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Computer

In current usage, a computer is a device which is used to process information according to a well-defined procedure.

The word was originally used to describe people who were employed to do arithmetic calculations, with or without mechanical aids. The famed Gottfried Wilhelm von Leibniz himself complained of the time he expended in performing calculations. Starting in the 1950s computing machine was used to refer to the machines themselves; finally, the shorter word computer took over the term computing machine. Originally, computing was almost exclusively related to arithmetical problems, but modern computers are used for many tasks unrelated to mathematics, as their cost has declined, their performance has increased and their size is reduced.

Within such a definition sit mechanical devices such as the slide rule, the gamut of mechanical calculators from the abacus onwards, as well as all contemporary electronic computers. Terms better suited for such broad meanings ascribed to the word "computer" would be " information processor", "information processing system", or even "controller".

For more details, see the word "computer.

Table of contents
1 Definitions
2 How computers work
3 Uses of computers
4 The word "computer"
5 See also
6 External links

Definitions

However, the above definition includes many special-purpose devices that can compute only one or a limited range of functions. When considering modern computers, their most notable characteristic that distinguishes them from earlier computing devices is that, given the right programming, any computer can emulate the behaviour of any other (limited only by storage capacity and execution speed), and, indeed, it is believed that current machines can emulate any future computing devices we invent (though undoubtedly more slowly). In some sense, then, this threshold capability is a useful test for identifying "general-purpose" computers from earlier special-purpose devices. This "general-purpose" definition can be formalised into a requirement that a certain machine must be able to emulate the behaviour of a universal Turing machine. Machines meeting this definition are referred to as Turing-complete. While such machines are physically impossible as they require unlimited storage and zero crashing probability, the attribute Turing-complete is sometimes also used in a lax sense for machines that would be universal if they had more (infinite) storage and were absolutely reliable. The first such machine appeared in 1941: the program-controlled Z3 of Konrad Zuse (but its Turing-completeness was shown only much later, namely, in 1998). Other machines followed in a flurry of developments around the world. See the history of computing article for more details of this period.

Embedded computers

In the last 20 years or so, however, many household devices, notably including video game consoles but extending to mobile telephones, video cassette recorders, PDA's and myriad other household, industrial, automotive, and other electronic devices, all contain computer-like circuitry capable of meeting the above Turing-completeness requirement (with the proviso that the programming of these devices is often hardwired into a ROM chip which would need to be replaced to change the programming of the machine). These computers inside other special-purpose devices are commonly referred to as "microcontrollers" or "embedded computers". Therefore, many restrict the definition of computers to devices whose primary purpose is information processing, rather than being a part of a larger system such as a telephone, microwave oven, or aircraft (but see: avionics), and can be adapted for a variety of purposes by the user without physical modification. Mainframe computers, minicomputers, and personal computers are the main types of computers meeting this definition.

Personal computers

Finally, many people who are unfamiliar with other forms of computers use the term exclusively to refer to personal computers (PCs).

How computers work

While the technologies used in digital computers have changed dramatically since the first computers of the 1940s (see History of computing hardware for more details), most still use the von Neumann architecture proposed in the early 1940s by John von Neumann.

Von Neumann's 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."

Memory

In this system, memory is a sequence of numbered cells, each containing a small piece of information. The information may be an instruction to tell the computer what to do. The cell may contain data that the computer needs to perform the instruction. Any cell may contain either, and indeed what is at one time data might be instructions later.

In general, the contents of a memory cell can be changed at any time - it is a scratchpad rather than a stone tablet.

The size of each cell, and the number of cells, varies greatly from computer to computer, and the technologies used to implement memory have varied greatly - from electromechanical relays, to mercury-filled tubes (and later springs) in which acoustic pulses were formed, to matrices of permanent magnets, to individual transistors, to integrated circuits with millions of capacitors on a single chip.

Processing

The arithmetic and logical unit, or ALU, is the device that performs elementary operations such as arithmetic operations (addition, subtraction, and so on), logical operations (AND, OR, NOT), and comparison operations (for example, comparing the contents of two bytes for equality). This unit is where the "real work" is done.

The control unit keeps track of which bytes in memory contain the current instruction that the computer is performing, telling the ALU what operation to perform and retrieving the information (from memory) that it needs to perform it, and transfers the result back to the appropriate memory location. Once that occurs, the control unit goes to the next instruction (typically located in the next slot (memory address), unless the instruction is a jump instruction informing the computer that the next instruction is located in another location),This is a very importent part of a computer , it comes in sd ram, ddr ram,rd ram.

Input and output

The I/O allows the computer to obtain information from the outside world, and send the results of its work back there. There is an incredibly broad range of I/O devices, from the familiar keyboardss, monitors and floppy disk drives, to the more unusual such as webcams.

What all input devices have in common is that they encode (convert) information of some type into data which can further be processed by the digital computer system. Output devices on the other hand, decode the data into information which can be understood by the computer user. In this sense, a digital computer system is an example of a data processing system.

Instructions

The instructions discussed above are not the rich instructions of a human language. A computer only has a limited number of well-defined, simple instructions. Typical sorts of instructions supported by most computers are "copy the contents of cell 123, and place the copy in cell 456", "add the contents of cell 666 to cell 042, and place the result in cell 013", and "if the contents of cell 999 are 0, your next instruction is at cell 345".

Instructions are represented within the computer as numbers - the code for "copy" might be 001, for example. The particular instruction set that a specific computer supports is known as that computer's machine language. In practice, people do not normally write the instructions for computers directly in machine language but rather use a "high level" programming language which is then translated into the machine language automatically by special computer programs (interpreterss and compilers). Some programming languages map very closely to the machine language, such as assembler (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).

Architecture

Contemporary computers put the ALU and control unit into a single integrated circuit known as the Central Processing Unit or CPU. Typically, the computer's memory is located on a few small integrated circuits near the CPU. The overwhelming majority of the computer's mass is either ancillary systems (for instance, to supply electrical power) or I/O devices.

Some larger computers differ from the above model in one major respect - they have multiple CPUs and control units working simultaneously. Additionally, a few computers, used mainly for research purposes and scientific computing, have differed significantly from the above model, but they have found little commercial application.

The functioning of a computer is therefore in principle quite straightforward. 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 the computer is turned off.

Programs

Computer programs are simply large lists of instructions for the computer to execute, perhaps with 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 2003) can execute around 2-3 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 clever people, "programmers." Good programmers develop sets of instructions to do common tasks (for instance, draw a dot on screen) and then make those sets of instructions available to other programmers.

Nowadays, most computers appear to execute several programs at the same time. This is usually referred to as multitasking. In reality, the CPU executes instructions from one program, then after a short period of time, it switches to a second program and executes some of its instructions. This small interval of time is often referred to as a time slice. This creates the illusion of multiple programs being executed simultaneously by sharing the CPU's time between the programs. This is similar to how a movie is simply a rapid succession of still frames. The operating system is the program that usually controls this time sharing.

Operating system

A computer always needs at least one program running at all times to operate. Under normal operation this program is the operating system (OS). The operating system decides which programs are run, when, and what resources (such as memory or I/O) they get to use. The operating system also provides a layer of abstraction over the hardware, and gives access by providing services to other programs, such as code ("drivers") which allow programmers to write programs for a machine without needing to know the intimate details of all attached electronic devices.

Uses of computers

The first digital computers, with their large size and cost, mainly performed scientific calculation. ENIAC, an early US computer originally designed to calculate ballistics firing tables for artillery, calculated neutron cross-sectional densities to see if the hydrogen bomb would work properly (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). (Interestingly, many of the most powerful supercomputers available today are also used for nuclear weapons simulations.) The CSIR Mk I, the first Australian computer, evaluated rainfall patterns for the catchment of the Snowy Mountains Scheme, a large hydroelectric generation project. Others were used in cryptanalysis, for example the world's first programmable (though not turing-complete) digital electronic computer, Colossus, built during World War II. However, early visionaries also anticipated that programming would allow chess playing, moving pictures and other uses.

People in governments and large corporations also used computers to automate many of the data collection and processing tasks previously performed by humans - for example, maintaining and updating accounts and inventories. In academia, scientists of all sorts began to use computers for their own analyses. Continual reductions in the costs of computers saw them adopted by ever-smaller organizations. Businesses, organizations, and governments often employ a large number of small computers to accomplish tasks that were previously done by an expensive, large mainframe computer. Collections of the smaller computers in one location is referred to as a server farm.

With the invention of the microprocessor in the 1970s, it became possible to produce very inexpensive computers. Personal computers became popular for many tasks, including keeping books, writing and printing documents. Calculating forecasts and other repetitive math with spreadsheets, communicating with e-mail and, the Internet. However, computers' wide availability and easy customization has seen them used for many other purposes.

At the same time, small computers, usually with fixed programming, began to find their way into other devices such as home appliances, automobiles, aeroplanes, and industrial equipment. These embedded processors controlled the behaviour of such devices more easily, allowing more complex control behaviours (for instance, the development of anti-lock brakes in cars). By the start of the twenty-first century, most electrical devices, most forms of powered transport, and most factory production lines are controlled by computers. Most engineers predict that this trend will continue.

The word "computer"

Over the years there have been several slightly different meanings to the word computer, and several different words for the thing we now usually call a computer.

For instance "computer" was once commonly used to mean a person employed to do arithmetic calculations, with or without mechanical aids. According to the Barnhart Concise Dictionary of Etymology, the word came into use in English in 1646 as a word for a "person who computes" and then by 1897 also for a mechanical calculating machine. During World War II it referred to U.S. and British servicewomen whose job it was to calculate the trajectories of large artillery shells with such machines.

Charles Babbage designed some of the first computing machines, including the Difference Engine and the Analytical Engine. Due to technological problems the Difference Engine was not completed in his lifetime, but a working model based on his drawings was constructed for the Science Museum in London in 1993. The Analytical Engine was never completed at all.

Various simple mechanical devices such as the slide rule and abacus have also been called computers. In some cases they were referred to as "analog computers", as they represented numbers by continuous physical quantities rather than by discrete binary digits. What are now called simply "computers" were once commonly called "digital computers" to distinguish them from these other devices (which are still used in the field of analog signal processing, for example).

In thinking of other words for the computer, it is worth noting that in other languages the word chosen does not always have the same literal meaning as the English language word. In French for example, the word is ordinateur, which means approximately "organizer", or "sorting machine". The Spanish word is ordenador , with the same meaning, although in some countries they use the anglicism computadora. In Portuguese, it assumes the form computador from the verb computar, which means "to compute", "to calculate". In Italian, computers are usually referred to with the anglicism computer, but there is also the term calcolatore, calculator — emphasizing its computational uses over logical ones like sorting — and elaboratore, elaborator. In Swedish, a computer is called dator from "data". At least in the 1950s, they were called matematikmaskin ("mathematics machine"). In Finnish computer is called tietokone which means "information machine". The Icelandic language's name for a computer is more poetic, their word tölva, a portmanteau meaning "number prophetess". In Chinese, a computer is called 电脑 (diànnǎo) or an "electric brain". In English, other words and phrases have been used, such as "data processing machine".

Computer types

See also

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Alandale Training Corp.
Provides training in Windows, MS Office, Act and Lotus Notes. Located in Vancouver, British Columbia, Canada.
http://www.alandaletraining.com

Anderson Computer Instruction
Microsoft Office and Windows software application tutorials. Located in Somersworth, New Hampshire.
http://www.anderson-computer.com/

Computer Teach, Inc.
Computer classes of 5 or less. Basics to Advanced. Internet, Word, Excel, Programming. Children's classes also available. Located in New York, United States.
http://www.computerteach.net

Alaska Computer Training Center
Instructor led and computer-based training. Located in Alaska, United States.
http://www.aktraining.com

Brainware
Computer training and consulting, including web and software services. Located in India.
http://www.brainware-india.com/

Computer Learning Niche
Offering a variety of personalized programs. Located in Maryland, United States.
http://www.pc-learningniche.com

Inspired Training
Offers training and consultancy services. Located in the United Kingdom.
http://www.c-holt.co.uk

Keystar Computer Training
Courses in Word, Excel, Powerpoint and other major Microsoft Applications. We can tailor courses to suit your individual needs. Located in the United Kingdom.
http://www.keystar.co.uk

Victorian Software Services
Fully certified corporate computer training in all Office, MYOB, VB, Macromedia and Web design programs. Located in Australia.
http://www.vicsoft.com.au

VSC Computer Lab
Low cost computer training to provide needed job skills. Located in Texas, United States.
http://www.angelfire.com/tx5/rbward42/index.htm

CANDO Tech Center
Provides career-oriented, technically advanced training and related services. Located in Wyoming, United States.
http://www.candotechcenter.org

Computer Training Resources
Specializing in a variety of Microsoft technologies. Located in the United Kingdom.
http://www.ctres.com

Customized Computer Training
Tailor made and scheduled computer software training. Located in the United Kingdom.
http://www.customisedcomputertraining.com

Insight Training
Provides IT training to companies or individuals. Located in the United Kingdom.
http://www.uk-insight.co.uk

IT For Life
Software and hardware training courses, self study. Located in the United Kingdom.
http://www.it4life.i12.com

McCarthy-Helstrom and Associates
Microsoft Office training with strong emphasis on practical business applications. Located in Indiana, United States.
http://www.mccarthy-helstrom.com

Microcentre
Microsoft Office and other desktop training from an independent Scottish company with over 20 years experience in the field. Located in the United Kingdom.
http://www.microcentre.co.uk

Zenith Training Systems
Computer and software training organization. Located in the United Kingdom.
http://www.zenith-training.com

Z Tech Training
Computer training and consulting solution provider. Located in New Jersey, United States.
http://www.ztechtraining.com

Comfort Computer College
Basic Computer Instruction on three levels: Beginner, Intermediate, Advanced, Windows, Microsoft Office Suite, Internet and email. Located in Texas, United States.
http://www.comfortcomputercollege.homestead.com/

Comtek International Inc.
Provides end user computer training and consulting to individuals, small businesses and corporations. Located in Ontario, Canada.
http://members.rogers.com/comtek.international/default.htm

BL&A, Inc. Training Consultants
Microsoft Office 97/200 training and MOUS certifications. Located in Maryland, United States.
http://www.blatraining.com

Freelance Services
Training and web design. Located in the United Kingdom.
http://www.freelance-services.com

Aptech
Offering a variety of computer courses. Located in India.
http://www.aptech-education.com

Software Solutions and Designs
Specializes in business applications computer based training and offers networking and web development consulting.
http://www.ssdplus.com/

Capital Computer Services
Specializes in using technology to assist small to medium sized businesses with their needs. Located in Washington, United States.
http://home.comcast.net/~capitalcomputerservices/ccs/index.htm

Monroe Business Associates
Quality end user application training. Located in Connecticut, United States.
http://www.monroebusiness.com

Jo Buxton IT Training and Consultancy
Training and consultancy for small business systems, office automation and courses. Located in the United Kingdom.
http://www.jobuxton.co.uk

One Step Training
One Step Training provides concise, effective, onsite, 90 minute computer segments MS Office training to small and large businesses. Located in Rhode Island, United States.
http://www.onesteptraining.com

Pass Computer Training
Scheduled and tailored courses on a range of Microsoft Office and accountancy software packages. Located in the United Kingdom.
http://www.pass.co.uk

System Information Technology
ECDL, Microsoft Office, Word, Excel, Access, PowerPoint, internet, email. Located in the United Kingdom.
http://www.system-it.co.uk

Systems Integration and Support
Providers of application training including Microsoft Office 97, 2000, Project 98, 2000, Windows 98 / ME / 2000. Located in Maryland, United States.
http://www.sis-md.com

Tagg
Specialist provider of tailor made training packages in Microsoft and Corel for all levels of user. Located in the United Kingdom.
http://www.tagg.co.uk

Wall to Wall Computer Services
Computer consultation, support, and application training. Located in Australia.
http://www.walltowall.com.au

Wordtrax Computer Training
Provides private lessons and small class groups for beginners and advanced level computer courses. Also the ECDL at Basic and Advanced level. Located in Ireland.
http://www.wordtrax.ie

Zone IT Training
Instructor-led computer training company. Accredited Test Centre for ECDL and MOUS. Also training on Sage and A+ Certification. Located in the United Kingdom.
http://www.zone-it.co.uk

Software Tuition Services
Tailored computer tuition for home users, small businesses and larger businesses. Located in the United Kingdom.
http://www.softwaretuitionservices.com

Bonnie Sussman
Personalized computer lessons in your home or office. Online support and personal attention. Located in Florida, United States.
http://bonniesussman.com

DH Innovative Technologies
We train & troubleshoot at home or work. We make housecalls. Located in Michigan, United States.
http://www.dhit.net

Stuart Wilkins
Computer training and support. I teach friendly, professional classes on many different aspects of PC's and Macintoshes. Located in Australia.
http://www.stuartwilkins.net

Guru Software
We offer onsite software training in Microsoft Project, in Microsoft Office at all levels, and Macromedia applications. Located in California, United States.
http://www.gurusoftware.com/

Certified Fluid Consultants
We provide desktop computer training as well as industrial training. Computer room rental also available. Located in Ohio, United States.
http://cfc-solar.com

Crystal Multimedia
Computer training and consultancy. We have helped one person enterprises, professionals and large companies to make the most of IT. Located in the United Kingdom.
http://www.crystalmm.com



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