A computer is a device that can be instructed to carry out sequences of arithmetic or logical operations automatically via computer programming. Modern computers have the ability to follow generalized sets of operations, called programs. These programs enable computers to perform an extremely wide range of tasks. A "complete" computer including the hardware, the operating system (main software), and peripheral equipment required and used for "full" operation can be referred to as a computer system. This term may as well be used for a group of computers that are connected and work together, in particular a computer network or computer cluster.
Computers are used as control systems for a wide variety of industrial and consumer devices. This includes simple special purpose devices like microwave ovens and remote controls, factory devices such as industrial robots and computer-aided design, and also general purpose devices like personal computers and mobile devices such as smartphones.
Early computers were only conceived as calculating devices. Since ancient times, simple manual devices like the abacus aided people in doing calculations. Early in the Industrial Revolution, some mechanical devices were built to automate long tedious tasks, such as guiding patterns for looms. More sophisticated electrical machines did specialized analog calculations in the early 20th century. The first digital electronic calculating machines were developed during World War II.  The speed, power, and versatility of computers have been increasing dramatically ever since then.
Conventionally, a modern computer consists of at least one processing element, typically a central processing unit (CPU), and some form of memory. The processing element carries out arithmetic and logical operations, and a sequencing and control unit can change the order of operations in response to stored information. Peripheral devices include input devices (keyboards, mice, joystick, etc.), output devices (monitor screens, printers, etc.), and input/output devices that perform both functions (e.g., the 2000s-era touchscreen). Peripheral devices allow information to be retrieved from an external source and they enable the result of operations to be saved and retrieved.

Etymology

According to the Oxford English Dictionary, the first known use of the word "computer" was in 1613 in a book called The Yong Mans Gleanings by English writer Richard Braithwait: "I haue [sic] read the truest computer of Times, and the best Arithmetician that euer [sic] breathed, and he reduceth thy dayes into a short number." This usage of the term referred to a human computer, a person who carried out calculations or computations. The word continued with the same meaning until the middle of the 20th century. Originally, women were often hired as "human computers" because they could be paid less than their male counterparts.[1] By 1943, most human computers were women.[2] From the end of the 19th century the word began to take on its more familiar meaning, a machine that carries out computations.[3]
The Online Etymology Dictionary gives the first attested use of "computer" in the "1640s, [meaning] "one who calculates,"; this is an "...agent noun from compute (v.)". The Online Etymology Dictionary states that the use of the term to mean "calculating machine" (of any type) is from 1897."  The Online Etymology Dictionary indicates that the "modern use" of the term, to mean "programmable digital electronic computer" dates from "...1945 under this name; [in a] theoretical [sense] from 1937, as Turing machine".[4]
History

Pre-20th century

Devices have been used to aid computation for thousands of years, mostly using one-to-one correspondence with fingers. The earliest counting device was probably a form of tally stick. Later record keeping aids throughout the Fertile Crescent included calculi (clay spheres, cones, etc.) which represented counts of items, probably livestock or grains, sealed in hollow unbaked clay containers.[5][6] The use of counting rods is one example.

The abacus was initially used for arithmetic tasks. The Roman abacus was developed from devices used in Babylonia as early as 2400 BC. Since then, many other forms of reckoning boards or tables have been invented. In a medieval European counting house, a checkered cloth would be placed on a table, and markers moved around on it according to certain rules, as an aid to calculating sums of money.

The Antikythera mechanism is believed to be the earliest mechanical analog "computer", according to Derek J. de Solla Price.[7] It was designed to calculate astronomical positions. It was discovered in 1901 in the Antikythera wreck off the Greek island of Antikythera, between Kythera and Crete, and has been dated to c.100 BC. Devices of a level of complexity comparable to that of the Antikythera mechanism would not reappear until a thousand years later.
Many mechanical aids to calculation and measurement were constructed for astronomical and navigation use. The planisphere was a star chart invented by Abū Rayhān al-Bīrūnī in the early 11th century.[8] The astrolabe was invented in the Hellenistic world in either the 1st or 2nd centuries BC and is often attributed to Hipparchus. A combination of the planisphere and dioptra, the astrolabe was effectively an analog computer capable of working out several different kinds of problems in spherical astronomy. An astrolabe incorporating a mechanical calendar computer[9][10] and gear-wheels was invented by Abi Bakr of Isfahan, Persia in 1235.[11] Abū Rayhān al-Bīrūnī invented the first mechanical geared lunisolar calendar astrolabe,[12] an early fixed-wired knowledge processing machine[13] with a gear train and gear-wheels,[14] c.1000 AD.
The sector, a calculating instrument used for solving problems in proportion, trigonometry, multiplication and division, and for various functions, such as squares and cube roots, was developed in the late 16th century and found application in gunnery, surveying and navigation.
The planimeter was a manual instrument to calculate the area of a closed figure by tracing over it with a mechanical linkage.

The slide rule was invented around 1620–1630, shortly after the publication of the concept of the logarithm. It is a hand-operated analog computer for doing multiplication and division. As slide rule development progressed, added scales provided reciprocals, squares and square roots, cubes and cube roots, as well as transcendental functions such as logarithms and exponentials, circular and hyperbolic trigonometry and other functions. Slide rules with special scales are still used for quick performance of routine calculations, such as the E6B circular slide rule used for time and distance calculations on light aircraft.
In the 1770s, Pierre Jaquet-Droz, a Swiss watchmaker, built a mechanical doll (automaton) that could write holding a quill pen. By switching the number and order of its internal wheels different letters, and hence different messages, could be produced. In effect, it could be mechanically "programmed" to read instructions. Along with two other complex machines, the doll is at the Musée d'Art et d'Histoire of Neuchâtel, Switzerland, and still operates.[15]
The tide-predicting machine invented by Sir William Thomson in 1872 was of great utility to navigation in shallow waters. It used a system of pulleys and wires to automatically calculate predicted tide levels for a set period at a particular location.
The differential analyser, a mechanical analog computer designed to solve differential equations by integration, used wheel-and-disc mechanisms to perform the integration. In 1876, Lord Kelvin had already discussed the possible construction of such calculators, but he had been stymied by the limited output torque of the ball-and-disk integrators.[16] In a differential analyzer, the output of one integrator drove the input of the next integrator, or a graphing output. The torque amplifier was the advance that allowed these machines to work. Starting in the 1920s, Vannevar Bush and others developed mechanical differential analyzers.
First computing device

Charles Babbage, an English mechanical engineer and polymath, originated the concept of a programmable computer. Considered the "father of the computer",[17] he conceptualized and invented the first mechanical computer in the early 19th century. After working on his revolutionary difference engine, designed to aid in navigational calculations, in 1833 he realized that a much more general design, an Analytical Engine, was possible. The input of programs and data was to be provided to the machine via punched cards, a method being used at the time to direct mechanical looms such as the Jacquard loom. For output, the machine would have a printer, a curve plotter and a bell. The machine would also be able to punch numbers onto cards to be read in later. The Engine incorporated an arithmetic logic unit, control flow in the form of conditional branching and loops, and integrated memory, making it the first design for a general-purpose computer that could be described in modern terms as Turing-complete.[18][19]
The machine was about a century ahead of its time. All the parts for his machine had to be made by hand– this was a major problem for a device with thousands of parts. Eventually, the project was dissolved with the decision of the British Government to cease funding. Babbage's failure to complete the analytical engine can be chiefly attributed to difficulties not only of politics and financing, but also to his desire to develop an increasingly sophisticated computer and to move ahead faster than anyone else could follow. Nevertheless, his son, Henry Babbage, completed a simplified version of the analytical engine's computing unit (the mill) in 1888. He gave a successful demonstration of its use in computing tables in 1906.
Analog computers

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.[20] The first modern analog computer was a tide-predicting machine, invented by Sir William Thomson in 1872. The differential analyser, a mechanical analog computer designed to solve differential equations by integration using wheel-and-disc mechanisms, was conceptualized in 1876 by James Thomson, the brother of the more famous Lord Kelvin.[16]
The art of mechanical analog computing reached its zenith with the differential analyzer, built by H. L. Hazen and Vannevar Bush at MIT starting in 1927. This built on the mechanical integrators of James Thomson and the torque amplifiers invented by H. W. Nieman. A dozen of these devices were built before their obsolescence became obvious. By the 1950s, the success of digital electronic computers had spelled the end for most analog computing machines, but analog computers remained in use during the 1950s in some specialized applications such as education (control systems) and aircraft (slide rule).
Digital computers

Electromechanical
By 1938, the United States Navy had developed an electromechanical analog computer small enough to use aboard a submarine. This was the Torpedo Data Computer, which used trigonometry to solve the problem of firing a torpedo at a moving target. During World War II similar devices were developed in other countries as well.

Early digital computers were electromechanical; electric switches drove mechanical relays to perform the calculation. These devices had a low operating speed and were eventually superseded by much faster all-electric computers, originally using vacuum tubes. The Z2, created by German engineer Konrad Zuse in 1939, was one of the earliest examples of an electromechanical relay computer.[21]
In 1941, Zuse followed his earlier machine up with the Z3, the world's first working electromechanical programmable, fully automatic digital computer.[22][23] The Z3 was built with 2000 relays, implementing a 22bit word length that operated at a clock frequency of about 5–10Hz.[24] Program code was supplied on punched film while data could be stored in 64 words of memory or supplied from the keyboard. It was quite similar to modern machines in some respects, pioneering numerous advances such as floating point numbers. Rather than the harder-to-implement decimal system (used in Charles Babbage's earlier design), using a binary system meant that Zuse's machines were easier to build and potentially more reliable, given the technologies available at that time.[25] The Z3 was Turing complete.[26][27]
Vacuum tubes and digital electronic circuits

Purely electronic circuit elements soon replaced their mechanical and electromechanical equivalents, at the same time that digital calculation replaced analog. The engineer Tommy Flowers, working at the Post Office Research Station in London in the 1930s, began to explore the possible use of electronics for the telephone exchange. Experimental equipment that he built in 1934 went into operation five years later, converting a portion of the telephone exchange network into an electronic data processing system, using thousands of vacuum tubes.[20] In the US, John Vincent Atanasoff and Clifford E. Berry of Iowa State University developed and tested the Atanasoff–Berry Computer (ABC) in 1942,[28] the first "automatic electronic digital computer".[29] This design was also all-electronic and used about 300 vacuum tubes, with capacitors fixed in a mechanically rotating drum for memory.[30]

During World War II, the British at Bletchley Park achieved a number of successes at breaking encrypted German military communications. The German encryption machine, Enigma, was first attacked with the help of the electro-mechanical bombes which were often run by women.[31][32] To crack the more sophisticated German Lorenz SZ 40/42 machine, used for high-level Army communications, Max Newman and his colleagues commissioned Flowers to build the Colossus.[30] He spent eleven months from early February 1943 designing and building the first Colossus.[33] After a functional test in December 1943, Colossus was shipped to Bletchley Park, where it was delivered on 18 January 1944[34] and attacked its first message on 5 February.[30]
Colossus was the world's first electronic digital programmable computer.[20] It used a large number of valves (vacuum tubes). It had paper-tape input and was capable of being configured to perform a variety of boolean logical operations on its data, but it was not Turing-complete. Nine Mk II Colossi were built (The Mk I was converted to a Mk II making ten machines in total). Colossus Mark I contained 1,500 thermionic valves (tubes), but Mark II with 2,400 valves, was both 5 times faster and simpler to operate than Mark I, greatly speeding the decoding process.[35][36]

The U.S.-built ENIAC[37] (Electronic Numerical Integrator and Computer) was the first electronic programmable computer built in the US. Although the ENIAC was similar to the Colossus, it was much faster, more flexible, and it was Turing-complete. Like the Colossus, a "program" on the ENIAC was defined by the states of its patch cables and switches, a far cry from the stored program electronic machines that came later. Once a program was written, it had to be mechanically set into the machine with manual resetting of plugs and switches. The programmers of the ENIAC were six women, often known collectively as the "ENIAC girls".[38][39]
It combined the high speed of electronics with the ability to be programmed for many complex problems. It could add or subtract 5000 times a second, a thousand times faster than any other machine. It also had modules to multiply, divide, and square root. High speed memory was limited to 20 words (about 80 bytes). Built under the direction of John Mauchly and J. Presper Eckert at the University of Pennsylvania, ENIAC's development and construction lasted from 1943 to full operation at the end of 1945. The machine was huge, weighing 30 tons, using 200 kilowatts of electric power and contained over 18,000 vacuum tubes, 1,500 relays, and hundreds of thousands of resistors, capacitors, and inductors.[40]
Modern computers
Concept of modern computer
The principle of the modern computer was proposed by Alan Turing in his seminal 1936 paper,[41] On Computable Numbers. Turing proposed a simple device that he called "Universal Computing machine" and that is now known as a universal Turing machine. He proved that such a machine is capable of computing anything that is computable by executing instructions (program) stored on tape, allowing the machine to be programmable. The fundamental concept of Turing's design is the stored program, where all the instructions for computing are stored in memory. Von Neumann acknowledged that the central concept of the modern computer was due to this paper.[42] Turing machines are to this day a central object of study in theory of computation. Except for the limitations imposed by their finite memory stores, modern computers are said to be Turing-complete, which is to say, they have algorithm execution capability equivalent to a universal Turing machine.
Stored programs

Early computing machines had fixed programs. Changing its function required the re-wiring and re-structuring of the machine.[30] With the proposal of the stored-program computer this changed. A stored-program computer includes by design an instruction set and can store in memory a set of instructions (a program) that details the computation. The theoretical basis for the stored-program computer was laid by Alan Turing in his 1936 paper. In 1945, Turing joined the National Physical Laboratory and began work on developing an electronic stored-program digital computer. His 1945 report "Proposed Electronic Calculator" was the first specification for such a device. John von Neumann at the University of Pennsylvania also circulated his First Draft of a Report on the EDVAC in 1945.[20]
The Manchester Baby was the world's first stored-program computer. It was built at the Victoria University of Manchester by Frederic C. Williams, Tom Kilburn and Geoff Tootill, and ran its first program on 21 June 1948.[43] It was designed as a testbed for the Williams tube, the first random-access digital storage device.[44] Although the computer was considered "small and primitive" by the standards of its time, it was the first working machine to contain all of the elements essential to a modern electronic computer.[45] As soon as the Baby had demonstrated the feasibility of its design, a project was initiated at the university to develop it into a more usable computer, the Manchester Mark 1. Grace Hopper was the first person to develop a compiler for programming language.[2]
The Mark 1 in turn quickly became the prototype for the Ferranti Mark 1, the world's first commercially available general-purpose computer.[46] Built by Ferranti, it was delivered to the University of Manchester in February 1951. At least seven of these later machines were delivered between 1953 and 1957, one of them to Shell labs in Amsterdam.[47] In October 1947, the directors of British catering company J. Lyons & Company decided to take an active role in promoting the commercial development of computers. The LEO I computer became operational in April 1951[48] and ran the world's first regular routine office computer job.
Transistors

The bipolar transistor was invented in 1947. From 1955 onwards transistors replaced vacuum tubes in computer designs, giving rise to the "second generation" of computers.
Compared to vacuum tubes, transistors have many advantages: they are smaller, and require less power than vacuum tubes, so give off less heat. Silicon junction transistors were much more reliable than vacuum tubes and had longer, indefinite, service life. Transistorized computers could contain tens of thousands of binary logic circuits in a relatively compact space.
At the University of Manchester, a team under the leadership of Tom Kilburn designed and built a machine using the newly developed transistors instead of valves.[49] Their first transistorised computer and the first in the world, was operational by 1953, and a second version was completed there in April 1955. However, the machine did make use of valves to generate its 125kHz clock waveforms and in the circuitry to read and write on its magnetic drum memory, so it was not the first completely transistorized computer. That distinction goes to the Harwell CADET of 1955,[50] built by the electronics division of the Atomic Energy Research Establishment at Harwell.[50][51]
Integrated circuits
The next great advance in computing power came with the advent of the integrated circuit.
The idea of the integrated circuit was first conceived by a radar scientist working for the Royal Radar Establishment of the Ministry of Defence, Geoffrey W.A. Dummer. Dummer presented the first public description of an integrated circuit at the Symposium on Progress in Quality Electronic Components in Washington,D.C. on 7 May 1952.[52]
The first practical ICs were invented by Jack Kilby at Texas Instruments and Robert Noyce at Fairchild Semiconductor.[53] Kilby recorded his initial ideas concerning the integrated circuit in July 1958, successfully demonstrating the first working integrated example on 12 September 1958.[54] In his patent application of 6 February 1959, Kilby described his new device as "a body of semiconductor material... wherein all the components of the electronic circuit are completely integrated".[55][56] Noyce also came up with his own idea of an integrated circuit half a year later than Kilby.[57] His chip solved many practical problems that Kilby's had not. Produced at Fairchild Semiconductor, it was made of silicon, whereas Kilby's chip was made of germanium.
This new development heralded an explosion in the commercial and personal use of computers and led to the invention of the microprocessor. While the subject of exactly which device was the first microprocessor is contentious, partly due to lack of agreement on the exact definition of the term "microprocessor", it is largely undisputed that the first single-chip microprocessor was the Intel 4004,[58] designed and realized by Ted Hoff, Federico Faggin, and Stanley Mazor at Intel.[59]
Mobile computers
The first mobile computers were heavy and ran from mains power. The 50lb IBM 5100 was an early example. Later portables such as the Osborne 1 and Compaq Portable were considerably lighter, but still needed to be plugged in. The first laptops, such as the Grid Compass, removed this requirement by incorporating batteries – and with the continued miniaturization of computing resources and advancements in portable battery life, portable computers grew in popularity in the 2000s.[60] The same developments allowed manufacturers to integrate computing resources into cellular phones.
These smartphones and tablets run on a variety of operating systems and soon became the dominant computing device on the market, with manufacturers reporting having shipped an estimated 237 million devices in 2Q 2013.[61]
Types
Computers are typically classified based on their uses:
Based on uses
Analog computer
Digital computer
Hybrid computer
Based on sizes
Smartphone
Microcomputer
Workstation
Personal computer
Laptop
Minicomputer
Mainframe computer
Supercomputer
Tablet computer
Hardware

The term hardware covers all of those parts of a computer that are tangible physical objects. Circuits, computer chips, graphic cards, sound cards, memory (RAM), motherboard, displays, power supplies, cables, keyboards, printers and "mice" input devices are all hardware.
History of computing hardware

Other hardware topics

A general purpose computer has four main components: the arithmetic logic unit (ALU), the control unit, the memory, and the input and output devices (collectively termed I/O). These parts are interconnected by buses, often made of groups of wires.
Inside each of these parts are thousands to trillions of small electrical circuits which can be turned off or on by means of an electronic switch. Each circuit represents a bit (binary digit) of information so that when the circuit is on it represents a "1", and when off it represents a "0" (in positive logic representation). The circuits are arranged in logic gates so that one or more of the circuits may control the state of one or more of the other circuits.
Input devices
When unprocessed data is sent to the computer with the help of input devices, the data is processed and sent to output devices. The input devices may be hand-operated or automated. The act of processing is mainly regulated by the CPU. Some examples of input devices are:
Computer keyboard
Digital camera
Digital video
Graphics tablet
Image scanner
Joystick
Microphone
Mouse
Overlay keyboard
Real-time clock
Trackball
Touchscreen
Output devices
The means through which computer gives output are known as output devices. Some examples of output devices are:
Computer monitor
Printer
PC speaker
Projector
Sound card
Video card
Control unit

The control unit (often called a control system or central controller) manages the computer's various components; it reads and interprets (decodes) the program instructions, transforming them into control signals that activate other parts of the computer.[63] Control systems in advanced computers may change the order of execution of some instructions to improve performance.
A key component common to all CPUs is the program counter, a special memory cell (a register) that keeps track of which location in memory the next instruction is to be read from.[64]
The control system's function is as follows—note that this is a simplified description, and some of these steps may be performed concurrently or in a different order depending on the type of CPU:
Read the code for the next instruction from the cell indicated by the program counter.
Decode the numerical code for the instruction into a set of commands or signals for each of the other systems.
Increment the program counter so it points to the next instruction.
Read whatever data the instruction requires from cells in memory (or perhaps from an input device). The location of this required data is typically stored within the instruction code.
Provide the necessary data to an ALU or register.
If the instruction requires an ALU or specialized hardware to complete, instruct the hardware to perform the requested operation.
Write the result from the ALU back to a memory location or to a register or perhaps an output device.
Jump back to step (1).
Since the program counter is (conceptually) just another set of memory cells, it can be changed by calculations done in the ALU. Adding 100 to the program counter would cause the next instruction to be read from a place 100 locations further down the program. Instructions that modify the program counter are often known as "jumps" and allow for loops (instructions that are repeated by the computer) and often conditional instruction execution (both examples of control flow).
The sequence of operations that the control unit goes through to process an instruction is in itself like a short computer program, and indeed, in some more complex CPU designs, there is another yet smaller computer called a microsequencer, which runs a microcode program that causes all of these events to happen.
Central processing unit (CPU)
The control unit, ALU, and registers are collectively known as a central processing unit (CPU). Early CPUs were composed of many separate components but since the mid-1970s CPUs have typically been constructed on a single integrated circuit called a microprocessor.
Arithmetic logic unit (ALU)

The ALU is capable of performing two classes of operations: arithmetic and logic.[65] The set of arithmetic operations that a particular ALU supports may be limited to addition and subtraction, or might include multiplication, division, trigonometry functions such as sine, cosine, etc., and square roots. Some can only operate on whole numbers (integers) while others use floating point to represent real numbers, albeit with limited precision. However, any computer that is capable of performing just the simplest operations can be programmed to break down the more complex operations into simple steps that it can perform. Therefore, any computer can be programmed to perform any arithmetic operation—although it will take more time to do so if its ALU does not directly support the operation. An ALU may also compare numbers and return boolean truth values (true or false) depending on whether one is equal to, greater than or less than the other ("is 64 greater than 65?"). Logic operations involve Boolean logic: AND, OR, XOR, and NOT. These can be useful for creating complicated conditional statements and processing boolean logic.
Superscalar computers may contain multiple ALUs, allowing them to process several instructions simultaneously.[66] Graphics processors and computers with SIMD and MIMD features often contain ALUs that can perform arithmetic on vectors and matrices.
Memory

A computer's memory can be viewed as a list of cells into which numbers can be placed or read. Each cell has a numbered "address" and can store a single number. The computer can be instructed to "put the number 123 into the cell numbered 1357" or to "add the number that is in cell 1357 to the number that is in cell 2468 and put the answer into cell 1595." The information stored in memory may represent practically anything. Letters, numbers, even computer instructions can be placed into memory with equal ease. Since the CPU does not differentiate between different types of information, it is the software's responsibility to give significance to what the memory sees as nothing but a series of numbers.
In almost all modern computers, each memory cell is set up to store binary numbers in groups of eight bits (called a byte). Each byte is able to represent 256 different numbers (28 = 256); either from 0 to 255 or −128 to +127. To store larger numbers, several consecutive bytes may be used (typically, two, four or eight). When negative numbers are required, they are usually stored in two's complement notation. Other arrangements are possible, but are usually not seen outside of specialized applications or historical contexts. A computer can store any kind of information in memory if it can be represented numerically. Modern computers have billions or even trillions of bytes of memory.
The CPU contains a special set of memory cells called registers that can be read and written to much more rapidly than the main memory area. There are typically between two and one hundred registers depending on the type of CPU. Registers are used for the most frequently needed data items to avoid having to access main memory every time data is needed. As data is constantly being worked on, reducing the need to access main memory (which is often slow compared to the ALU and control units) greatly increases the computer's speed.
Computer main memory comes in two principal varieties:
random-access memory or RAM
read-only memory or ROM
RAM can be read and written to anytime the CPU commands it, but ROM is preloaded with data and software that never changes, therefore the CPU can only read from it. ROM is typically used to store the computer's initial start-up instructions. In general, the contents of RAM are erased when the power to the computer is turned off, but ROM retains its data indefinitely. In a PC, the ROM contains a specialized program called the BIOS that orchestrates loading the computer's operating system from the hard disk drive into RAM whenever the computer is turned on or reset. In embedded computers, which frequently do not have disk drives, all of the required software may be stored in ROM. Software stored in ROM is often called firmware, because it is notionally more like hardware than software. Flash memory blurs the distinction between ROM and RAM, as it retains its data when turned off but is also rewritable. It is typically much slower than conventional ROM and RAM however, so its use is restricted to applications where high speed is unnecessary.[67]
In more sophisticated computers there may be one or more RAM cache memories, which are slower than registers but faster than main memory. Generally computers with this sort of cache are designed to move frequently needed data into the cache automatically, often without the need for any intervention on the programmer's part.
Input/output (I/O)

I/O is the means by which a computer exchanges information with the outside world.[68] Devices that provide input or output to the computer are called peripherals.[69] On a typical personal computer, peripherals include input devices like the keyboard and mouse, and output devices such as the display and printer. Hard disk drives, floppy disk drives and optical disc drives serve as both input and output devices. Computer networking is another form of I/O.
I/O devices are often complex computers in their own right, with their own CPU and memory. A graphics processing unit might contain fifty or more tiny computers that perform the calculations necessary to display 3D graphics. Modern desktop computers contain many smaller computers that assist the main CPU in performing I/O. A 2016-era flat screen display contains its own computer circuitry.
Multitasking

While a computer may be viewed as running one gigantic program stored in its main memory, in some systems it is necessary to give the appearance of running several programs simultaneously. This is achieved by multitasking i.e. having the computer switch rapidly between running each program in turn.[70] One means by which this is done is with a special signal called an interrupt, which can periodically cause the computer to stop executing instructions where it was and do something else instead. By remembering where it was executing prior to the interrupt, the computer can return to that task later. If several programs are running "at the same time". then the interrupt generator might be causing several hundred interrupts per second, causing a program switch each time. Since modern computers typically execute instructions several orders of magnitude faster than human perception, it may appear that many programs are running at the same time even though only one is ever executing in any given instant. This method of multitasking is sometimes termed "time-sharing" since each program is allocated a "slice" of time in turn.[71]
Before the era of inexpensive computers, the principal use for multitasking was to allow many people to share the same computer. Seemingly, multitasking would cause a computer that is switching between several programs to run more slowly, in direct proportion to the number of programs it is running, but most programs spend much of their time waiting for slow input/output devices to complete their tasks. If a program is waiting for the user to click on the mouse or press a key on the keyboard, then it will not take a "time slice" until the event it is waiting for has occurred. This frees up time for other programs to execute so that many programs may be run simultaneously without unacceptable speed loss.
Multiprocessing

Some computers are designed to distribute their work across several CPUs in a multiprocessing configuration, a technique once employed only in large and powerful machines such as supercomputers, mainframe computers and servers. Multiprocessor and multi-core (multiple CPUs on a single integrated circuit) personal and laptop computers are now widely available, and are being increasingly used in lower-end markets as a result.
Supercomputers in particular often have highly unique architectures that differ significantly from the basic stored-program architecture and from general purpose computers.[72] They often feature thousands of CPUs, customized high-speed interconnects, and specialized computing hardware. Such designs tend to be useful only for specialized tasks due to the large scale of program organization required to successfully utilize most of the available resources at once. Supercomputers usually see usage in large-scale simulation, graphics rendering, and cryptography applications, as well as with other so-called "embarrassingly parallel" tasks.
Software

Software refers to parts of the computer which do not have a material form, such as programs, data, protocols, etc. Software is that part of a computer system that consists of encoded information or computer instructions, in contrast to the physical hardware from which the system is built. Computer software includes computer programs, libraries and related non-executable data, such as online documentation or digital media. It is often divided into system software and application software]] Computer hardware and software require each other and neither can be realistically used on its own. When software is stored in hardware that cannot easily be modified, such as with BIOS ROM in an IBM PC compatible computer, it is sometimes called "firmware".

Languages
There are thousands of different programming languages—some intended to be general purpose, others useful only for highly specialized applications.

Programs
The defining feature of modern computers which distinguishes them from all other machines is that they can be programmed. That is to say that some type of instructions (the program) can be given to the computer, and it will process them. Modern computers based on the von Neumann architecture often have machine code in the form of an imperative programming language. In practical terms, a computer program may be just a few instructions or extend to many millions of instructions, as do the programs for word processors and web browsers for example. A typical modern computer can execute billions of instructions per second (gigaflops) and rarely makes a mistake over many years of operation. Large computer programs consisting of several million instructions may take teams of programmers years to write, and due to the complexity of the task almost certainly contain errors.
Stored program architecture

This section applies to most common RAM machine–based computers.
In most cases, computer instructions are simple: add one number to another, move some data from one location to another, send a message to some external device, etc. These instructions are read from the computer's memory and are generally carried out (executed) in the order they were given. However, there are usually specialized instructions to tell the computer to jump ahead or backwards to some other place in the program and to carry on executing from there. These are called "jump" instructions (or branches). Furthermore, jump instructions may be made to happen conditionally so that different sequences of instructions may be used depending on the result of some previous calculation or some external event. Many computers directly support subroutines by providing a type of jump that "remembers" the location it jumped from and another instruction to return to the instruction following that jump instruction.
Program execution might be likened to reading a book. While a person will normally read each word and line in sequence, they may at times jump back to an earlier place in the text or skip sections that are not of interest. Similarly, a computer may sometimes go back and repeat the instructions in some section of the program over and over again until some internal condition is met. This is called the flow of control within the program and it is what allows the computer to perform tasks repeatedly without human intervention.
Comparatively, a person using a pocket calculator can perform a basic arithmetic operation such as adding two numbers with just a few button presses. But to add together all of the numbers from 1 to 1,000 would take thousands of button presses and a lot of time, with a near certainty of making a mistake. On the other hand, a computer may be programmed to do this with just a few simple instructions. The following example is written in the MIPS assembly language:

  begin:
  addi $8, $0, 0           # initialize sum to 0
  addi $9, $0, 1           # set first number to add = 1
  loop:
  slti $10, $9, 1000       # check if the number is less than 1000
  beq $10, $0, finish      # if odd number is greater than n then exit
  add $8, $8, $9           # update sum
  addi $9, $9, 1           # get next number
  j loop                   # repeat the summing process
  finish:
  add $2, $8, $0           # put sum in output register

Once told to run this program, the computer will perform the repetitive addition task without further human intervention. It will almost never make a mistake and a modern PC can complete the task in a fraction of a second.
Machine code
In most computers, individual instructions are stored as machine code with each instruction being given a unique number (its operation code or opcode for short). The command to add two numbers together would have one opcode; the command to multiply them would have a different opcode, and so on. The simplest computers are able to perform any of a handful of different instructions; the more complex computers have several hundred to choose from, each with a unique numerical code. Since the computer's memory is able to store numbers, it can also store the instruction codes. This leads to the important fact that entire programs (which are just lists of these instructions) can be represented as lists of numbers and can themselves be manipulated inside the computer in the same way as numeric data. The fundamental concept of storing programs in the computer's memory alongside the data they operate on is the crux of the von Neumann, or stored program, architecture. In some cases, a computer might store some or all of its program in memory that is kept separate from the data it operates on. This is called the Harvard architecture after the Harvard Mark I computer. Modern von Neumann computers display some traits of the Harvard architecture in their designs, such as in CPU caches.
While it is possible to write computer programs as long lists of numbers (machine language) and while this technique was used with many early computers,[73] it is extremely tedious and potentially error-prone to do so in practice, especially for complicated programs. Instead, each basic instruction can be given a short name that is indicative of its function and easy to remember– a mnemonic such as ADD, SUB, MULT or JUMP. These mnemonics are collectively known as a computer's assembly language. Converting programs written in assembly language into something the computer can actually understand (machine language) is usually done by a computer program called an assembler.

Programming language

Programming languages provide various ways of specifying programs for computers to run. Unlike natural languages, programming languages are designed to permit no ambiguity and to be concise. They are purely written languages and are often difficult to read aloud. They are generally either translated into machine code by a compiler or an assembler before being run, or translated directly at run time by an interpreter. Sometimes programs are executed by a hybrid method of the two techniques.
Low-level languages

Machine languages and the assembly languages that represent them (collectively termed low-level programming languages) tend to be unique to a particular type of computer. For instance, an ARM architecture computer (such as may be found in a smartphone or a hand-held videogame) cannot understand the machine language of an x86 CPU that might be in a PC.[74]
High-level languages

Although considerably easier than in machine language, writing long programs in assembly language is often difficult and is also error prone. Therefore, most practical programs are written in more abstract high-level programming languages that are able to express the needs of the programmer more conveniently (and thereby help reduce programmer error). High level languages are usually "compiled" into machine language (or sometimes into assembly language and then into machine language) using another computer program called a compiler.[75] High level languages are less related to the workings of the target computer than assembly language, and more related to the language and structure of the problem(s) to be solved by the final program. It is therefore often possible to use different compilers to translate the same high level language program into the machine language of many different types of computer. This is part of the means by which software like video games may be made available for different computer architectures such as personal computers and various video game consoles.
Program design

Program design of small programs is relatively simple and involves the analysis of the problem, collection of inputs, using the programming constructs within languages, devising or using established procedures and algorithms, providing data for output devices and solutions to the problem as applicable. As problems become larger and more complex, features such as subprograms, modules, formal documentation, and new paradigms such as object-oriented programming are encountered. Large programs involving thousands of line of code and more require formal software methodologies.
The task of developing large software systems presents a significant intellectual challenge. Producing software with an acceptably high reliability within a predictable schedule and budget has historically been difficult; the academic and professional discipline of software engineering concentrates specifically on this challenge.
Bugs

Errors in computer programs are called "bugs". They may be benign and not affect the usefulness of the program, or have only subtle effects. But in some cases, they may cause the program or the entire system to "hang", becoming unresponsive to input such as mouse clicks or keystrokes, to completely fail, or to crash. Otherwise benign bugs may sometimes be harnessed for malicious intent by an unscrupulous user writing an exploit, code designed to take advantage of a bug and disrupt a computer's proper execution. Bugs are usually not the fault of the computer. Since computers merely execute the instructions they are given, bugs are nearly always the result of programmer error or an oversight made in the program's design.[76]
Admiral Grace Hopper, an American computer scientist and developer of the first compiler, is credited for having first used the term "bugs" in computing after a dead moth was found shorting a relay in the Harvard Mark II computer in September 1947.[77]
Networking and the Internet

Computers have been used to coordinate information between multiple locations since the 1950s. The U.S. military's SAGE system was the first large-scale example of such a system, which led to a number of special-purpose commercial systems such as Sabre.[78] In the 1970s, computer engineers at research institutions throughout the United States began to link their computers together using telecommunications technology. The effort was funded by ARPA (now DARPA), and the computer network that resulted was called the ARPANET.[79] 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. 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 almost ubiquitous. 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. "Wireless" networking, often utilizing mobile phone networks, has meant networking is becoming increasingly ubiquitous even in mobile computing environments.

Unconventional computers

A computer does not need to be electronic, nor even have a processor, nor RAM, nor even a hard disk. While popular usage of the word "computer" is synonymous with a personal electronic computer, the modern[80] definition of a computer is literally: "A device that computes, especially a programmable [usually] electronic machine that performs high-speed mathematical or logical operations or that assembles, stores, correlates, or otherwise processes information."[81] Any device which processes information qualifies as a computer, especially if the processing is purposeful.
Future
There is active research to make computers out of many promising new types of technology, such as optical computers, DNA computers, neural computers, and quantum computers. Most computers are universal, and are able to calculate any computable function, and are limited only by their memory capacity and operating speed. However different designs of computers can give very different performance for particular problems; for example quantum computers can potentially break some modern encryption algorithms (by quantum factoring) very quickly.
Computer architecture paradigms
There are many types of computer architectures:
Quantum computer vs. Chemical computer
Scalar processor vs. Vector processor
Non-Uniform Memory Access (NUMA) computers
Register machine vs. Stack machine
Harvard architecture vs. von Neumann architecture
Cellular architecture
Of all these abstract machines, a quantum computer holds the most promise for revolutionizing computing.[82] Logic gates are a common abstraction which can apply to most of the above digital or analog paradigms. The ability to store and execute lists of instructions called programs makes computers extremely versatile, distinguishing them from calculators. The Church–Turing thesis is a mathematical statement of this versatility: any computer with a minimum capability (being Turing-complete) is, in principle, capable of performing the same tasks that any other computer can perform. Therefore, any type of computer (netbook, supercomputer, cellular automaton, etc.) is able to perform the same computational tasks, given enough time and storage capacity.
Artificial intelligence
A computer will solve problems in exactly the way it is programmed to, without regard to efficiency, alternative solutions, possible shortcuts, or possible errors in the code. Computer programs that learn and adapt are part of the emerging field of artificial intelligence and machine learning. Artificial intelligence based products generally fall into two major categories: rule based systems and pattern recognition systems. Rule based systems attempt to represent the rules used by human experts and tend to be expensive to develop. Pattern based systems use data about a problem to generate conclusions. Examples of pattern based systems include voice recognition, font recognition, translation and the emerging field of on-line marketing.
Professions and organizations
As the use of computers has spread throughout society, there are an increasing number of careers involving computers.

The need for computers to work well together and to be able to exchange information has spawned the need for many standards organizations, clubs and societies of both a formal and informal nature.

See also

Glossary of computers
Computability theory
Computer insecurity
Computer security
Glossary of computer hardware terms
History of computer science
List of computer term etymologies
List of fictional computers
List of pioneers in computer science
Pulse computation
TOP500 (list of most powerful computers)
Unconventional computing

Notes

 Cite uses deprecated parameter |registration= (help)

 Cite uses deprecated parameter |subscription= (help)

 Media related to Computers at Wikimedia Commons
Wikiversity has a quiz on this article


The European jackal (Canis aureus moreoticus), also known as the Caucasian jackal[2] or reed wolf[3] is a subspecies of golden jackal native to Southeast Europe, Asia Minor and the Caucasus. It was first described in 1833 by French naturalist Isidore Geoffroy Saint-Hilaire during the Morea expedition.[4] In Europe, there are an estimated 70,000 jackals.[5] Though mostly found in scattered populations within Eastern Europe, its range has grown to encompass parts of its former Eastern European range, as well as in Western Europe, which is thought to be attributable to a decline in grey wolf populations.[6][7][8]
Physical description
The European jackal is the largest of the golden jackals, with animals of both sexes measuring 120–125cm (47–49in) in total length and 10–13kg (20–29lb) in body weight.[8] One adult male in North-Eastern Italy is recorded to have reached 14.9kg (33lb).[9] The fur is coarse, and is generally brightly coloured with blackish tones on the back. The thighs, upper legs, ears and forehead are bright reddish chestnut.[2] Jackals in Northern Dalmatia have broader than average skulls, which is thought to result from human induced isolation from other populations, thus resulting in a new morphotype.[10]
Diet
In the Caucasus, jackals mainly hunt hares, small rodents, pheasants, partridges, ducks, coots, moorhens and passerines. They readily eat lizards, snakes, frogs, fish, molluscs and insects. During the winter period, they will kill many nutrias and waterfowl. During such times, jackals will surplus kill and cache what they do not eat. Jackals will feed on fruits such as pears, hawthorn, dogwood and the cones of Common Medlars.[2] European jackals tend not to be as damaging to livestock as wolves and red foxes are, though they can become a serious nuisance to small sized stock when in high numbers.[8] The highest number of livestock damages occurred in southern Bulgaria: 1,053 attacks on small stock, mainly sheep and lambs, were recorded between 1982–87, along with some damages to newborn deer in game farms.
In Greece, rodents, insects, carrion, and fruits comprise the jackal’s diet, though they rarely eat garbage, due to large numbers of stray dogs preventing them access to places with high human density.[8] Jackals in Turkey have been known to eat the eggs of the endangered green sea-turtle.[11] In Hungary, their most frequent prey are common voles and bank voles.[12] In Dalmatia, mammals (the majority being even-toed ungulates and lagomorphs) made up 50.3% of the golden jackal's diet, fruit seeds (14% each being common fig and common grape vine, while 4.6% are Juniperus oxycedrus) and vegetables 34.1%, insects (16% orthopteras, 12% beetles, and 3% dictyopteras) 29.5%, birds and their eggs 24.8%, artificial food 24%, and branches, leaves, and grass 24%.[13] Information on the diet of jackals in North-Eastern Italy is scant, but it is certain that they prey on small roe deer and hares.[9]
Distribution
The jackal's current European range mostly encompasses the Balkan region, where habitat loss and mass poisoning caused it to become extinct in many areas during the 1960s, with core populations only occurring in scattered regions such as Strandja, the Dalmatian Coast, Aegean Macedonia and the Peloponnese.  It recolonised its former territories in Bulgaria during 1962, following legislative protection, and subsequently expanded its range into Romania and Serbia. Individual jackals further expanded into Italy, Slovenia, Austria, Hungary and Slovakia during the 1980s.[14] The golden jackal is listed as an Annex V species in the EU Habitats Directive and as such has legal protection in Estonia, Greece and all other EU member states.

Bulgaria has the largest jackal population in Europe, which went through a 33–fold increase in range from the early sixties to mid-eighties. Factors aiding this increase include the replacement of natural forests with dense scrub, an increase in animal carcasses from state game farms, reductions in wolf populations and the abandonment of poisoning campaigns. In the early 1990s, it was estimated that up to 5000 jackals populated Bulgaria. The population increased in 1994, and appears to have stabilised.[15]
In Greece, golden jackals are the rarest of the three wild extant canids there, having disappeared from Central Greece, Western Greece and Corfu and are now limited to discontinuous, isolated population clusters in Peloponnese, Phocis, Samos Island, Halkidiki and North-eastern Greece. Currently, the largest population cluster is located in Nestos, north-eastern Greece. Although listed as "vulnerable" in the Red Data Book for Greek Vertebrates, the species has neither been officially declared as a game species nor as a protected one.[16]
Jackal populations have been increasing in Serbia since the late 1970s, and occur mainly in north-eastern Serbia and lower Srem. Jackals are especially common near Negotin and Bela Palanka (close to the border with Bulgaria), where during the 1990s, about 500 specimens were shot.[15] In Croatia, a 2007 survey reported 19 jackal packs in the north-western part of Ravni kotari and two on Vir Island.[17] Golden jackals are listed as a protected species in Slovenia, where they were first spotted in 1952 and have also established permanent territories there.[18] In 2005, a probably vagrant female was accidentally shot near Gornji Grad in the Upper Savinja Valley, Northern Slovenia.[6] In 2009, two territorial groups of golden jackal were recorded in the Ljubljana Marshes area, Central Slovenia. It seems that the species continues to expand towards Central Europe.[18] Jackal populations in Albania however are on the verge of extinction with possible occurrence in only three lowland wetland locations along the Adriatic Sea.[15]

In Hungary, where they are sometimes called "reed wolves",[3] golden jackals had disappeared in the 1950s through hunting and habitat destruction, only to return in the late 1970s, with the first breeding pairs being detected near the southern border in Transdanubia, then between the River Danube and Tisza. Golden jackals have increased greatly in number year by year, with some estimates indicating that they now outnumber red foxes. The sighting of a jackal near the Austrian border in the summer of 2007 indicated that they have spread throughout the country.[19][20]
In Italy the species is found in the wild in Friuli Venezia Giulia, Veneto and Trentino-Alto Adige/Südtirol. In the High Adriatic Hinterland, its distribution has been recently updated by Lapini et al. (2009).[21] In 1984 Canis aureus had reached the Province of Belluno, in 1985 a pack reproduced near Udine (this group was eliminated in 1987), a road-killed jackal was collected in Veneto in 1992, and their presence was then confirmed in the Province of Gorizia and in the hinterland of the Gulf of Trieste. In these cases, the specimens were usually roaming male subadults, though a family-group was discovered in Agordino in 1994.[22] A young dead female was discovered on 10 December 2009 in Carnia, indicating that the species' range has continued to expand. Moreover, in the late summer of 2009, the species was also signalled in Trentino-Alto Adige/Südtirol, where it has likely reached the Puster Valley.[23] The Italian branch of WWF estimates that jackal numbers in Italy may be underestimated.[22] The golden jackal is a protected animal in Italy.[23]
A dead adult was found close to the road near Podolí (Uherské Hradiště District) in the Czech Republic, on 19 March 2006.[24]
Recently, an isolated population was confirmed in western Estonia, much further north than their common range. Whether they were an introduced population or came through natural migration was unknown.[25] Environmental Board of Estonia classified it as an invasive species, and subject to extermination campaigns.[26] However, studies confirmed that animals reached Estonia naturally from Caucasus through Ukraine, which means they are not considered invasive species any more and are treated equally with other hunted animals.[27][28] Number of jackals has grown quickly in coastal areas of Estonia. In 2016 jackals killed over 100 sheep in Estonia and during hunting season in winter 2016/2017 32 jackals were killed. Due to quick growth of jackal population their hunting season was extended by two months for next year, up to six months in a year.[27][29]
In Turkey, Romania, the North Black Sea coast, and the Caucasus region, the status of jackals is largely unknown. There are indications of expanding populations in Romania and the north-western Black Sea coast, and reports of decline in Turkey.[15]
In Denmark the carcass of a roadkilled golden jackal was discovered in September 2015 near Karup in Central Jutland.[30] In August 2016 a live golden jackal was spotted and photographed in Lille Vildmose.[31]
The species' presence in Poland was confirmed in 2015 through a necropsy on a roadkilled male found in the northwest and camera trapping of two live specimens in the east.[32]
In the Netherlands, presence of a single individual was confirmed through a camera trap at the Veluwe in 2016.[33]
A golden jackal was photographed in late 2017 in Haute-Savoie, southeastern France.[34]
Origins and presence in European tradition
Although present in Europe, jackals are rarely featured in European folklore or literature. The ancient Greek philosopher and scientist Aristotle, in 4th century BCE wrote that jackals avoid lions and dogs, but they are friendly to people, not being afraid of them. He also mentioned that they change appearance from summer to winter.  Being a rare and elusive animal, the European jackal was historically often assumed to be an invasive species wherever it made its presence known: In Dalmatia, it was widely believed by the inhabitants of Korčula Island that African jackals were introduced to the island by the Republic of Venice to inflict damage on the Republic of Ragusa. When in 1929 a male jackal appeared on Premuda, the islanders believed that it was brought to the island "out of a sheer malice." An African origin for Dalmatian jackals has been proven unlikely, as their skulls bear few similarities to those of African jackals, but are similar to those of jackals in Asia Minor.[10] Sir William Jardine thought that jackals were first transported to Europe through the Muslim conquests.[35] However, the fossil record indicates that the golden jackal likely colonised the European continent from Asia during the Upper Holocene[6] or late Pleistocene.[21]
In 2015, during an attempt to understand the genetic identity of the rapidly expanding jackal populations in Europe, an international team of researchers examined 15 microsatellite markers and a 406 base-pair fragment of the mtDNA control region from the tissue samples of 97 specimens throughout Europe and Asia Minor. The results showed that European jackals have much lower haplotype diversity than those in Israel (where they have admixed with dogs, grey wolves and African wolves), and that they mostly descend from populations originating from the Caucasus. The highest level of haplotype diversity was found in Peloponnesian jackals, which may represent a relict population of Europe's original golden jackals prior to their extirpation elsewhere. Particular attention was paid to the genetics of Baltic jackals, as all Baltic states class the animal as an artificially introduced invasive species subject to extermination. It was found that jackals in Estonia originate from the south-eastern European population, whereas those in Lithuania are of Caucasian origin; this was concluded to render the hypothesis of an artificial introduction unlikely, and that their presence in both states was consistent with the natural northward expansion of both southeastern and eastern European populations.[36]
Surveys taken in the High Adriatic Hinterland indicate that the totality of people with first hand experience of jackals (hunters, game keepers and local people) regularly mistook red foxes affected by sarcoptic mange (or in a problematic state of moult) for golden jackals. The sighting of a true golden jackal however, was always referred to as a wolf, or a little wolf. This was verified both with photo-trapping sessions and with track studies, confirming previous observations on this matter. This erroneous and controversial perception of the golden jackal may be due to the fact that its presence is still not traditional, neither in Italian and Slovenian human culture, nor in hunting and game keeping traditions.[21]


The Great Wall of China is a series of fortifications made of stone, brick, tamped earth, wood, and other materials, generally built along an east-to-west line across the historical northern borders of China to protect the Chinese states and empires against the raids and invasions of the various nomadic groups of the Eurasian Steppe with an eye to expansion. Several walls were being built from as early as the 7th centuryBC;[2] these were later joined together and made bigger by Qin Shi Huang (220–206BC), the first Emperor of China. Little of that wall remains.[3] Later on, many successive dynasties have repaired, maintained, and newly built multiple stretches of border walls. The most well-known of the walls were built during the Ming Dynasty (1368–1644).
Apart from defense, other purposes of the Great Wall have included border controls, allowing the imposition of duties on goods transported along the Silk Road, regulation or encouragement of trade and the control of immigration and emigration. Furthermore, the defensive characteristics of the Great Wall were enhanced by the construction of watch towers, troop barracks, garrison stations, signaling capabilities through the means of smoke or fire, and the fact that the path of the Great Wall also served as a transportation corridor.
The frontier walls built by different dynasties have multiple courses. Collectively, they stretch from Dandong in the east to Lop Lake in the west, from present-day Sino-Russian border in the north to Qinghai in the south; along an arc that roughly delineates the edge of Mongolian steppe. A comprehensive archaeological survey, using advanced technologies, has concluded that the walls built by the Ming dynasty measure 8,850km (5,500mi).[4] This is made up of 6,259km (3,889mi) sections of actual wall, 359km (223mi) of trenches and 2,232km (1,387mi) of natural defensive barriers such as hills and rivers.[4] Another archaeological survey found that the entire wall with all of its branches measures out to be 21,196km (13,171mi).[5] Today, the Great Wall is generally recognized as one of the most impressive architectural feats in history.[6]
Names
The collection of fortifications known as the Great Wall of China has historically had a number of different names in both Chinese and English.
In Chinese histories, the term "Long Wall(s)" (長城, changcheng) appears in Sima Qian's Records of the Grand Historian, where it referred both to the separate great walls built between and north of the Warring States and to the more unified construction of the First Emperor.[7] The Chinese character 城, meaning city or fortress, is a phono-semantic compound of the "earth" radical 土 and phonetic 成, whose Old Chinese pronunciation has been reconstructed as *deŋ.[8] It originally referred to the rampart which surrounded traditional Chinese cities and was used by extension for these walls around their respective states; today, however, it is much more often the Chinese word for "city".[9]
The longer Chinese name "Ten-Thousand Mile Long Wall" (萬里長城, Wanli Changcheng) came from Sima Qian's description of it in the Records, though he did not name the walls as such. The AD493 Book of Song quotes the frontier general Tan Daoji referring to "the long wall of 10,000 miles", closer to the modern name, but the name rarely features in pre-modern times otherwise.[10] The traditional Chinese mile (里, lǐ) was an often irregular distance that was intended to show the length of a standard village and varied with terrain but was usually standardized at distances around a third of an English mile (540m).[11] Since China's metrication in 1930, it has been exactly equivalent to 500 metres or 1,600 feet,[12] which would make the wall's name describe a distance of 5,000km (3,100mi). However, this use of "ten-thousand" (wàn) is figurative in a similar manner to the Greek and English myriad and simply means "innumerable" or "immeasurable".[13]
Because of the wall's association with the First Emperor's supposed tyranny, the Chinese dynasties after Qin usually avoided referring to their own additions to the wall by the name "Long Wall".[14] Instead, various terms were used in medieval records, including "frontier(s)" (塞, sāi),[15] "rampart(s)" (垣, yuán),[15] "barrier(s)" (障, zhàng),[15] "the outer fortresses" (外堡, wàibǎo),[16] and "the border wall(s)" (t 邊牆, s 边墙, biānqiáng).[14] Poetic and informal names for the wall included "the Purple Frontier" (紫塞, Zǐsāi)[17] and "the Earth Dragon" (t 土龍, s 土龙, Tǔlóng).[18] Only during the Qing period did "Long Wall" become the catch-all term to refer to the many border walls regardless of their location or dynastic origin, equivalent to the English "Great Wall".[19]
The current English name evolved from accounts of "the Chinese wall" from early modern European travelers.[19] By the 19th century,[19] "The Great Wall of China" had become standard in English, French, and German, although other European languages continue to refer to it as "the Chinese wall."[13]
History

Early walls

The Chinese were already familiar with the techniques of wall-building by the time of the Spring and Autumn period between the 8th and 5th centuriesBC.[20] During this time and the subsequent Warring States period, the states of Qin, Wei, Zhao, Qi, Yan, and Zhongshan[21][22] all constructed extensive fortifications to defend their own borders. Built to withstand the attack of small arms such as swords and spears, these walls were made mostly by stamping earth and gravel between board frames.
King Zheng of Qin conquered the last of his opponents and unified China as the First Emperor of the Qin dynasty ("Qin Shi Huang") in 221BC.  Intending to impose centralized rule and prevent the resurgence of feudal lords, he ordered the destruction of the sections of the walls that divided his empire among the former states. To position the empire against the Xiongnu people from the north, however, he ordered the building of new walls to connect the remaining fortifications along the empire's northern frontier. "Build and move on" was a central guiding principle in constructing the wall, implying that the Chinese were not erecting a permanently fixed border.[23] Transporting the large quantity of materials required for construction was difficult, so builders always tried to use local resources. Stones from the mountains were used over mountain ranges, while rammed earth was used for construction in the plains. There are no surviving historical records indicating the exact length and course of the Qin walls. Most of the ancient walls have eroded away over the centuries, and very few sections remain today. The human cost of the construction is unknown, but it has been estimated by some authors that hundreds of thousands,[24] if not up to a million, workers died building the Qin wall.[25][26] Later, the Han,[27] the Sui, and the Northern dynasties all repaired, rebuilt, or expanded sections of the Great Wall at great cost to defend themselves against northern invaders.[28] The Tang and Song dynasties did not undertake any significant effort in the region.[28] The Liao, Jin, and Yuan dynasties, who ruled Northern China throughout most of the 10th–13th centuries, constructed defensive walls in the 12th century but those were located much to the north of the Great Wall as we know it, within China's province of Inner Mongolia and in Mongolia itself.[29]
Ming era

The Great Wall concept was revived again under the Ming in the 14th century,[30] and following the Ming army's defeat by the Oirats in the Battle of Tumu. The Ming had failed to gain a clear upper hand over the Mongolian tribes after successive battles, and the long-drawn conflict was taking a toll on the empire. The Ming adopted a new strategy to keep the nomadic tribes out by constructing walls along the northern border of China. Acknowledging the Mongol control established in the Ordos Desert, the wall followed the desert's southern edge instead of incorporating the bend of the Yellow River.
Unlike the earlier fortifications, the Ming construction was stronger and more elaborate due to the use of bricks and stone instead of rammed earth. Up to 25,000 watchtowers are estimated to have been constructed on the wall.[31] As Mongol raids continued periodically over the years, the Ming devoted considerable resources to repair and reinforce the walls. Sections near the Ming capital of Beijing were especially strong.[32] Qi Jiguang between 1567 and 1570 also repaired and reinforced the wall, faced sections of the ram-earth wall with bricks and constructed 1,200 watchtowers from Shanhaiguan Pass to Changping to warn of approaching Mongol raiders.[33] During the 1440s–1460s, the Ming also built a so-called "Liaodong Wall". Similar in function to the Great Wall (whose extension, in a sense, it was), but more basic in construction, the Liaodong Wall enclosed the agricultural heartland of the Liaodong province, protecting it against potential incursions by Jurched-Mongol Oriyanghan from the northwest and the Jianzhou Jurchens from the north. While stones and tiles were used in some parts of the Liaodong Wall, most of it was in fact simply an earth dike with moats on both sides.[34]
Towards the end of the Ming, the Great Wall helped defend the empire against the Manchu invasions that began around 1600. Even after the loss of all of Liaodong, the Ming army held the heavily fortified Shanhai Pass, preventing the Manchus from conquering the Chinese heartland. The Manchus were finally able to cross the Great Wall in 1644, after Beijing had already fallen to Li Zicheng's rebels. Before this time, the Manchus had crossed the Great Wall multiple times to raid, but this time it was for conquest. The gates at Shanhai Pass were opened on May 25 by the commanding Ming general, Wu Sangui, who formed an alliance with the Manchus, hoping to use the Manchus to expel the rebels from Beijing.[35] The Manchus quickly seized Beijing, and eventually defeated both the rebel-founded Shun dynasty and the remaining Ming resistance, establishing the Qing dynasty rule over all of China.[36]
Under Qing rule, China's borders extended beyond the walls and Mongolia was annexed into the empire, so constructions on the Great Wall were discontinued. On the other hand, the so-called Willow Palisade, following a line similar to that of the Ming Liaodong Wall, was constructed by the Qing rulers in Manchuria. Its purpose, however, was not defense but rather to prevent Han Chinese migration into Manchuria.[37]
Foreign accounts

None of the Europeans who visited Yuan China or Mongolia, such as Marco Polo, Giovanni da Pian del Carpine, William of Rubruck, Giovanni de' Marignolli and Odoric of Pordenone, mentioned the Great Wall.[39][40]
The North African traveler Ibn Battuta, who also visited China during the Yuan dynasty ca. 1346, had heard about China's Great Wall, possibly before he had arrived in China.[41] He wrote that the wall is "sixty days' travel" from Zeitun (modern Quanzhou) in his travelogue Gift to Those Who Contemplate the Wonders of Cities and the Marvels of Travelling.  He associated it with the legend of the wall mentioned in the Qur'an,[42] which Dhul-Qarnayn (commonly associated with Alexander the Great) was said to have erected to protect people near the land of the rising sun from the savages of Gog and Magog.  However, Ibn Battuta could find no one who had either seen it or knew of anyone who had seen it, suggesting that although there were remnants of the wall at that time, they weren't significant.[43]
Soon after Europeans reached Ming China by ship in the early 16th century, accounts of the Great Wall started to circulate in Europe, even though no European was to see it for another century. Possibly one of the earliest European descriptions of the wall and of its significance for the defense of the country against the "Tartars" (i.e. Mongols), may be the one contained in João de Barros's 1563 Asia.[44] Other early accounts in Western sources include those of Gaspar da Cruz, Bento de Goes, Matteo Ricci, and Bishop Juan González de Mendoza.[45] In 1559, in his work "A Treatise of China and the Adjoyning Regions," Gaspar da Cruz offers an early discussion of the Great Wall.[45] Perhaps the first recorded instance of a European actually entering China via the Great Wall came in 1605, when the Portuguese Jesuit brother Bento de Góis reached the northwestern Jiayu Pass from India.[46] Early European accounts were mostly modest and empirical, closely mirroring contemporary Chinese understanding of the Wall,[47] although later they slid into hyperbole,[48] including the erroneous but ubiquitous claim that the Ming Walls were the same ones that were built by the First Emperor in the 3rd centuryBC.[48]
When China opened its borders to foreign merchants and visitors after its defeat in the First and Second Opium Wars, the Great Wall became a main attraction for tourists. The travelogues of the later 19th century further enhanced the reputation and the mythology of the Great Wall,[49] such that in the 20th century, a persistent misconception exists about the Great Wall of China being visible from the Moon or even Mars.[50]

Course

Although a formal definition of what constitutes a "Great Wall" has not been agreed upon, making the full course of the Great Wall difficult to describe in its entirety,[51] the course of the main Great Wall line following Ming constructions can be charted.
The Jiayu Pass, located in Gansu province, is the western terminus of the Ming Great Wall. Although Han fortifications such as Yumen Pass and the Yang Pass exist further west, the extant walls leading to those passes are difficult to trace. From Jiayu Pass the wall travels discontinuously down the Hexi Corridor and into the deserts of Ningxia, where it enters the western edge of the Yellow River loop at Yinchuan. Here the first major walls erected during the Ming dynasty cuts through the Ordos Desert to the eastern edge of the Yellow River loop. There at Piantou Pass (t 偏頭關, s 偏头关, Piāntóuguān) in Xinzhou, Shanxi province, the Great Wall splits in two with the "Outer Great Wall" (t 外長城, s 外长城, Wài Chǎngchéng) extending along the Inner Mongolia border with Shanxi into Hebei province, and the "inner Great Wall" (t 內長城, s 內长城, Nèi Chǎngchéng) running southeast from Piantou Pass for some 400km (250mi), passing through important passes like the Pingxing Pass and Yanmen Pass before joining the Outer Great Wall at Sihaiye (四海冶, Sìhǎiyě), in Beijing's Yanqing County.
The sections of the Great Wall around Beijing municipality are especially famous: they were frequently renovated and are regularly visited by tourists today. The Badaling Great Wall near Zhangjiakou is the most famous stretch of the Wall, for this is the first section to be opened to the public in the People's Republic of China, as well as the showpiece stretch for foreign dignitaries.[52] South of Badaling is the Juyong Pass; when used by the Chinese to protect their land, this section of the wall had many guards to defend China's capital Beijing. Made of stone and bricks from the hills, this portion of the Great Wall is 7.8m (25ft 7in) high and 5m (16ft 5in) wide.
One of the most striking sections of the Ming Great Wall is where it climbs extremely steep slopes in Jinshanling. There it runs 11km (7mi) long, ranges from 5 to 8m (16ft 5in to 26ft 3in) in height, and 6m (19ft 8in) across the bottom, narrowing up to 5m (16ft 5in) across the top. Wangjinglou (t 望京樓, s 望京楼, Wàngjīng Lóu) is one of Jinshanling's 67 watchtowers, 980m (3,220ft) above sea level. Southeast of Jinshanling is the Mutianyu Great Wall which winds along lofty, cragged mountains from the southeast to the northwest for 2.25km (1.40mi). It is connected with Juyongguan Pass to the west and Gubeikou to the east. This section was one of the first to be renovated following the turmoil of the Cultural Revolution.[53]
At the edge of the Bohai Gulf is Shanhai Pass, considered the traditional end of the Great Wall and the "First Pass Under Heaven". The part of the wall inside Shanhai Pass that meets the sea is named the "Old Dragon Head". 3km (2mi) north of Shanhai Pass is Jiaoshan Great Wall (焦山長城), the site of the first mountain of the Great Wall.[54] 15km (9mi) northeast from Shanhaiguan is Jiumenkou (t 九門口, s 九门口, Jiǔménkǒu), which is the only portion of the wall that was built as a bridge. Beyond Jiumenkou, an offshoot known as the Liaodong Wall continues through Liaoning province and terminates at the Hushan Great Wall, in the city of Dandong near the North Korean border.[55]
In 2009, 180km of previously unknown sections of the wall concealed by hills, trenches and rivers were discovered with the help of infrared range finders and GPS devices.[56] In March and April 2015 nine sections with a total length of more than 10km (6mi), believed to be part of the Great Wall, were discovered along the border of Ningxia autonomous region and Gansu province.[57]
Characteristics

Before the use of bricks, the Great Wall was mainly built from rammed earth, stones, and wood. During the Ming, however, bricks were heavily used in many areas of the wall, as were materials such as tiles, lime, and stone. The size and weight of the bricks made them easier to work with than earth and stone, so construction quickened. Additionally, bricks could bear more weight and endure better than rammed earth. Stone can hold under its own weight better than brick, but is more difficult to use. Consequently, stones cut in rectangular shapes were used for the foundation, inner and outer brims, and gateways of the wall. Battlements line the uppermost portion of the vast majority of the wall, with defensive gaps a little over 30cm (12in) tall, and about 23cm (9.1in) wide. From the parapets, guards could survey the surrounding land.[58] Communication between the army units along the length of the Great Wall, including the ability to call reinforcements and warn garrisons of enemy movements, was of high importance. Signal towers were built upon hill tops or other high points along the wall for their visibility. Wooden gates could be used as a trap against those going through. Barracks, stables, and armories were built near the wall's inner surface.[58]
Condition

While portions north of Beijing and near tourist centers have been preserved and even extensively renovated, in many other locations the Wall is in disrepair. Those parts might serve as a village playground or a source of stones to rebuild houses and roads.[59] Sections of the Wall are also prone to graffiti and vandalism, while inscribed bricks were pilfered and sold on the market for up to 50 renminbi.[60] Parts have been destroyed because the Wall is in the way of construction.[61] A 2012 report by the National Cultural Heritage Administration states that 22% of the Ming Great Wall has disappeared, while 1,961km (1,219mi) of wall have vanished.[60]  More than 60km (37mi) of the wall in Gansu province may disappear in the next 20 years, due to erosion from sandstorms. In some places, the height of the wall has been reduced from more than 5m (16ft 5in) to less than 2m (6ft 7in). Various square lookout towers that characterize the most famous images of the wall have disappeared. Many western sections of the wall are constructed from mud, rather than brick and stone, and thus are more susceptible to erosion.[62] In 2014 a portion of the wall near the border of Liaoning and Hebei province was repaired with concrete. The work has been much criticized.[63]
Visibility from space
From the Moon
One of the earliest known references to the myth that the Great Wall can be seen from the moon appears in a letter written in 1754 by the English antiquary William Stukeley. Stukeley wrote that, "This mighty wall [Hadrian's wall] of four score miles [130km] in length is only exceeded by the Chinese Wall, which makes a considerable figure upon the terrestrial globe, and may be discerned at the Moon."[64] The claim was also mentioned by Henry Norman in 1895 where he states "besides its age it enjoys the reputation of being the only work of human hands on the globe visible from the Moon."[65] The issue of "canals" on Mars was prominent in the late 19th century and may have led to the belief that long, thin objects were visible from space. The claim that the Great Wall is visible from the moon also appears in 1932's Ripley's Believe It or Not! strip[66] and in Richard Halliburton's 1938 book Second Book of Marvels.
The claim that the Great Wall is visible from the moon has been debunked many times,[67] but is still ingrained in popular culture.[68] The wall is a maximum 9.1m (29ft 10in) wide, and is about the same color as the soil surrounding it. Based on the optics of resolving power (distance versus the width of the iris: a few millimeters for the human eye, meters for large telescopes) only an object of reasonable contrast to its surroundings which is 110km (70mi) or more in diameter (1 arc-minute) would be visible to the unaided eye from the Moon, whose average distance from Earth is 384,393km (238,851mi). The apparent width of the Great Wall from the Moon is the same as that of a human hair viewed from 3km (2mi) away. To see the wall from the Moon would require spatial resolution 17,000 times better than normal (20/20) vision.[69] Unsurprisingly, no lunar astronaut has ever claimed to have seen the Great Wall from the Moon.
From low Earth orbit

A more controversial question is whether the Wall is visible from low Earth orbit (an altitude of as little as 160km (100mi)). NASA claims that it is barely visible, and only under nearly perfect conditions; it is no more conspicuous than many other man-made objects.[70] Other authors have argued that due to limitations of the optics of the eye and the spacing of photoreceptors on the retina, it is impossible to see the wall with the naked eye, even from low orbit, and would require visual acuity of 20/3 (7.7 times better than normal).[69]
Astronaut William Pogue thought he had seen it from Skylab but discovered he was actually looking at the Grand Canal of China near Beijing. He spotted the Great Wall with binoculars, but said that "it wasn't visible to the unaided eye." U.S. Senator Jake Garn claimed to be able to see the Great Wall with the naked eye from a space shuttle orbit in the early 1980s, but his claim has been disputed by several U.S. astronauts. Veteran U.S. astronaut Gene Cernan has stated: "At Earth orbit of 100 to 200 miles [160 to 320km] high, the Great Wall of China is, indeed, visible to the naked eye." Ed Lu, Expedition 7 Science Officer aboard the International Space Station, adds that, "it's less visible than a lot of other objects. And you have to know where to look."
In 2001, Neil Armstrong stated about the view from Apollo 11: "I do not believe that, at least with my eyes, there would be any man-made object that I could see. I have not yet found somebody who has told me they've seen the Wall of China from Earth orbit.... I've asked various people, particularly Shuttle guys, that have been many orbits around China in the daytime, and the ones I've talked to didn't see it."[71]
In October 2003, Chinese astronaut Yang Liwei stated that he had not been able to see the Great Wall of China. In response, the European Space Agency (ESA) issued a press release reporting that from an orbit between 160 and 320km (100 and 200mi), the Great Wall is visible to the naked eye. In an attempt to further clarify things, the ESA published a picture of a part of the "Great Wall" photographed from low orbit. However, in a press release a week later, they acknowledged that the "Great Wall" in the picture was actually a river.[69]
Leroy Chiao, a Chinese-American astronaut, took a photograph from the International Space Station that shows the wall. It was so indistinct that the photographer was not certain he had actually captured it. Based on the photograph, the China Daily later reported that the Great Wall can be seen from 'space' with the naked eye, under favorable viewing conditions, if one knows exactly where to look.[72] However, the resolution of a camera can be much higher than the human visual system, and the optics much better, rendering photographic evidence irrelevant to the issue of whether it is visible to the naked eye.[69]
Gallery

"The First Mound" – at Jiayuguan, the western terminus of the Ming wall
The Great Wall of China at Badaling
The Juyongguan area of the Great Wall accepts numerous tourists each day
A portion of the Great Wall of China at Simatai, overlooking the gorge
Mutianyu Great Wall, China. This is atop the wall on a section that has not been restored
The Old Dragon Head, the Great Wall where it meets the sea in the vicinity of Shanhaiguan
Gateway of Gubeikou Fortress at Gubeikou

See also

Defense of the Great Wall
Gates of Alexander
Great Wall of China hoax
Great Wall Marathon
Great Wall of Qi
Huangya Pass
Jiankou
List of World Heritage Sites in China
Miaojiang Great Wall
Roman military frontiers and fortifications

Notes

 Cite uses deprecated parameter |dead-url= (help)

Further reading

Arnold, H.J.P, "The Great Wall: Is It or Isn't It?" Astronomy Now, 1995.
Beckwith, Christopher I. (2009): Empires of the Silk Road: A History of Central Eurasia from the Bronze Age to the Present. Princeton: Princeton University Press. ISBN978-0-691-13589-2.
Luo, Zewen, et al. and Baker, David, ed. (1981). The Great Wall. Maidenhead: McGraw-Hill Book Company (UK). ISBN0-07-070745-6

Michaud, Roland and Sabrina (photographers), & Michel Jan, The Great Wall of China. Abbeville Press, 2001. ISBN0-7892-0736-2
Schafer, Edward H. (1985) The Golden Peaches of Samarkand. Berkeley: University of California Press. ISBN978-0-520-05462-2.

– organization focused on conservation

 (in Chinese)
 on In Our Time at the BBC

 Geographic data related to  at OpenStreetMap


List of ''Guiding Light'' cast members
This is a list of actors and actresses who have had roles on the soap opera Guiding Light.

Cast
Final main cast

Recurring cast

Former cast members

Deceased cast members

(*) = Radio
 This television-related list is incomplete; you can help by .

Who was the lead actress on Guiding Light?