. Supercomputers used in research and defence
applications attain speeds of billions of cycles per second.
Digital
computer speed and calculating power are further enhanced by
the amount of data handled during each cycle. If a computer
checks only one switch at a time, that switch can represent
only two commands or numbers; thus ON would symbolize one
operation or number, and OFF would symbolize another. By
checking groups of switches linked as a unit, however, the
computer increases the number of operations it can recognize
at each cycle. For example, a computer that checks two
switches at one time can represent four numbers (0-3) or can
execute one of four instructions at each cycle, one for each
of the following switch patterns: OFF-OFF (0); OFF-ON (1); ON-OFF
(2); or ON-ON (3).
III.
HISTORY
The
first adding machine, a precursor of the digital computer, was
devised in 1642 by the French scientist, mathematician, and
philosopher Blaise Pascal. This device employed a series of
ten-toothed wheels, each tooth representing a digit from 0 to
9. The wheels were connected so that numbers could be added to
each other by advancing the wheels by a correct number of
teeth. In the 1670s the German philosopher and mathematician
Gottfried Wilhelm Leibniz improved on this machine by devising
one that could also multiply.
The
French inventor Joseph-Marie Jacquard, in designing an
automatic loom, used thin, perforated wooden boards to control
the weaving of complicated designs. During the 1880s the
American statistician Herman Hollerith conceived the idea of
using perforated cards, similar to Jacquard’s boards, for
processing data. Employing a system that passed punched cards
over electrical contacts, he was able to compile statistical
information for the 1890 United States census.
A)
The Analytical Engine
Also
in the 19th century, the British mathematician and inventor
Charles Babbage worked out the principles of the modern
digital computer. He conceived a number of machines, such as
the Difference Engine, that were designed to handle
complicated mathematical problems. Many historians consider
Babbage and his associate, the mathematician Augusta Ada Byron,
Countess of Lovelace, the true pioneers of the modern digital
computer. One of Babbage’s designs, the Analytical Engine,
had many features of a modern computer. It had an input stream
in the form of a deck of punched cards, a "store"
for saving data, a "mill" for arithmetic operations,
and a printer that made a permanent record. Babbage failed to
put this idea into practice, though it may well have been
technically possible at that date.
B)
Early Computers
Analogue
computers began to be built in the late 19th century. Early
models calculated by means of rotating shafts and gears.
Numerical approximations of equations too difficult to solve
in any other way were evaluated with such machines. Lord
Kelvin built a mechanical tide predictor that was a
specialized analogue computer. During World Wars I and II,
mechanical and, later, electrical analogue computing systems
were used as torpedo course predictors in submarines and as
bombsight controllers in aircraft. Another system was designed
to predict spring floods in the Mississippi River basin.
C)
Electronic Computers
During
World War II a team of scientists and mathematicians, working
at Bletchley Park, north of London, created one of the first
all-electronic digital computers: Colossus. By December 1943,
Colossus, which incorporated 1,500 vacuum tubes, was
operational. It was used by the team headed by Alan Turing, in
the largely successful attempt to crack German radio messages
enciphered in the Enigma code.
Independently
of this, in the United States, a prototype electronic machine
had been built as early as 1939, by John Atanasoff and
Clifford Berry at Iowa State College. This prototype and later
research were completed quietly and later overshadowed by the
development of the Electronic Numerical Integrator And
Computer ( ENIAC) in 1945. ENIAC was granted a patent, which
was overturned decades later, in 1973, when the machine was
revealed to have incorporated principles first used in the
Atanasoff-Berry Computer (ABC).
ENIAC contained
18,000 vacuum tubes and had a speed of several hundred
multiplications per minute, but originally its program was
wired into the processor and had to be manually altered. Later
machines were built with program storage, based on the ideas
of the Hungarian-American mathematician John von Neumann. The
instructions, like the data, were stored within a "memory",
freeing the computer from the speed limitations of the paper-tape
reader during execution and permitting problems to be solved
without rewiring the computer. See Von Neumann
Architecture.
The
use of the transistor in computers in the late 1950s marked
the advent of smaller, faster, and more versatile logical
elements than were possible with vacuum-tube machines. Because
transistors use much less power and have a much longer life,
this development alone was responsible for the improved
machines called second-generation computers. Components became
smaller, as did inter-component spacings, and the system
became much less expensive to build.
D)
Integrated Circuits
Late
in the 1960s the integrated circuit, or IC, was introduced,
making it possible for many transistors to be fabricated on
one silicon substrate, with interconnecting wires plated in
place. The IC resulted in a further reduction in price, size,
and failure rate. The microprocessor became a reality in the
mid-1970s with the introduction of the large-scale integrated
(LSI) circuit and, later, the very large-scale integrated (VLSI)
circuit (microchip), with many thousands of interconnected
transistors etched into a single silicon substrate.
To
return, then, to the switching capabilities of a modern
computer: computers in the 1970s were generally able to handle
eight switches at a time. That is, they could deal with eight binary
digits, or bits, of data, at every cycle. A group of
eight bits is called a byte, each byte containing 256 possible
patterns of ONs and OFFs (or 1s and 0s). Each pattern is the
equivalent of an instruction, a part of an instruction, or a
particular type of datum, such as a number or a character or a
graphics symbol. The pattern 11010010, for example, might be
binary data—in this case, the decimal number 210 (see
Number Systems)—or it might be an instruction telling the
computer to compare data stored in its switches to data stored
in a certain memory-chip location.
The
development of processors that can handle 16, 32, and 64 bits
of data at a time has increased the speed of computers. The
complete collection of recognizable patterns—the total list
of operations—of which a computer is capable is called its
instruction set. Both factors—the number of bits that can be
handled at one time, and the size of instruction sets—continue
to increase with the ongoing development of modern digital
computers.
IV.
HARDWARE
Modern
digital computers are all conceptually similar, regardless of
size. Nevertheless, they can be divided into several
categories on the basis of cost and performance: the personal
computer or microcomputer, a relatively low-cost machine,
usually of desk-top size (though "laptops" are small
enough to fit in a briefcase, and "palmtops" can fit
into a pocket); the workstation, a microcomputer with enhanced
graphics and communications capabilities that make it
especially useful for office work; the minicomputer, generally
too expensive for personal use, with capabilities suited to a
business, school, or laboratory; and the mainframe computer, a
large, expensive machine with the capability of serving the
needs of major business enterprises, government departments,
scientific research establishments, or the like (the largest
and fastest of these are called supercomputers).
A digital
computer is not a single machine: rather, it is a system
composed of five distinct elements: (1) a central processing
unit; (2) input devices; (3) memory storage devices; (4)
output devices; and (5) a communications network, called a
bus, which links all the elements of the system and connects
the system to the external world.
A)
Central Processing Unit (CPU)
The
CPU may be a single chip or a series of chips that perform
arithmetic and logical calculations and that time and control
the operations of the other elements of the system.
Miniaturization and integration techniques made possible the
development of the microprocessor, a CPU chip that
incorporates additional circuitry and memory. The result is
smaller computers and reduced support circuitry.
Microprocessors are used in personal computers.
Most
CPU chips and microprocessors are composed of four functional
sections: (1) an arithmetic/logic unit; (2) registers; (3) a
control section; and (4) an internal bus. The arithmetic/logic
unit gives the chip its calculating ability and permits
arithmetical and logical operations. The registers are
temporary storage areas that hold data, keep track of
instructions, and hold the location and results of these
operations. The control section has three principal duties. It
times and regulates the operations of the entire computer
system; its instruction decoder reads the patterns of data in
a designated register and translates the pattern into an
activity, such as adding or comparing; and its interrupt unit
indicates the order in which individual operations use the CPU,
and regulates the amount of CPU time that each operation may
consume.
The
last segment of a CPU chip or microprocessor is its internal
bus, a network of communication lines that connects the
internal elements of the processor and also leads to external
connectors that link the processor to the other elements of
the computer system. The three types of CPU buses are: (1) a
control bus consisting of a line that senses input signals and
another line that generates control signals from within the
CPU; (2) the address bus, a one-way line from the processor
that handles the location of data in memory addresses; and (3)
the data bus, a two-way transfer line that both reads data
from memory and writes new data into memory.
B)
Input Devices
These
devices enable a computer user to enter data, commands, and
programs into the CPU. The most common input device is the
keyboard. Information typed at the typewriter-like keyboard is
translated by the computer into recognizable patterns. Other
input devices include the mouse, which translates physical
motion into motion on a computer video display screen; the
joystick, which performs the same function, and is favoured
for computer games; the trackball, which replaces the mouse on
laptops; scanners, which "read" words or symbols on
a printed page and translate them into electronic patterns
that the computer can manipulate and store; light pens, which
can be used to "write" directly on the monitor
screen; and voice recognition systems, which take spoken words
and translate them into digital signals for the computer.
Storage devices can also be used to input data into the
processing unit.
C)
Storage Devices
Computer
systems can store data internally (in memory) and externally
(on storage devices). Internally, instructions or data can be
temporarily stored in silicon RAM (Random Access Memory) chips
that are mounted directly on the computer’s main circuit
board, or in chips mounted on peripheral cards that plug into
the computer’s main circuit board. These RAM chips consist
of millions of switches that are sensitive to changes in
electric current. So-called static RAM chips hold their data
as long as current flows through the circuit, whereas dynamic
RAM (DRAM) chips need high or low voltages applied at regular
intervals—every two milliseconds or so—if they are not to
lose their information.
Another
type of internal memory consists of silicon chips on which all
switches are already set. The patterns on these ROM (Read-Only
Memory) chips form commands, data, or programs that the
computer needs to function correctly. RAM chips are like
pieces of paper that can be written on, erased, and used again;
ROM chips are like a book, with its words already set on each
page. 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.
D)
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.
E)
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.
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