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A computer is a device or machine for processing information according to a program — a compiled list of instructions. The information to be processed may represent numbers, text, pictures, or sound, amongst many other types.

Originally, a "computer" was a person who performed numerical calculations under the direction of a mathematician, often with the aid of a variety of mechanical calculating devices from the abacus onward. An example of an early computing device was the Antikythera mechanism, an ancient Greek device for calculating the movements of planets, dating from about 87 BC. The technology responsible for this mysterious device seems to have been lost at some point.

With the reinvigoration of European mathematics and engineering in the end of the Middle Ages, a succession of mechanical calculating devices were constructed based on technology developed for clockwork by the early 17th century. A considerable number of technologies that would later prove vital for the digital computer were developed in the 19th and early 20th centuries, such as the punched card, and the valve. In the 19th century, Charles Babbage was the first to conceptualise and design a fully programmable computer as early as 1837, but due to a combination of the limits of the technology of the time, limited finance, and an inability to resist tinkering with his design (a trait that would in time doom thousands of computer-related engineering projects) ensured that the device was never actually constructed.

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

The Colossus computer was used to break German ciphers during World War II.A succession of steadily more powerful and flexible computing devices were constructed in the 1930's and 1940's, gradually adding the key features of modern computers: the use of digital electronics (essentially invented by Claude Shannon in 1937), and more flexible programmability. Defining one point along this road as "the first computer" is exceedingly difficult. Notable achievements include Konrad Zuse's Z machines; the electro-mechanical Z3 was arguably the first universal computer, but it was completely impractical to use in this manner; the secret British Colossus computer, which had limited programmability but demonstrated that a device using thousands of valves could be made reliable; and the American ENIAC - a general purpose machine, but with an inflexible architecture that meant reprogramming it essentially required it to be rewired.

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

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

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

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

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

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

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

The control system ties this all together. Its job is to read instructions and data from memory or the I/O devices, decode the instructions, providing the ALU with the correct inputs according to the instructions, "tell" the ALU what operation to perform on those inputs, and send the results back to the memory or to the I/O devices. One key component of the control system is a counter that keeps track of what the address of the current instruction is; typically this is incremented each time an instruction is executed, unless the instruction itself indicates that the next instruction should be at some other location (allowing the computer to repeatedly execute the same instructions). Physically, since the 1980's the ALU and control unit have been located on a single integrated circuit called a Central Processing Unit or CPU.

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

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

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

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

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

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

Tubes, transistors, and transistors on integrated circuits can be and are used as the "storage" component of the stored-program architecture, using a circuit design known as a flip-flop, and indeed flip-flops are used for small amounts of very high-speed storage. However, few computer designs have used flip-flops for the bulk of their storage needs. Instead, earliest computers stored data in Williams tubes - essentially, projecting some dots on a TV screen and reading them again, or mercury delay lines where the data was stored as sound pulses 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 is a general term for the devices by which a computer is sent information from the outside world, including instructions on what it is to do, and how it sends back the results of its computations; these can either be for the purpose of viewing by people, or perhaps for the purposes of controlling other machines; in a robot, for instance, the controlling computer's major output device is the robot itself.

The first generation of computers were typically equipped with a fairly limited range of input devices; a punch card reader or something similar was used to input instructions and data into the computers memory, and some kind of printer, usually a modified teletype, was used to record the results. Over the years, though, a huge variety of other devices have been added. For the personal computer, for instance, Keyboards, and mice, are the primary ways people directly enter information into the computer, and monitors are a major way information from the computer is presented back to the computer user, though printers and some kind of sound-generating device are also very commonly used. There are 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. Two of the most prominent classes of I/O device are secondary storage devices such as hard disks, CD-ROMs, key drives and the like; these represent comparatively slow, but high-capacity devices where information can be stored for later retrieval. Second is devices to access computer networks; the ability to transfer data between computers has opened up a huge range of capabilities for the computer. Collectively, the global Internet lets millions of computers transfer information of all types between each other.

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

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

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

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

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

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

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

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

Computer applications

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

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

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

Networking and the Internet
In the 1970s, computer engineers at research institutions throughout the US began to link their computers together using telecommunications technology. This effort was funded by ARPA, and the computer network that it produced was called the ARPANET. The technologies that made the Arpanet possible spread and evolved. In time, the network spread beyond academic 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". That is, 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 email and the World Wide Web, combined with the development of cheap, fast networking technologies like Ethernet (on a local scale) and ADSL saw computer networking become ubiquitous in the developed world.

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