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For the magazine by IEEE Computer Society, see Computer
(magazine)

A computer is a machine for manipulating data according to a list of
instructions - aprogram.

Computers can be extremely versatile. In fact, they are universal
information processing machines. According to the Church-Turing
thesis, a computer with a certain minimum threshold capability is in
principle capable of performing the tasks of any other computer, from
those of a personal digital
assistant to a supercomputer, as long as
time is not a factor. Therefore, the same computer designs have been adapted for
tasks from processing company payrolls to controlling industrial robots. Modern electronic
computers also have enormous speed and capacity for information processing
compared to earlier designs, and they have become exponentially more powerful
over the years (a phenomenon known as Moore's Law).

Computers are available in many physical forms. The original computers were
the size of a large room, and such enormous computing facilities still exist for
specialized scientific computation — supercomputers — and for
the transaction
processing requirements of large companies, generally called mainframes.
Smaller computers for individual use, called personal computers,
and their portable equivalent, the laptop computer, are
ubiquitous information-processing and communication tools and
are perhaps what most non-experts think of as "a computer". However, the most
common form of computer in use today is the embedded computer,
small computers used to control another device. Embedded computers control
machines from fighter planes to digital cameras.

Contents

1 History of
computing

2 How computers work: the
stored program architecture

3 Digital circuits

3.1 I/O devices

3.1.1 Instructions

3.2 Programs

3.2.1 Libraries and
operating systems

4 Computer
applications

4.1 Networking and the
Internet

5 Computing professions and
disciplines

6 See also

7 External links

[edit]

History of computing

Main article: History of
computing

ENIAC — a very important milestone
in computing history

Originally, the term "computer" referred to a person who performed
numerical calculations under the direction of a mathematician, possibly
with the aid of a variety of mechanical
calculating devices such as the abacus onward. An example of an
early computing device was the Antikythera
mechanism, an ancient Greek device for calculating the movements of planets, dating from
about 87 BCE. The end of the Middle Ages saw a
reinvigoration of European mathematics and
engineering, and by the early 17th century a succession of mechanical
calculating devices had been constructed using clockwork technology. [citation
needed]

Charles Babbage was the
first to conceptualize and design a fully programmable computer as early as
1837, but due to a combination of the limits of the technology of the time,
limited finance, and an inability to resist tinkering with his design, the
device was never actually constructed in his lifetime. A number of technologies
that would later prove useful in computing, such as the punch card and the vacuum
tube had appeared by the end of the 19th century, and large-scale automated
data processing using punch cards was performed by tabulating machines designed
by Hermann Hollerith.

During the first half of the 20th century, many scientific computing needs
were met by increasingly sophisticated, special-purpose analog computers, which
used a direct physical or electrical model of the
problem as a basis for computation. These became increasingly rare after the
development of the digital computer.

A succession of steadily more powerful and flexible computing devices were
constructed in the 1930s and 1940s, gradually adding the key features of modern
computers, such as the use of digital electronics (invented by Claude
Shannon in 1937) and more flexible programmability. Defining one point along
this road as "the first computer" is exceedingly difficult. Notable achievements
include the Atanasoff Berry
Computer, a special-purpose machine that used valve-driven (vacuum tube)
computation, binary numbers,
and regenerative memory; the secret British Colossus computer
(1944), which had limited programmability but demonstrated that a device using
thousands of valves could be made reliable and reprogrammed electronically; the
American ENIAC
(1946) — which was one of the first general purpose machine, but still used the
decimal system and incorporated an inflexible architecture that meant
reprogramming it essentially required it to be rewired and Konrad
Zuse's Z machines, with the electromechanical Z3 (1941) being the first working
machine featuring automatic binary arithmetic and feasible programability.

The team who developed ENIAC, recognizing its flaws, came up with a far more
flexible and elegant design, which has become known as the stored program
architecture, which is the basis from which virtually all modern computers
were derived. A number of projects to develop computers based on the stored
program architecture commenced in the late 1940s; the first of these to be up
and running was the Small-Scale
Experimental Machine, but the EDSAC was perhaps the first
practical version.

The Apple II, an early personal
computer

Valve-driven computer designs were in use throughout the 1950s, but were
eventually replaced with transistor-based computers,
which were smaller, faster, cheaper, and much more reliable, thus allowing them
to be commercially produced, in the 1960s. By the 1970s, the adoption of integrated circuit
technology had enabled computers to be produced at a low enough cost to allow
individuals to own a personal computer of
the type familiar today.

[edit]

How computers work: the stored program architecture

While the technologies used in computers have changed dramatically since the
first electronic, general-purpose, computers of the 1940s, most still use the stored program
architecture (sometimes called the von Neumann architecture; as the article
describes the primary inventors were probably ENIAC designers J. Presper Eckert and
John William
Mauchly). The design made the universal computer a practical reality.

The architecture describes a computer with four main sections: the arithmetic and
logic unit (ALU), the control circuitry, the memory, and the input
and output devices (collectively termed I/O). These parts are interconnected by
a bundle of wires (a "bus") and are usually
driven by a timer or clock (although other events could drive the
control circuitry).

Conceptually, a computer's memory can be viewed as a list of cells. Each cell
has a numbered "address" and can store a small, fixed amount of information.
This information can either be an instruction, telling the computer what to do,
or data, the information which the computer is to process using the instructions
that have been placed in the memory. In principle, any cell can be used to store
either instructions or data.

The ALU is in many
senses the heart of the computer. It is capable of performing two classes of
basic operations: arithmetic operations, the core of which is the ability to add
or subtract two numbers but also encompasses operations like "multiply this
number by 2" or "divide by 2" (for reasons which will become clear later), as
well as some others. The second class of ALU operations involves
comparison operations, which, given two numbers, can determine if they
are equal, and if not, which is of greater magnitude.

The I/O systems are the means by which the computer receives information from
the outside world, and reports its results back to that world. On a typical
personal computer, input devices include objects like the keyboard and mouse, and output devices
include computer monitors, printers and the
like, but as will be discussed later a huge variety of devices can be connected
to a computer and serve as I/O devices.

The control system ties this all together. Its job is to read instructions
and data from memory or the I/O devices, decode the instructions, providing the
ALU with the correct inputs according to the instructions, "tell" the ALU what
operation to perform on those inputs, and send the results back to the memory or
to the I/O devices. One key component of the control system is a counter that
keeps track of what the address of the current instruction is; typically, this
is incremented each time an instruction is executed, unless the instruction
itself indicates that the next instruction should be at some other location
(allowing the computer to repeatedly execute the same instructions).

Since the 1980s the ALU and control unit (collectively called a central processing
unit or CPU) have typically been located on a single integrated circuit
called a microprocessor.

The functioning of such a computer is in principle quite straightforward.
Typically, on each clock cycle, the computer fetches instructions and data from
its memory. The instructions are executed, the results are stored, and the next
instruction is fetched. This procedure repeats until a halt instruction
is encountered.

Larger computers, such as some minicomputers, mainframe computers,
servers, differ
from the model above in one significant aspect; rather than one CPU they often
have a number of them. Supercomputers often have
highly unusual architectures significantly different from the basic
stored-program architecture, sometimes featuring thousands of CPUs, but such
designs tend to be useful only for specialized tasks.

[edit]

Digital circuits

The conceptual design above could be implemented using a variety of different
technologies. As previously mentioned, a stored program computer could be
designed entirely of mechanical components like Babbage's. However, digital circuits allow
Boolean logic and arithmetic using binary
numerals to be implemented using relays — essentially, electrically
controlled switches. Shannon's famous thesis showed how relays could be arranged
to form units called logic gates, implementing
simple Boolean operations. Others soon figured out that vacuum tubes — electronic
devices, could be used instead. Vacuum tubes were originally used as a signal amplifier for
radio and other applications, but were used in digital electronics as a very
fast switch; when electricity is provided to one of the pins, current can flow
through between the other two.

Through arrangements of logic gates, one can build digital circuits to do
more complex tasks, for instance, an adder, which
implements in electronics the same method — in computer terminology, an algorithm — to
add two numbers together that children are taught — add one column at a time,
and carry what's left over. Eventually, through combining circuits together, a
complete ALU and control system can be built up. This does require a
considerable number of components. CSIRAC, one of the earliest
stored-program computers, is probably close to the smallest practically useful
design. It had about 2,000 valves, some of which were "dual components", so this
represented somewhere between 2 and 4,000 logic components.

Vacuum tubes had severe limitations for the construction of large numbers of
gates. They were expensive, unreliable (particularly when used in such large
quantities), took up a lot of space, and used a lot of electrical power, and,
while incredibly fast compared to a mechanical switch, had limits to the speed
at which they could operate. Therefore, by the 1960s they were replaced by the
transistor, a new device
which performed the same task as the tube but was much smaller, faster
operating, reliable, used much less power, and was far cheaper.

Integrated circuits
are the basis of modern digital computing hardware.

In the 1960s and 1970s, the transistor itself was gradually replaced by the
integrated circuit,
which placed multiple transistors (and other components) and the wires
connecting them on a single, solid piece of silicon. By the 1970s, the entire
ALU and control unit, the combination becoming known as a CPU, were being placed on
a single "chip" called a microprocessor. Over the
history of the integrated circuit, the number of components that can be placed
on one has grown enormously. The first IC's contained a few tens of components;
as of 2005, modern microprocessors such from AMD and Intel contain over 100 million
transistors. Furthermore, The 45nm SRAM chip announced in 2006 by Intel has more
than 1 billion transistors.

Tubes, transistors, and transistors on integrated circuits can be used as the
"storage" component of the stored-program architecture, using a circuit design
known as a flip-flop, and
indeed flip-flops are used for small amounts of very high-speed storage.
However, few computer designs have used flip-flops for the bulk of their storage
needs. Instead, earliest computers stored data in Williams tubes —
essentially, projecting some dots on a TV screen and reading them again, or mercury delay lines
where the data was stored as sound pulses traveling slowly (compared to the
machine itself) along long tubes filled with mercury. These somewhat ungainly
but effective methods were eventually replaced by magnetic memory devices, such
as magnetic core
memory, where electrical currents were used to introduce a permanent (but
weak) magnetic field in some ferrous material, which could then be read to
retrieve the data. Eventually, DRAM was introduced. A DRAM unit is
a type of integrated circuit containing huge banks of an electronic component
called a capacitor which can store an
electrical charge for a period of time. The level of charge in a capacitor could
be set to store information, and then measured to read the information when
required.

[edit]

I/O devices

I/O (short for input/output) is a general term for devices that send
computers information from the outside world and that return the results of
computations. These results can either be viewed directly by a user, or they can
be sent to another machine, whose control has been assigned to the computer: In
a robot, for
instance, the controlling computer's major output device is the robot
itself.

The first generation of computers were equipped with a fairly limited range
of input devices. A punch card reader, or
something similar, was used to enter instructions and data into the computer's
memory, and some kind of printer, usually a modified teletype, was used to record
the results. Over the years, a huge variety of other devices have been added.
For the personal computer, for instance, keyboards and mice are the primary ways
people directly enter information into the computer; and monitors are the
primary way in which information from the computer is presented back to the
user, though printers, speakers, and headphones are common, too. There is a huge
variety of other devices for obtaining other types of input. One example is the
digital camera, which can
be used to input visual information. There are two prominent classes of I/O
devices. The first class is that of secondary storage
devices, such as hard disks, CD-ROMs, key
drives and the like, which represent comparatively slow, but high-capacity
devices, where information can be stored for later retrieval; the second class
is that of devices used to access computer networks. The
ability to transfer data between computers has opened up a huge range of
capabilities for the computer. The global Internet allows millions of
computers to transfer information of all types between each other.

[edit]

Instructions

The instructions interpreted by the control unit, and executed by the ALU,
are not nearly as rich as a human language. A computer responds only to a
limited number of instructions, which are precisely defined, simple, and
unambiguous. Typical sorts of instructions supported by most computers are "copy
the contents of memory cell 5 and place the copy in cell 10", "add the contents
of cell 7 to the contents of cell 13 and place the result in cell 20", "if the
contents of cell 999 are 0, the next instruction is at cell 30". All computer
instructions fall into one of four categories: 1) moving data from one location
to another; 2) executing arithmetic and logical processes on data; 3) testing
the condition of data; and 4) altering the sequence of operations.

Instructions are represented within the computer as binary code — a
base two system of counting. For example, the code for one kind of "copy"
operation in the Intel line of microprocessors is 10110000. The particular
instruction set that a specific computer supports is known as that computer's machine language.

To slightly oversimplify, if two computers have CPUs that respond to the same
set of instructions identically, software from one can run on the other without
modification. This easy portability of existing software creates a great
incentive to stick with existing designs, only switching for the most compelling
of reasons, and has gradually narrowed the number of distinct instruction set
architectures in the marketplace.

[edit]

Programs

Computer programs are
simply lists of instructions for the computer to execute. These can range from
just a few instructions which perform a simple task, to a much more complex
instruction list which may also include tables of data. Many computer programs
contain millions of instructions, and many of those instructions are executed
repeatedly. A typical modern PC (in the year 2005)
can execute around 3 billion instructions per second. Computers do not gain
their extraordinary capabilities through the ability to execute complex
instructions. Rather, they do millions of simple instructions arranged by people
known as programmers.

In practice, people do not normally write the instructions for computers
directly in machine language. Such programming is incredibly tedious and highly
error-prone, making programmers very unproductive. Instead, programmers describe
the desired actions in a "high level" programming
language which is then translated into the machine language automatically by
special computer programs (interpreters and
compilers).
Some programming languages map very closely to the machine language, such as Assembly Language (low
level languages); at the other end, languages like Prolog are based on abstract
principles far removed from the details of the machine's actual operation (high
level languages). The language chosen for a particular task depends on the
nature of the task, the skill set of the programmers, tool availability and,
often, the requirements of the customers (for instance, projects for the US
military were often required to be in the Ada programming
language).

Computer software
is an alternative term for computer programs; it is a more inclusive phrase and
includes all the ancillary material accompanying the program needed to do useful
tasks. For instance, a video game
includes not only the program itself, but also data representing the pictures,
sounds, and other material needed to create the virtual environment of the game.
A computer
application is a piece of computer software provided to many computer users,
often in a retail environment. The stereotypical modern example of an
application is perhaps the office suite, a set of
interrelated programs for performing common office tasks.

Going from the extremely simple capabilities of a single machine language
instruction to the myriad capabilities of application programs means that many
computer programs are extremely large and complex. A typical example is the Firefox web browser,
created from roughly 2 million lines of computer code in the C++ programming
language; there are many projects of even bigger scope, built by large teams
of programmers. The management of this enormous complexity is key to making such
projects possible; programming languages, and programming practices, enable the
task to be divided into smaller and smaller subtasks until they come within the
capabilities of a single programmer in a reasonable period.

Nevertheless, the process of developing software remains slow, unpredictable,
and error-prone; the discipline of software
engineering has attempted, with some partial success, to make the process
quicker and more productive and improve the quality of the end product.

[edit]

Libraries and operating systems

Soon after the development of the computer, it was discovered that certain
tasks were required in many different programs; an early example was computing
some of the standard mathematical functions. For the purposes of efficiency,
standard versions of these were collected in libraries and made available to all
who required them. A particularly common task set related to handling the gritty
details of "talking" to the various I/O devices, so libraries for these were
quickly developed.

By the 1960s, with computers in wide industrial use for many purposes, it
became common for them to be used for many different jobs within an
organization. Soon, special software to automate the scheduling and execution of
these many jobs became available. The combination of managing "hardware" and
scheduling jobs became known as the "operating system"; the
classic example of this type of early operating system was OS/360 by IBM.

The next major development in operating systems was timesharing — the idea that
multiple users could use the machine "simultaneously" by keeping all of their
programs in memory, executing each user's program for a short time so as to
provide the illusion that each user had their own computer. Such a development
required the operating system to provide each user's programs with a "virtual
machine" such that one user's program could not interfere with another's (by
accident or design). The range of devices that operating systems had to manage
also expanded; a notable one was hard disks; the idea of
individual "files" and a hierarchical structure of "directories" (now often
called folders) greatly simplified the use of these devices for permanent
storage. Security access controls, allowing computer users access only to files,
directories and programs they had permissions to use, were also common.

Perhaps the last major addition to the operating system were tools to provide
programs with a standardized graphical user
interface. While there are few technical reasons why a GUI has to be tied to
the rest of an operating system, it allows the operating system vendor to
encourage all the software for their operating system to have a similar looking
and acting interface.

Outside these "core" functions, operating systems are usually shipped with an
array of other tools, some of which may have little connection with these
original core functions but have been found useful by enough customers for a
provider to include them. For instance, Apple's Mac OS X ships with a digital video
editor application.

Not all operating systems provide all of the above functions; operating
systems for smaller computers typically provide fewer, such as the highly
minimal operating systems for early microcomputers. Embedded computers may
have a specialized operating system, or sometimes none at all. Instead, the
custom programs written for their task perform all necessary functions that
would be performed by an operating system in less specialized roles.

[edit]

Computer applications

Computer-controlled robots are now common in industrial
manufacture.

Computer-generated
imagery (CGI) is a central ingredient in motion picture visual effects. The
seawater creature in The Abyss (1989) marked the acceptance of CGI
in the visual effects industry.

Furby: many modern, mass-produced
toys would not be possible without cheap embedded computers.

The first digital computers, with their large size and cost, mainly performed
scientific calculations, often to support military objectives. The ENIAC was originally
designed to calculate ballistics-firing tables for artillery, but it was also
used to calculate neutron cross-sectional densities to help in the design of the
hydrogen bomb. This
calculation, performed in December, 1945 through January, 1946 and involving
over a million punch cards of data, showed the design then under
consideration would fail. (Many of the most powerful supercomputers available
today are also used for nuclear weapons simulations.) The CSIR Mk I, the first
Australian stored-program computer, evaluated rainfall patterns for the catchment area of the Snowy Mountains Scheme,
a large hydroelectric generation
project. Others were used in cryptanalysis, for example
the first programmable (though not general-purpose) digital electronic computer,
Colossus, built in
1943 during World War II. Despite this
early focus of scientific and military engineering applications, computers were
quickly used in other areas.

From the beginning, stored program computers were applied to business
problems. The LEO, a stored
program-computer built by J. Lyons and Co. in the
United Kingdom, was
operational and being used for inventory management and other purposes 3 years
before IBM built their
first commercial stored-program computer. Continual reductions in the cost and
size of computers saw them adopted by ever-smaller organizations. Moreover, with
the invention of the microprocessor in the
1970s, it became possible to produce inexpensive computers. In the 1980s, personal computers
became popular for many tasks, including book-keeping, writing and
printing documents, calculating forecasts and other repetitive mathematical
tasks involving spreadsheets.

As computers have become less expensive, they have been used extensively in
the creative arts as well. Sound, still pictures, and video are now routinely
created (through synthesizers, computer graphics and
computer animation),
and near-universally edited by computer. They have also been used for
entertainment, with the video game
becoming a huge industry.

Computers have been used to control mechanical devices since they became
small and cheap enough to do so; indeed, a major spur for integrated circuit
technology was building a computer small enough to guide the Apollo
missions and the Minuteman missile, two
of the first major applications for embedded computers. Today, it is almost
rarer to find a powered mechanical device not controlled by a computer
than to find one that is at least partly so. Perhaps the most famous
computer-controlled mechanical devices are robots, machines with more-or-less
human appearance and some subset of their capabilities. Industrial robots have
become commonplace in mass production, but
general-purpose human-like robots have not lived up to the promise of their
fictional counterparts and remain either toys or research projects.

Robotics, indeed, is the physical expressions of the field of artificial
intelligence, a discipline whose exact boundaries are fuzzy but to some
degree involves attempting to give computers capabilities that they do not
currently possess but humans do. Over the years, methods have been developed to
allow computers to do things previously regarded as the exclusive domain of
humans — for instance, "read" handwriting, play chess, or perform symbolic
integration. However, progress on creating a computer that exhibits
"general" intelligence comparable to a human has been extremely slow.

[edit]

Networking and the Internet

Computers have been used to coordinate information in multiple locations
since the 1950s, with the US military's SAGE system the first large-scale
example of such a system, which led to a number of special-purpose commercial
systems like Sabre.

In the 1970s, computer engineers at research institutions throughout the US
began to link their computers together using telecommunications technology. This
effort was funded by ARPA,
and the computer network that
it produced was called the ARPANET. The technologies that
made the Arpanet possible spread and evolved. In time, the network spread beyond
academic and military institutions and became known as the Internet. The emergence of
networking involved a redefinition of the nature and boundaries of the computer.
In the phrase of John Gage and Bill Joy (of
Sun Microsystems), "the
network is the computer". Computer operating systems and applications were
modified to include the ability to define and access the resources of other
computers on the network, such as peripheral devices, stored information, and
the like, as extensions of the resources of an individual computer. Initially
these facilities were available primarily to people working in high-tech
environments, but in the 1990s the spread of applications like e-mail and the World
Wide Web, combined with the development of cheap, fast networking
technologies like Ethernet and ADSL saw computer networking become
ubiquitous almost everywhere. In fact, the number of computers that are
networked is growing phenomenally. A very large proportion of personal computers
regularly connect to the Internet to communicate and
receive information.

[edit]

Computing professions and disciplines

In the developed world, virtually every profession makes use of
computers. However, certain professional and academic disciplines have evolved
that specialize in techniques to construct, program, and use computers.
Terminology for different professional disciplines is still somewhat fluid and
new fields emerge from time to time: however, some of the major groupings are as
follows:

Computer
engineering is the branch of electronic
engineering devoted to the physical construction of computers and their
attendant components.

Computer science is
an academic study of the processes related to computation, such as developing
efficient algorithms to perform
specific tasks. It tackles questions as to whether problems can be solved at
all using a computer, how efficiently they can be solved, and how to construct
efficient programs to compute solutions. A huge array of specialties has
developed within computer science to investigate different classes of
problems.

Software
engineering concentrates on methodologies and practices to allow the
development of reliable software systems while minimizing, and reliably
estimating, costs and timelines.

Information systems
concentrates on the use and deployment of computer systems in a wider
organizational (usually business) context.

Many disciplines have developed at the intersection of computers with
other professions; one of many examples is experts in geographical
information systems who apply computer technology to problems of managing
geographical information.

There are two major professional societies dedicated to computers, the Association
for Computing Machinery and IEEE Computer Society.

[edit]

See also

Look up Computer
in Wiktionary, the free
dictionary

Wikiquote has a collection of
quotations related to:

Computer

Wikimedia Commons has
media related to:

Computer

Association
for Computing Machinery

IEEE Computer
Society

Computer hardware

Computability
theory

Computer datasheet

Computer expo

Computer science

Computer types: analog computer, hybrid computer, supercomputer, desktop, laptop, desknote, roll-away computer,
embedded computer,
cart computer, tablet pc,
handheld computer,
subnotebook, and server

Computing

Computers in
fiction

Computer security
and Computer
insecurity challenges such as: malware, phishing, spam, and how to solve them, such
as firewall, computer security
audit

Digital

History of
computing

List of computing
topics

Personal computer

Word processing

Computer
programming

Quantum computer

[edit]

External links

Discuss Computers / Computing

Computer History

Apple Computers

Purchasing A Computer

Retrieved from "http://en.wikipedia.org/wiki/Computer"

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