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[[Image:RCX_Computer.png|thumb|right|225px|A [[Lego]] RCX Computer is an example of an [[embedded computer]] used to control mechanical devices. It is fully programmable.]]
A '''computer''' is a [[machine]] for manipulating [[data]] according to a list of [[instruction (computer science)|instructions]] known as a [[computer program|program]].
Computers are 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. Therefore, computers with capabilities ranging from those of a [[personal digital assistant]] to a [[supercomputer]] may all perform the same tasks, as long as time and [[Memory (computers)|memory]] capacity are not considerations. Therefore, the same computer designs may be adapted for tasks ranging from processing company [[payroll]]s to controlling [[Unmanned space mission|unmanned spaceflights]]. Due to [[technological]] advancement, modern electronic computers are exponentially more capable than those of preceding generations (a phenomenon partially described by [[Moore's Law]]).
Computers take numerous physical forms. Early electronic computers were the size of a large room, while entire modern embedded computers may be smaller than a deck of [[playing card]]s. Even today, enormous computing facilities still exist for specialized [[scientific computing|scientific computation]] and for the [[transaction processing]] requirements of large organizations. Smaller computers designed for individual use are called [[personal computer]]s. Along with its portable equivalent, the [[laptop computer]], the personal computer is the ubiquitous information processing and [[communication]] tool, and is usually what is meant by "a computer". However, the most common form of computer in use today is the [[embedded computer]]. Embedded computers are usually relatively simple and physically small computers used to control another device. They may control machines from [[fighter aircraft]] to [[industrial robot]]s to [[digital camera]]s.
==History of computing==
{{main|History of computing}}
[[Image:Eniac.jpg|thumb|right|225px|[[ENIAC]] was a milestone in computing history.]]
Originally, the term "computer" referred to a [[human computer|person who performed numerical calculations]], often with the aid of a [[mechanical calculating device]] or [[analog computer]]. Examples of these early devices, the ancestors of the computer, included the [[abacus]] and the [[Antikythera mechanism]], an [[ancient Greece|ancient Greek]] device for calculating the movements of [[planet]]s which dates from about 87 BC.<ref name="antikythera">{{cite web | author=Phillips, Tony | publisher=American Mathematical Society | year=2000 | title=The Antikythera Mechanism I | url=http://www.math.sunysb.edu/~tony/whatsnew/column/antikytheraI-0400/kyth1.html|accessdate=2006-04-05}}</ref> The end of the [[Middle Ages]] saw a reinvigoration of European mathematics and engineering, and [[Wilhelm Schickard]]'s 1623 device was the first of a number of mechanical calculators constructed by European engineers.<ref name="Schickard">{{cite web | year=Unknown | publisher=computerhistory.org | title=Visible Storage | url=http://www.computerhistory.org/VirtualVisibleStorage/artifact_main.php?tax_id=01.01.06.00|accessdate=2006-04-05}}</ref>
In [[1801]], [[Joseph Marie Jacquard]] made an improvement to the existing loom designs that used a series of punched paper cards as a program to weave intricate patterns. The resulting [[Jacquard loom]] is not considered a true computer but it was an important step in the development of modern digital computers.
[[Charles Babbage]] was the first to conceptualize and design a fully programmable computer as early as 1820, 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. By the end of the 19th century a number of technologies that would later prove useful in computing had appeared, such as the [[punch card]] and the [[vacuum tube]], 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 computer]]s, which used a direct mechanical or [[electricity|electrical]] model of the problem as a basis for computation (they became increasingly rare after the development of the programmable 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.
The use of digital electronics was introduced by [[Claude Shannon]] in 1937<ref name="shannon">Shannon, Claude Elwood (1940). [http://hdl.handle.net/1721.1/11173 A symbolic analysis of relay and switching circuits]. Massachusetts Institute of Technology: Thesis (M.S.)</ref> in his thesis [[A Symbolic Analysis of Relay and Switching Circuits]]. Here he introduced switches for implementing logic and aritmethic. He came up with the idea while studying the [[relay]] circuits of [[Vannevar Bush]]'s [[Differential Analyzer]].<ref>{http://scienceworld.wolfram.com/biography/Shannon.html Biography of Claude Elwood Shannon] - URL retrieved [[September 26]], [[2006]]</ref>. In November the same year [[George Stibitz]] completed a relay-based computer implementing the idea (the "Model K"). This point marked the beginning of binary [[digital circuit]] design and the use of [[logic gates]]. Precursors of this idea were [[Almon Strowger]], who patented a device containing a logic gate switch circuit, [[Nikola Tesla]] who filed for patents of devices containing logic gate circuits in 1898 (see [[List of Tesla patents]]), and [[Lee De Forest]]'s modification, in 1907, who replaced relays with vacuum tubes.
Defining one point along this road as "the first digital electronic computer" is exceedingly difficult.
On [[May 12]],[[1941]] [[Konrad Zuse]] completed his electromechanical [[Z3]], being the first working machine featuring automatic [[Binary numeral system|binary]] arithmetic and feasible programmability (therefore the first digital operational programmable computer, although not electronic); other notable achievements include the [[Atanasoff-Berry Computer]] (shown working around Summer 1941), a special-purpose machine that used valve-driven (vacuum tube) computation, [[Binary numeral system|binary]] numbers, and regenerative memory; the secret British [[Colossus computer]] (demonstrated in 1943), which had limited programmability but demonstrated that a device using thousands of valves could be both made reliable and reprogrammed electronically; the [[Harvard Mark I]], a large-scale electromechanical computer with limited programmability (shown working around 1944); the decimal-based American [[ENIAC]] (1944) — which was the first ''general purpose'' electronic computer, but originally had an inflexible architecture that meant reprogramming it essentially required it to be rewired.
{{Early computer characteristics}}
The team who developed ENIAC, recognizing its flaws, came up with a far more flexible and elegant design, which has become known as the [[Von Neumann architecture]] (or "stored program architecture"). This stored program architecture became the basis for virtually all modern computers. A number of projects to develop computers based on the stored program architecture commenced in the mid to late-1940s; the first of these were completed in Britain. The first to be up and running was the [[Small-Scale Experimental Machine]], but the [[EDSAC]] was perhaps the first practical version that was developed.
Valve (tube) 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 [[personal computer]]s.
==How computers work: the stored program architecture==
[[Image:Personal computer, exploded 5.svg|thumb|right|200px|An exploded view of a modern [[personal computer]]:
<ol>
<li>[[Computer display|Display]]
<li>[[Motherboard]]
<li>[[Central processing unit|CPU]] ([[Microprocessor]])
<li>[[Primary storage]] ([[Random access memory|RAM]])
<li>[[Expansion card]]s
<li>[[Power supply]]
<li>[[Optical disc|Optical disc drive]]
<li>[[Secondary storage]] ([[Hard disk|HD]])
<li>[[Computer keyboard|Keyboard]]
<li>[[Mouse (computing)|Mouse]]
</ol>]]
While the technologies used in computers have changed dramatically since the first [[electronics|electronic]], ''general-purpose computers'' of the 1940s, most still use the [[von Neumann architecture|stored program architecture]] (sometimes called the ''von Neumann architecture''). The design made the universal computer a practical reality.
<!-- this isn't what the stored program architecture is... rewrite -->
The architecture describes a computer with four main sections: the [[arithmetic and logic unit]] (ALU), the [[control unit|control circuitry]], the [[computer storage|memory]], and the input and output devices (collectively termed ''I/O''). These parts are interconnected by bundles of wires (called "[[computer bus|bus]]es" when the same bundle supports more than one data path) and are usually driven by a timer or [[Clock signal|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. The first is arithmetic operations; for instance, adding or subtracting two numbers together. The set of arithmetic operations may be very limited; indeed, some designs do not directly support multiplication and division operations (instead, users support multiplication and division through programs that perform multiple additions, subtractions, and other digit manipulations). The second class of ALU operations involves ''comparison'' operations: given two numbers, determining if they are equal, or if not equal which is larger.
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 [[computer mouse|mouse]], and output devices include [[computer monitor]]s, [[Computer printer|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.
The set of instructions interpreted by the control unit, and executed by the ALU, are limited in number, precisely defined, and very simple operations. Broadly, they fit into one or more of four categories: 1) moving data from one location to another (an example might be an instruction that "tells" the CPU to "copy the contents of memory cell 5 and place the copy in cell 10"). 2) executing arithmetic and logical processes on data (for instance, "add the contents of cell 7 to the contents of cell 13 and place the result in cell 20"). 3) testing the condition of data ("if the contents of cell 999 are 0, the next instruction is at cell 30"). 4) altering the sequence of operations (the previous example alters the sequence of operations, but instructions such as "the next instruction is at cell 100" are also standard).
Instructions, like data, are represented within the computer as [[Binary numeral system|binary]] code — a base two system of counting. For example, the code for one kind of "copy" operation in the [[Intel]] [[X86 architecture|x86]] line of microprocessors is 10110000.<ref>{{cite web | author=Unknown|title=IA-32 architecture
one byte opcodes|publisher= sandpile.org| year=Unknown | url=http://www.sandpile.org/ia32/opc_1.htm | accessdate=2006-04-09}}</ref> The particular instruction set that a specific computer supports is known as that computer's [[machine language]]. Using an already-popular machine language makes it much easier to run existing software on a new machine; consequently, in markets where commercial software availability is important suppliers have converged on one or a very small number of distinct machine languages.
More powerful computers such as [[minicomputer]]s, [[mainframe computer]]s and [[Server (computing)|servers]] may differ from the model above by dividing their work between more than one main CPU. [[Multiprocessing|Multiprocessor]] and [[Multi-core (computing)|multicore]] personal and laptop computers are also beginning to become available.<ref>{{cite web | author=Kanellos, Michael | title=Intel: 15 dual-core projects under way | publisher= CNET Networks, Inc.| year=2005 | url=http://news.com.com/Intel+15+dual-core+projects+under+way/2100-1006_3-5594773.html | accessdate=2006-07-15}}</ref><ref>{{cite web | author=Chen, Anne | title=Laptops Leap Forward in Power and Battery Life | publisher= Ziff Davis Publishing Holdings Inc. | year=2006 | url=http://www.eweek.com/article2/0,1895,1948898,00.asp | accessdate=2006-07-15}}</ref>
[[Supercomputer]]s 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. At the other end of the size scale, some [[microcontroller]]s use the [[Harvard architecture]] that ensures that program and data memory are logically separate.
==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 [[Charles Babbage|Babbage]]'s devices or the [[Digi-Comp I]]. However, [[digital circuits]] allow [[Boolean logic]] and [[binary arithmetic|arithmetic using binary numerals]] to be implemented using [[relay]]s — essentially, electrically controlled switches. Shannon's famous thesis showed how relays could be arranged to form units called [[logic gate]]s, implementing simple Boolean operations. Others soon figured out that [[vacuum tube]]s — 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 (electronics)|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",<ref>The last of the first : CSIRAC : Australia's first computer, Doug McCann and Peter Thorne, ISBN 0-7340-2024-4.</ref> so this represented somewhere between 2,000 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.
[[Image:InternalIntegratedCircuit2.JPG|thumb|right|225px|[[Integrated circuit]]s 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 (computer)|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 2006, the [[Intel Core Duo]] processor contains 151 million transistors.<ref name="toms-tcount">{{cite web | author=Thon, Harald and Töpel, Bert | publisher=Tom's Hardware |title=Will Core Duo Notebooks Trade Battery Life For Quicker Response? | year=January 16, 2006 | url=http://www.tomshardware.com/2006/01/16/will_core_duo_notebooks_trade_battery_life_for_quicker_response/ | accessdate=2006-04-09}}</ref>
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 (electronics)|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 tube]]s — essentially, projecting some dots on a TV screen and reading them again, or [[mercury delay line]]s where the data was stored as sound pulses travelling 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.
==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, other devices have been added. For the personal computer, for instance, [[computer keyboard|keyboards]] and [[computer mouse|mice]] are the primary ways people directly enter information into the computer; and [[Computer monitor|monitors]] are the primary way in which information from the computer is presented back to the user, though [[computer printer|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 disk]]s, [[CD-ROM]]s, [[USB flash drive|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 network]]s. 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.
==Programs==
[[Computer program]]s 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 [[personal computer|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 [[programmer]]s.
In practice, people do not normally write the instructions for computers directly in machine language. Such programming is time-consuming and error-prone, making programmers less productive. 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 ([[Interpreter (computing)|interpreters]] and [[compiler]]s). 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 [[Computer and video games|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 [[Windows XP]], created from roughly 40 million [[Source lines of code|lines of computer code]] in the [[C++]] [[programming language]];<ref name="WindowsXP-size">Tanenbaum, Andrew S. ''Modern Operating Systems'' (2nd ed.). Prentice Hall. ISBN 0-13-092641-8.</ref> 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 success, to make the process quicker and more productive and improve the quality of the end product.
====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]].<ref name="ibm-pr">{{cite press release | publisher = IBM Data Processing Division | date = April 7, 1964 | title = System/360 Announcement | url=http://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_PR360.html}}</ref>
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 disk]]s; 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 was 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 [[Video editing software|digital video editor]] application.
Operating systems for smaller computers may not provide all of these functions. The operating systems for early [[microcomputer]]s with limited memory and processing capability did not, and [[Embedded computer]]s typically have specialized operating systems or no operating system at all, with their custom application programs performing the tasks that might otherwise be delegated to an operating system.
==Computer applications==
{{wrapper}}
|[[Image:Industrial_Robotics_in_car_production.jpg|200px|thumb|Computer-controlled robots are now common in industrial manufacture.]]
|
|[[Image:Abyss.jpg|200px|thumb|[[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.]]
|
|[[Image:Furby_new.jpg|150px|thumb|Many modern, mass-produced toys like [[Furby]] would not be possible without low-cost embedded computers.]]
{{end}}
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]]<ref>{{cite web | title=Classical Super / Runaway Super | year=Unknown | publisher=Globalsecurity.org|url=http://www.globalsecurity.org/wmd/intro/classical-super.htm|accessdate=2006-04-05}}</ref> significantly speeding up its development. (Many of the most powerful [[supercomputer]]s available today are also used for [[nuclear weapon]]s [[simulation]]s.) The [[CSIRAC|CSIR Mk I]], the first Australian stored-program computer, was amongst many other tasks used for the evaluation of rainfall patterns for the [[catchment area]] of the [[Snowy Mountains]] Scheme, a large [[hydroelectric]] generation project<ref>The last of the first : CSIRAC : Australia's first computer, Doug McCann and Peter Thorne, ISBN 0-7340-2024-4.</ref> Others were used in [[cryptanalysis]], for example the first programmable (though not general-purpose) digital electronic computer, [[Colossus 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 computer|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 [[spreadsheet]]s.
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 [[Computer and video games|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 program|Apollo missions]]<ref>{{cite web | author=Brown, Alexander | title=Integrated Circuits in the Apollo Guidance Computer | year=August 22, 2002 | url=http://hrst.mit.edu/hrs/apollo/ic | accessdate=2006-04-05}}</ref><ref>{{cite web | year=Unknown | title=Technological Innovation and the ICBM | publisher=Smithsonian Institution|url=http://www.hrw.com/science/si-science/earth/spacetravel/spacerace/SpaceRace/sec200/sec270.html|accessdate=2006-04-05}}</ref> 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 [[robot]]s, 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 expression 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.
===Networking and the Internet===
Computers have been used to coordinate information in multiple locations since the 1950s, with the US military's [[Semi Automatic Ground Environment|SAGE]] system the first large-scale example of such a system, which led to a number of special-purpose commercial systems like [[Sabre (computer system)|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 [[Advanced Research Projects Agency|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.<ref>{{cite web | title=North America Internet Usage Stats | publisher=Internet World Stats | year=April 3, 2006|url=http://www.internetworldstats.com/america.htm#us|accessdate=2006-04-05}}</ref> "Wireless" networking, often utilizing [[mobile phone]] networks, has meant networking is becoming increasingly ubiquitous even in mobile computing environments. Wi-Fi is also a popular application, involving the wireless transfer of data through the internet. Wi-Fi is commonly used with laptops and can even be used with modern video game consoles.
==Alternative computing models==
Despite the massive gains in speed and capacity over the history of the digital computer, there are many tasks for which current computers are inadequate. For some of these tasks, conventional computers are fundamentally inadequate, because the time taken to find a solution grows very quickly as the size of the problem to be solved expands. Therefore, there has been research interest in some computer models that use biological processes, or the oddities of [[quantum physics]], to tackle these types of problems. For instance, [[DNA computing]] is proposed to use biological processes to solve certain problems. Because of the exponential division of cells, a DNA computing system could potentially tackle a problem in a massively parallel fashion. However, such a system is limited by the maximum practical mass of DNA that can be handled.
[[Quantum computer]]s, as the name implies, take advantage of the unusual world of quantum physics. If a practical quantum computer is ever constructed, there are a limited number of problems for which the quantum computer is fundamentally faster than a standard computer. However, these problems, relating to [[cryptography]] and, unsurprisingly, quantum physics simulations, are of considerable practical interest.
These alternative models for computation remain research projects at the present time, and will likely find application only for those problems where conventional computers are inadequate.
See also [[Unconventional computing]].
==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 [[electrical engineering]] that focuses both on hardware and software design, and the interaction between the two.
*[[Computer science]] is a traditional name of the academic study of the processes related to computers and computation, such as developing efficient [[algorithm]]s to perform specific class of 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 high quality software systems, while minimizing, and reliably estimating, costs and timelines.
*[[Information system]]s 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 [[Geographic information system|geographical information systems]] who apply computer technology to problems of managing geographical information.
There are three major professional societies dedicated to computers, the [[British Computer Society]] the [[Association for Computing Machinery]] and [[IEEE]] [[Computer Society]].
A '''computer''' is a [[machine]] for manipulating [[data]] according to a list of [[instruction (computer science)|instructions]] known as a [[computer program|program]].
Computers are 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. Therefore, computers with capabilities ranging from those of a [[personal digital assistant]] to a [[supercomputer]] may all perform the same tasks, as long as time and [[Memory (computers)|memory]] capacity are not considerations. Therefore, the same computer designs may be adapted for tasks ranging from processing company [[payroll]]s to controlling [[Unmanned space mission|unmanned spaceflights]]. Due to [[technological]] advancement, modern electronic computers are exponentially more capable than those of preceding generations (a phenomenon partially described by [[Moore's Law]]).
Computers take numerous physical forms. Early electronic computers were the size of a large room, while entire modern embedded computers may be smaller than a deck of [[playing card]]s. Even today, enormous computing facilities still exist for specialized [[scientific computing|scientific computation]] and for the [[transaction processing]] requirements of large organizations. Smaller computers designed for individual use are called [[personal computer]]s. Along with its portable equivalent, the [[laptop computer]], the personal computer is the ubiquitous information processing and [[communication]] tool, and is usually what is meant by "a computer". However, the most common form of computer in use today is the [[embedded computer]]. Embedded computers are usually relatively simple and physically small computers used to control another device. They may control machines from [[fighter aircraft]] to [[industrial robot]]s to [[digital camera]]s.
==History of computing==
{{main|History of computing}}
[[Image:Eniac.jpg|thumb|right|225px|[[ENIAC]] was a milestone in computing history.]]
Originally, the term "computer" referred to a [[human computer|person who performed numerical calculations]], often with the aid of a [[mechanical calculating device]] or [[analog computer]]. Examples of these early devices, the ancestors of the computer, included the [[abacus]] and the [[Antikythera mechanism]], an [[ancient Greece|ancient Greek]] device for calculating the movements of [[planet]]s which dates from about 87 BC.<ref name="antikythera">{{cite web | author=Phillips, Tony | publisher=American Mathematical Society | year=2000 | title=The Antikythera Mechanism I | url=http://www.math.sunysb.edu/~tony/whatsnew/column/antikytheraI-0400/kyth1.html|accessdate=2006-04-05}}</ref> The end of the [[Middle Ages]] saw a reinvigoration of European mathematics and engineering, and [[Wilhelm Schickard]]'s 1623 device was the first of a number of mechanical calculators constructed by European engineers.<ref name="Schickard">{{cite web | year=Unknown | publisher=computerhistory.org | title=Visible Storage | url=http://www.computerhistory.org/VirtualVisibleStorage/artifact_main.php?tax_id=01.01.06.00|accessdate=2006-04-05}}</ref>
In [[1801]], [[Joseph Marie Jacquard]] made an improvement to the existing loom designs that used a series of punched paper cards as a program to weave intricate patterns. The resulting [[Jacquard loom]] is not considered a true computer but it was an important step in the development of modern digital computers.
[[Charles Babbage]] was the first to conceptualize and design a fully programmable computer as early as 1820, 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. By the end of the 19th century a number of technologies that would later prove useful in computing had appeared, such as the [[punch card]] and the [[vacuum tube]], 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 computer]]s, which used a direct mechanical or [[electricity|electrical]] model of the problem as a basis for computation (they became increasingly rare after the development of the programmable 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.
The use of digital electronics was introduced by [[Claude Shannon]] in 1937<ref name="shannon">Shannon, Claude Elwood (1940). [http://hdl.handle.net/1721.1/11173 A symbolic analysis of relay and switching circuits]. Massachusetts Institute of Technology: Thesis (M.S.)</ref> in his thesis [[A Symbolic Analysis of Relay and Switching Circuits]]. Here he introduced switches for implementing logic and aritmethic. He came up with the idea while studying the [[relay]] circuits of [[Vannevar Bush]]'s [[Differential Analyzer]].<ref>{http://scienceworld.wolfram.com/biography/Shannon.html Biography of Claude Elwood Shannon] - URL retrieved [[September 26]], [[2006]]</ref>. In November the same year [[George Stibitz]] completed a relay-based computer implementing the idea (the "Model K"). This point marked the beginning of binary [[digital circuit]] design and the use of [[logic gates]]. Precursors of this idea were [[Almon Strowger]], who patented a device containing a logic gate switch circuit, [[Nikola Tesla]] who filed for patents of devices containing logic gate circuits in 1898 (see [[List of Tesla patents]]), and [[Lee De Forest]]'s modification, in 1907, who replaced relays with vacuum tubes.
Defining one point along this road as "the first digital electronic computer" is exceedingly difficult.
On [[May 12]],[[1941]] [[Konrad Zuse]] completed his electromechanical [[Z3]], being the first working machine featuring automatic [[Binary numeral system|binary]] arithmetic and feasible programmability (therefore the first digital operational programmable computer, although not electronic); other notable achievements include the [[Atanasoff-Berry Computer]] (shown working around Summer 1941), a special-purpose machine that used valve-driven (vacuum tube) computation, [[Binary numeral system|binary]] numbers, and regenerative memory; the secret British [[Colossus computer]] (demonstrated in 1943), which had limited programmability but demonstrated that a device using thousands of valves could be both made reliable and reprogrammed electronically; the [[Harvard Mark I]], a large-scale electromechanical computer with limited programmability (shown working around 1944); the decimal-based American [[ENIAC]] (1944) — which was the first ''general purpose'' electronic computer, but originally had an inflexible architecture that meant reprogramming it essentially required it to be rewired.
{{Early computer characteristics}}
The team who developed ENIAC, recognizing its flaws, came up with a far more flexible and elegant design, which has become known as the [[Von Neumann architecture]] (or "stored program architecture"). This stored program architecture became the basis for virtually all modern computers. A number of projects to develop computers based on the stored program architecture commenced in the mid to late-1940s; the first of these were completed in Britain. The first to be up and running was the [[Small-Scale Experimental Machine]], but the [[EDSAC]] was perhaps the first practical version that was developed.
Valve (tube) 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 [[personal computer]]s.
==How computers work: the stored program architecture==
[[Image:Personal computer, exploded 5.svg|thumb|right|200px|An exploded view of a modern [[personal computer]]:
<ol>
<li>[[Computer display|Display]]
<li>[[Motherboard]]
<li>[[Central processing unit|CPU]] ([[Microprocessor]])
<li>[[Primary storage]] ([[Random access memory|RAM]])
<li>[[Expansion card]]s
<li>[[Power supply]]
<li>[[Optical disc|Optical disc drive]]
<li>[[Secondary storage]] ([[Hard disk|HD]])
<li>[[Computer keyboard|Keyboard]]
<li>[[Mouse (computing)|Mouse]]
</ol>]]
While the technologies used in computers have changed dramatically since the first [[electronics|electronic]], ''general-purpose computers'' of the 1940s, most still use the [[von Neumann architecture|stored program architecture]] (sometimes called the ''von Neumann architecture''). The design made the universal computer a practical reality.
<!-- this isn't what the stored program architecture is... rewrite -->
The architecture describes a computer with four main sections: the [[arithmetic and logic unit]] (ALU), the [[control unit|control circuitry]], the [[computer storage|memory]], and the input and output devices (collectively termed ''I/O''). These parts are interconnected by bundles of wires (called "[[computer bus|bus]]es" when the same bundle supports more than one data path) and are usually driven by a timer or [[Clock signal|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. The first is arithmetic operations; for instance, adding or subtracting two numbers together. The set of arithmetic operations may be very limited; indeed, some designs do not directly support multiplication and division operations (instead, users support multiplication and division through programs that perform multiple additions, subtractions, and other digit manipulations). The second class of ALU operations involves ''comparison'' operations: given two numbers, determining if they are equal, or if not equal which is larger.
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 [[computer mouse|mouse]], and output devices include [[computer monitor]]s, [[Computer printer|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.
The set of instructions interpreted by the control unit, and executed by the ALU, are limited in number, precisely defined, and very simple operations. Broadly, they fit into one or more of four categories: 1) moving data from one location to another (an example might be an instruction that "tells" the CPU to "copy the contents of memory cell 5 and place the copy in cell 10"). 2) executing arithmetic and logical processes on data (for instance, "add the contents of cell 7 to the contents of cell 13 and place the result in cell 20"). 3) testing the condition of data ("if the contents of cell 999 are 0, the next instruction is at cell 30"). 4) altering the sequence of operations (the previous example alters the sequence of operations, but instructions such as "the next instruction is at cell 100" are also standard).
Instructions, like data, are represented within the computer as [[Binary numeral system|binary]] code — a base two system of counting. For example, the code for one kind of "copy" operation in the [[Intel]] [[X86 architecture|x86]] line of microprocessors is 10110000.<ref>{{cite web | author=Unknown|title=IA-32 architecture
one byte opcodes|publisher= sandpile.org| year=Unknown | url=http://www.sandpile.org/ia32/opc_1.htm | accessdate=2006-04-09}}</ref> The particular instruction set that a specific computer supports is known as that computer's [[machine language]]. Using an already-popular machine language makes it much easier to run existing software on a new machine; consequently, in markets where commercial software availability is important suppliers have converged on one or a very small number of distinct machine languages.
More powerful computers such as [[minicomputer]]s, [[mainframe computer]]s and [[Server (computing)|servers]] may differ from the model above by dividing their work between more than one main CPU. [[Multiprocessing|Multiprocessor]] and [[Multi-core (computing)|multicore]] personal and laptop computers are also beginning to become available.<ref>{{cite web | author=Kanellos, Michael | title=Intel: 15 dual-core projects under way | publisher= CNET Networks, Inc.| year=2005 | url=http://news.com.com/Intel+15+dual-core+projects+under+way/2100-1006_3-5594773.html | accessdate=2006-07-15}}</ref><ref>{{cite web | author=Chen, Anne | title=Laptops Leap Forward in Power and Battery Life | publisher= Ziff Davis Publishing Holdings Inc. | year=2006 | url=http://www.eweek.com/article2/0,1895,1948898,00.asp | accessdate=2006-07-15}}</ref>
[[Supercomputer]]s 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. At the other end of the size scale, some [[microcontroller]]s use the [[Harvard architecture]] that ensures that program and data memory are logically separate.
==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 [[Charles Babbage|Babbage]]'s devices or the [[Digi-Comp I]]. However, [[digital circuits]] allow [[Boolean logic]] and [[binary arithmetic|arithmetic using binary numerals]] to be implemented using [[relay]]s — essentially, electrically controlled switches. Shannon's famous thesis showed how relays could be arranged to form units called [[logic gate]]s, implementing simple Boolean operations. Others soon figured out that [[vacuum tube]]s — 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 (electronics)|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",<ref>The last of the first : CSIRAC : Australia's first computer, Doug McCann and Peter Thorne, ISBN 0-7340-2024-4.</ref> so this represented somewhere between 2,000 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.
[[Image:InternalIntegratedCircuit2.JPG|thumb|right|225px|[[Integrated circuit]]s 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 (computer)|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 2006, the [[Intel Core Duo]] processor contains 151 million transistors.<ref name="toms-tcount">{{cite web | author=Thon, Harald and Töpel, Bert | publisher=Tom's Hardware |title=Will Core Duo Notebooks Trade Battery Life For Quicker Response? | year=January 16, 2006 | url=http://www.tomshardware.com/2006/01/16/will_core_duo_notebooks_trade_battery_life_for_quicker_response/ | accessdate=2006-04-09}}</ref>
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 (electronics)|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 tube]]s — essentially, projecting some dots on a TV screen and reading them again, or [[mercury delay line]]s where the data was stored as sound pulses travelling 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.
==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, other devices have been added. For the personal computer, for instance, [[computer keyboard|keyboards]] and [[computer mouse|mice]] are the primary ways people directly enter information into the computer; and [[Computer monitor|monitors]] are the primary way in which information from the computer is presented back to the user, though [[computer printer|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 disk]]s, [[CD-ROM]]s, [[USB flash drive|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 network]]s. 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.
==Programs==
[[Computer program]]s 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 [[personal computer|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 [[programmer]]s.
In practice, people do not normally write the instructions for computers directly in machine language. Such programming is time-consuming and error-prone, making programmers less productive. 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 ([[Interpreter (computing)|interpreters]] and [[compiler]]s). 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 [[Computer and video games|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 [[Windows XP]], created from roughly 40 million [[Source lines of code|lines of computer code]] in the [[C++]] [[programming language]];<ref name="WindowsXP-size">Tanenbaum, Andrew S. ''Modern Operating Systems'' (2nd ed.). Prentice Hall. ISBN 0-13-092641-8.</ref> 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 success, to make the process quicker and more productive and improve the quality of the end product.
====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]].<ref name="ibm-pr">{{cite press release | publisher = IBM Data Processing Division | date = April 7, 1964 | title = System/360 Announcement | url=http://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_PR360.html}}</ref>
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 disk]]s; 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 was 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 [[Video editing software|digital video editor]] application.
Operating systems for smaller computers may not provide all of these functions. The operating systems for early [[microcomputer]]s with limited memory and processing capability did not, and [[Embedded computer]]s typically have specialized operating systems or no operating system at all, with their custom application programs performing the tasks that might otherwise be delegated to an operating system.
==Computer applications==
{{wrapper}}
|[[Image:Industrial_Robotics_in_car_production.jpg|200px|thumb|Computer-controlled robots are now common in industrial manufacture.]]
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|[[Image:Abyss.jpg|200px|thumb|[[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.]]
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|[[Image:Furby_new.jpg|150px|thumb|Many modern, mass-produced toys like [[Furby]] would not be possible without low-cost embedded computers.]]
{{end}}
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]]<ref>{{cite web | title=Classical Super / Runaway Super | year=Unknown | publisher=Globalsecurity.org|url=http://www.globalsecurity.org/wmd/intro/classical-super.htm|accessdate=2006-04-05}}</ref> significantly speeding up its development. (Many of the most powerful [[supercomputer]]s available today are also used for [[nuclear weapon]]s [[simulation]]s.) The [[CSIRAC|CSIR Mk I]], the first Australian stored-program computer, was amongst many other tasks used for the evaluation of rainfall patterns for the [[catchment area]] of the [[Snowy Mountains]] Scheme, a large [[hydroelectric]] generation project<ref>The last of the first : CSIRAC : Australia's first computer, Doug McCann and Peter Thorne, ISBN 0-7340-2024-4.</ref> Others were used in [[cryptanalysis]], for example the first programmable (though not general-purpose) digital electronic computer, [[Colossus 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 computer|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 [[spreadsheet]]s.
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 [[Computer and video games|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 program|Apollo missions]]<ref>{{cite web | author=Brown, Alexander | title=Integrated Circuits in the Apollo Guidance Computer | year=August 22, 2002 | url=http://hrst.mit.edu/hrs/apollo/ic | accessdate=2006-04-05}}</ref><ref>{{cite web | year=Unknown | title=Technological Innovation and the ICBM | publisher=Smithsonian Institution|url=http://www.hrw.com/science/si-science/earth/spacetravel/spacerace/SpaceRace/sec200/sec270.html|accessdate=2006-04-05}}</ref> 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 [[robot]]s, 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 expression 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.
===Networking and the Internet===
Computers have been used to coordinate information in multiple locations since the 1950s, with the US military's [[Semi Automatic Ground Environment|SAGE]] system the first large-scale example of such a system, which led to a number of special-purpose commercial systems like [[Sabre (computer system)|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 [[Advanced Research Projects Agency|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.<ref>{{cite web | title=North America Internet Usage Stats | publisher=Internet World Stats | year=April 3, 2006|url=http://www.internetworldstats.com/america.htm#us|accessdate=2006-04-05}}</ref> "Wireless" networking, often utilizing [[mobile phone]] networks, has meant networking is becoming increasingly ubiquitous even in mobile computing environments. Wi-Fi is also a popular application, involving the wireless transfer of data through the internet. Wi-Fi is commonly used with laptops and can even be used with modern video game consoles.
==Alternative computing models==
Despite the massive gains in speed and capacity over the history of the digital computer, there are many tasks for which current computers are inadequate. For some of these tasks, conventional computers are fundamentally inadequate, because the time taken to find a solution grows very quickly as the size of the problem to be solved expands. Therefore, there has been research interest in some computer models that use biological processes, or the oddities of [[quantum physics]], to tackle these types of problems. For instance, [[DNA computing]] is proposed to use biological processes to solve certain problems. Because of the exponential division of cells, a DNA computing system could potentially tackle a problem in a massively parallel fashion. However, such a system is limited by the maximum practical mass of DNA that can be handled.
[[Quantum computer]]s, as the name implies, take advantage of the unusual world of quantum physics. If a practical quantum computer is ever constructed, there are a limited number of problems for which the quantum computer is fundamentally faster than a standard computer. However, these problems, relating to [[cryptography]] and, unsurprisingly, quantum physics simulations, are of considerable practical interest.
These alternative models for computation remain research projects at the present time, and will likely find application only for those problems where conventional computers are inadequate.
See also [[Unconventional computing]].
==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 [[electrical engineering]] that focuses both on hardware and software design, and the interaction between the two.
*[[Computer science]] is a traditional name of the academic study of the processes related to computers and computation, such as developing efficient [[algorithm]]s to perform specific class of 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 high quality software systems, while minimizing, and reliably estimating, costs and timelines.
*[[Information system]]s 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 [[Geographic information system|geographical information systems]] who apply computer technology to problems of managing geographical information.
There are three major professional societies dedicated to computers, the [[British Computer Society]] the [[Association for Computing Machinery]] and [[IEEE]] [[Computer Society]].