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    Computer

A computer is a machine

    that manipulates data

    according to a list of

    instructions.

    The first devices that

    resemble modern computers date to the mid-20th century (19401945), although the computer concept and various machines similar to computers existed earlier. Early electronic computers were the size of a large room, consuming as much power as several hundred modern personal computers (PC). Modern computers are

    based on tiny integrated circuits and are millions to billions of times more capable while occupying a fraction of the space. Today, simple computers may be made small enough to fit into a wristwatch and be

    powered from a watch battery. Personal computers, in various forms,

    are icons of the Information Age and are what most people think of as "a computer"; however, the most common form of computer in use today is the embedded computer. Embedded computers are

    small, simple devices that are used to control other devices for

example, they may be found in machines ranging from fighter aircraft

    to industrial robots, digital cameras, and children's toys.

    The ability to store and execute lists of instructions called programs

    makes computers extremely versatile and distinguishes them from calculators. The ChurchTuring thesis is a mathematical statement

    of this versatility: any computer with a certain minimum capability is, in principle, capable of performing the same tasks that any other computer can perform. Therefore, computers with capability and complexity ranging from that of a personal digital assistant to a

    supercomputer are all able to perform the same computational tasks given enough time and storage capacity.

    Contents

    ; 1 History of computing

    ; 2 Stored program architecture

    o 2.1 Programs

    o 2.2 Example

    ; 3 How computers work

    o 3.1 Control unit

    o 3.2 Arithmetic/logic unit (ALU)

    o 3.3 Memory

    o 3.4 Input/output (I/O)

    o 3.5 Multitasking

    o 3.6 Multiprocessing

    o 3.7 Networking and the Internet

    ; 4 Further topics

    o 4.1 Hardware

    o 4.2 Software

    o 4.3 Programming languages

    o 4.4 Professions and organizations

    History of computing

    Main article: History of computer hardware

The Jacquard loom was one of the

    first programmable devices.

    It is difficult to identify any one device as the earliest computer, partly because the term "computer" has been subject to varying

    interpretations over time. Originally, the term "computer" referred to a person who performed numerical

calculations (a human computer), often with the aid of a mechanical

    calculating device.

    The history of the modern computer begins with two separate technologies - that of automated calculation and that of programmability.

    Examples of early mechanical calculating devices included the

    , the slide rule and arguably the astrolabe and the Antikythera abacus

    mechanism (which dates from about 150-100 BC). Hero of Alexandria

    (c. 1070 AD) built a mechanical theater which performed a play lasting 10 minutes and was operated by a complex system of ropes and drums that might be considered to be a means of deciding which parts of the mechanism performed which actions and when. This is the essence of programmability.

    The "castle clock", an astronomical clock invented by Al-Jazari in

    1206, is considered to be the earliest programmable analog computer.

    It displayed the zodiac, the solar and lunar orbits, a crescent

    moon-shaped pointer travelling across a gateway causing automatic

    doors to open every hour, and five robotic musicians who play music

    when struck by levers operated by a camshaft attached to a water

    wheel. The length of day and night could be re-programmed every

    day in order to account for the changing lengths of day and night throughout the year.

    The end of the Middle Ages saw a re-invigoration of European

    mathematics and engineering, and Wilhelm Schickard's 1623 device

    was the first of a number of mechanical calculators constructed by European engineers. However, none of those devices fit the modern definition of a computer because they could not be programmed. In 1801, Joseph Marie Jacquard made an improvement to the textile

    loom that used a series of punched paper cards as a template to

    allow his loom to weave intricate patterns automatically. The resulting Jacquard loom was an important step in the development of computers because the use of punched cards to define woven patterns can be viewed as an early, albeit limited, form of programmability.

    It was the fusion of automatic calculation with programmability that produced the first recognizable computers. In 1837, Charles Babbage

    was the first to conceptualize and design a fully programmable mechanical computer that he called "The Analytical Engine". Due to

    limited finances, and an inability to resist tinkering with the design, Babbage never actually built his Analytical Engine.

    Large-scale automated data processing of punched cards was performed for the U.S. Census in 1890 by tabulating machines

     and manufactured by the Computing designed by Herman Hollerith

    Tabulating Recording Corporation, which later became IBM. By the

    end of the 19th century a number of technologies that would later prove useful in the realization of practical computers had begun to appear: the punched card, Boolean algebra, the vacuum tube

    (thermionic valve) and the teleprinter.

    During the first half of the 20th century, many scientific computing needs were met by increasingly sophisticated analog computers,

    which used a direct mechanical or electrical model of the problem as

    a basis for computation. However, these were not programmable and generally lacked the versatility and accuracy of modern digital computers.

    Defining characteristics of some early digital computers of the 1940s

    (In the history of computing hardware)

    First Numeral Computing Turing

    Name Programming

    operationalsystem mechanism complete

    Zuse Z3 Program-controlled by

    May 1941Binary Electro -mechanical Yes (1998) (Germany) punched film stock

    AtanasoffBerry Not programmablesingle

    1942 BinaryElectronic No

    Computer (US) purpose

    Colossus Mark 1February Program-controlled by patch

    BinaryElectronic No

    (UK) 1944 cables and switches

    Program-controlled by Harvard Mark I 24-channel punched paper

    May 1944Decimal Electro -mechanical No

    IBM ASCC (US) tape (but no conditional

    branch)

    Colossus Mark 2 Program-controlled by patch

    June 1944BinaryElectronic No

    (UK) cables and switches

    Program-controlled by patch ENIAC (US) July 1946DecimalElectronic Yes

    cables and switches

    Manchester June 1948BinaryElectronic Yes Stored-program in Williams Small-Scale

Experimental cathode ray tube memory

    Machine (UK)

    Program-controlled by patch

    cables and switches plus a Modified ENIACSeptember primitive read-only stored

    DecimalElectronic Yes

    (US) 1948 programming mechanism

    using the Function Tables as

    program ROM

    Stored-program in mercury EDSAC (UK)May 1949 Binary Electronic Yes

    delay line memory

    Stored-program in Williams

    Manchester Mark October

    BinaryElectronic cathode ray tube memoryYes

    1 (UK) 1949

    and magnetic drum memory

    CSIRAC November Stored-program in mercury

    BinaryElectronic Yes

    (Australia) 1949 delay line memory

    A succession of steadily more powerful and flexible computing

    devices were constructed in the 1930s and 1940s, gradually adding

    the key features that are seen in modern computers. The use of digital electronics (largely invented by Claude Shannon in 1937) and

    more flexible programmability were vitally important steps, but defining one point along this road as "the first digital electronic

    . Notable achievements computer" is difficult (Shannon 1940)

    include:

    EDSAC was one of the first computers to implement the stored

    program (von Neumann) architecture.

    ; Konrad Zuse's electromechanical "Z machines". The Z3 (1941)

    was the first working machine featuring binary arithmetic,

    including floating point arithmetic and a measure of

    programmability. In 1998 the Z3 was proved to be Turing

    complete, therefore being the world's first operational

    computer.

    ; The non-programmable AtanasoffBerry Computer (1941)

    which used vacuum tube based computation, binary numbers,

    and regenerative capacitor memory.

    ; The secret British Colossus computers (1943), which had

    limited programmability but demonstrated that a device using

    thousands of tubes could be reasonably reliable and

    electronically reprogrammable. It was used for breaking

    German wartime codes.

    ; The Harvard Mark I (1944), a large-scale electromechanical

    computer with limited programmability.

    ; The U.S. Army's Ballistics Research Laboratory ENIAC (1946),

    which used decimal arithmetic and is sometimes called the first

    general purpose electronic computer (since Konrad Zuse's Z3

    of 1941 used electromagnets instead of electronics). Initially,

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