COMPUTER SYSTEM
05:18 - Posted by Hakimi - 0 comments

ComputerFrom Wikipedia, the free encyclopediaFor other uses, see Computer (disambiguation)."Computer technology" redirects here. For the company, see Computer Technology Limited.Conventionally, a computer consists of at least one processing element and some form of memory. The processing element carries out arithmetic and logic operations, and a sequencing and control unit that can change the order of operations based on stored information. Peripheral devices allow information to be retrieved from an external source, and the result of operations saved and retrieved.A computer is a general purpose device that can be programmed to carry out a finite set of arithmetic or logical operations. Since a sequence of operations can be readily changed, the computer can solve more than one kind of problem.The first electronic digital computers were developed between 1940 and 1945 in the United Kingdom and United States. Originally they were the size of a large room, consuming as much power as several hundred modern personal computers (PCs).[1] In this eramechanical analog computers were used for military applications.Modern computers based on integrated circuits are millions to billions of times more capable than the early machines, and occupy a fraction of the space.[2] Simple computers are small enough to fit into mobile devices, and mobile computers can be powered by smallbatteries. Personal computers in their various forms are icons of the Information Age and are what most people think of as "computers". However, the embedded computers found in many devices from mp3 players to fighter aircraft and from toys to industrial robots are the most numerous.History of computing
Main article: History of computing hardwareThe first use of the word "computer" was recorded in 1613, referring to a person who carried out calculations,
or computations, and the word continued with the same meaning until the middle of the 20th century.
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 history of the modern computer begins with two separate technologies, automated calculation and
programmability, but no single device can be identified as the earliest computer, partly because of the
inconsistent application of that term. A few devices are worth mentioning though, like some mechanical
aids to computing, which were very successful and survived for centuries until the advent of the
electronic calculator, like the Sumerian abacus, designed around 2500 BC[4] of which a descendant won a speed
competition against a modern desk calculating machine in Japan in 1946,[5] the slide rules, invented in the 1620s,
which were carried on five Apollo space missions, including to the moon[6] and arguably the astrolabe and the
 Antikythera mechanism, an ancient astronomical computer built by the Greeks around 80 BC.[7] The Greek
mathematician Hero of Alexandria (c. 10–70 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.[8] This is the essence of
programmability.
Around the end of the 10th century, the French monk Gerbert d'Aurillac brought back from Spain the drawings of a
machine invented by the Moors that answered either Yes or No to the questions it was asked.[9] Again in the
13th century, the monks Albertus Magnus and Roger Bacon built talking androids without any further development (Albertus Magnus complained that he had wasted forty years of his life when Thomas Aquinas, terrified by his machine, destroyed it).[10]In 1642, the Renaissance saw the invention of the mechanical calculator,[11] a device that could perform all four
arithmetic operations without relying on human intelligence.[12] The mechanical calculator was at the root of the
development of computers in two separate ways. Initially, it was in trying to develop more powerful and more
 flexible calculators[13] that the computer was first theorized by Charles Babbage[14][15] and then developed.[16]Secondly, development of a low-cost electronic calculator, successor to the mechanical calculator, resulted in
the development byIntel[17] of the first commercially available microprocessor integrated circuit.
In 1801, Joseph Marie Jacquard made an improvement to the textile loom by introducing a series of punched paper cards as a template which allowed 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.
The Most Famous Image in the Early History of Computing[18]This portrait of Jacquard was woven in silk on a Jacquard loom and required 24,000 punched cards to create (1839). It was only produced to order. Charles Babbage owned one of these portraits ; it inspired him in using perforated cards in his analytical engine[19]In the late 1880s, Herman Hollerith invented the recording of data on It was the
fusion of automatic calculation with programmability that
produced the first recognizable computers. In 1837, Charles Babbagewas the first to conceptualize and design a fully
programmablemechanical computer, his analytical engine.[20]Limited finances and Babbage's inability to resist tinkering with the
design meant that the device was never completed—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.
This machine was given to the Science museum in South Kensington in 1910.
a machine-readable medium. Earlier uses of machine-readable media
had been for control, not data. "After some initial trials with paper tape,
he settled on punched cards ..."[21] To process these punched cards
he invented the tabulator, and thekeypunch machines. These three
inventions were the foundation of the modern information processing
industry. Large-scale automated data processing of punched cards
was performed for the 1890 United States Census by Hollerith's
company, which later became the core of IBM. By the end of the 19th
century a number of ideas and technologies, that would later prove
useful in the realization of practical computers, had begun to appear:
Boolean algebra, thevacuum tube (thermionic valve), punched cards
and tape, and the teleprinter.
During the first half of the 20th century, many scientific computingneeds 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.
Alan Turing is widely regarded as the father of modern computer science. In 1936 Turing provided an influential
formalisation of the concept of thealgorithm and computation with the Turing machine, providing a blueprint for
 the electronic digital computer.[22] Of his role in the creation of the modern computer, Time magazine in naming
 Turing one of the 100 most influential people of the 20th century, states: "The fact remains that everyone who
taps at a keyboard, opening a spreadsheet or a word-processing program, is working on an incarnation of a
Turing machine".[22]


The Atanasoff–Berry Computer (ABC) was the world's first electronic digital computer, albeit notprogrammable.[23] Atanasoff is considered to be one of the fathers of the computer.[24] Conceived in 1937 by Iowa State College
physics professor John Atanasoff, and built with the assistance of graduate studentClifford Berry,[25] the machine
was not programmable, being designed only to solve systems of linear equations. The computer did employ parallel computation. A 1973 court ruling in a patent dispute found that the patent for the 1946 ENIAC computer derived from the Atanasoff–Berry Computer.
The first program-controlled computer was invented by Konrad Zuse, who built the Z3, an electromechanical
computing machine, in 1941.[26] The first programmable electronic computer was the Colossus, built in 1943 by
Tommy Flowers.
George Stibitz is internationally recognized as a father of the modern digital computer. While working at Bell Labs
 in November 1937, Stibitz invented and built a relay-based calculator he dubbed the
"Model K" (for "kitchen table", on which he had assembled it), which was the first to use binary circuits to
perform an arithmetic operation. Later models added greater sophistication including complex arithmetic and
programmability.[27]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 computer" is difficult.Shannon 1940 Notable achievements
include:
  • 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.[28]
  • regenerative memory allowed it to be much more compact than its peers (being approximately the size 
  • of a large desk or workbench), since intermediate results could be stored and then fed back into the 
  • same set of computation elements.
  • 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.[30]
  • essentially required rewiring to change its programming.
Computers using vacuum tubes as their electronic elements were in use throughout the 1950s, but by the
1960s had been largely replaced bysemiconductor transistor-based machines, which were smaller, faster,
cheaper to produce, required less power, and were more reliable. The first transistorised computer was
demonstrated at the University of Manchester in 1953.[31] In the 1970s, integrated circuit technology and the
subsequent creation of microprocessors, such as the Intel 4004, further decreased size and cost and further
increased speed and reliability of computers. By the late 1970s, many products such as video recorderscontained dedicated computers called microcontrollers, and they started to appear as a replacement to
mechanical controls in domestic appliances such as washing machines. The 1980s witnessed home computersand the now ubiquitous personal computer. With the evolution of the Internet, personal computers are becoming
as common as the television and the telephone in the household[citation needed].
Modern smartphones are fully programmable computers in their own right, and as of 2009 may well be the most
common form of such computers in existence[citation needed].
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. While some computers may have strange concepts "instructions" and "output"
(see quantum computing), 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.
Main articles: Computer program and Computer programmingThis 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. For example:
mov No. 0, sum ; set sum to 0
mov No. 1, num ; set num to 1
loop: add num, sum ; add num to sum
add No. 1, num ; add 1 to num
cmp num, #1000 ; compare num to 1000
ble loop ; if num <= 1000, go back to 'loop'
halt ; end of program. stop running
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 about a millionth
of a second.[32]Main article: software bug
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" – become 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.[33]Rear Admiral Grace Hopper 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.[34]In most computers, individual instructions are stored as machine code with each instruction being given a unique
number (its operation code or opcodefor 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,[35] 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.
Main article: Programming languageProgramming 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 languagesMain article: Low-level programming languageMachine 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 PDA or a hand-held videogame) cannot understand
the machine language of an Intel Pentium or the AMD Athlon 64 computer that might be in a PC.[36]Higher-level languagesMain article: High-level programming languageThough 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.[37] 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.
Question book-new.svgThis section does not cite anyreferences or sources. (July 2012)
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 engineeringconcentrates specifically on this challenge.
ComponentsMain articles: Central processing unit and MicroprocessorA 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 busses,
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.
The control unit, ALU, registers, and basic I/O (and often other hardware closely linked with these) 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.
Main articles: CPU design and 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 a series of control signals which
 activate other parts of the computer.[38] Control systems in advanced computers may change the order of some
 instructions so as 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.[39]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:
  1. Read the code for the next instruction from the cell indicated by the program counter.
  1. Decode the numerical code for the instruction into a set of commands or signals for each of the other 
  1. systems.
  1. Increment the program counter so it points to the next instruction.
  1. Read whatever data the instruction requires from cells in memory (or perhaps from an input device). 
  1. The location of this required data is typically stored within the instruction code.
  1. Provide the necessary data to an ALU or register.
  1. If the instruction requires an ALU or specialized hardware to complete, instruct the hardware to perform 
  1. the requested operation.
  1. Write the result from the ALU back to a memory location or to a register or perhaps an output device.
  1. 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.
Main article: Arithmetic logic unitThe ALU is capable of performing two classes of operations: arithmetic and logic.[40]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) whilst 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 returnboolean 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.[41] Graphics processors and computers with SIMD and MIMD features often contain ALUs that
can perform arithmetic on vectors and matrices.
Main article: Computer data storage
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 (2^8 = 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 complementnotation. 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 and read-only memoryor ROM. RAM can be read and written to anytime the CPU commands it, but ROM is pre-loaded 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 RAMwhenever 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.[42]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.
Main article: Input/output
I/O is the means by which a computer exchanges information with the outside world.[43] Devices that provide
input or output to the computer are calledperipherals.[44] On a typical personal computer, peripherals include
input devices like the keyboard and mouse, and output devices such as the displayand 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[citation needed]. Modern desktop computers contain many smaller computers that assist the main
CPU in performing I/O.Main article: Computer multitaskingWhile 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.[45]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.[46]Before the era of cheap 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.
Main article: 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 computersand 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.[47] 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.Main articles: Computer networking and InternetVisualization of a portion of the routes on 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 asSabre.[48]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.[49] 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 andADSL 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.
There are many types of computer architectures:
The quantum computer architecture holds the most promise to revolutionize computing.[50]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.
MisconceptionsA 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 computer, the 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."[51] Any device which processes information qualifies as a computer, especially if the
processing is purposeful.
Main article: Unconventional computingHistorically, computers evolved from mechanical computers and eventually from vacuum tubes to transistors.
However, conceptually computational systems as flexible as a personal computer can be built out of almost
anything. For example, a computer can be made out of billiard balls (billiard ball computer); an oft-quoted example.
More realistically, modern computers are made out oftransistors made of photolithographed semiconductors.
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.
Further topics
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.
Main articles: Computer hardware and Personal computer hardwareThe term hardware covers all of those parts of a computer that are tangible objects. Circuits, displays,
power supplies, cables, keyboards, printers and mice are all hardware.
Limited-function early computersFirst general-purpose computers
Semiconductors and microprocessorsStored program architectureBugsMachine codeProgramming languageProgram designControl unitArithmetic logic unit (ALU)MemoryInput/output (I/O)MultitaskingMultiprocessingNetworking and the InternetComputer architecture paradigmsRequired technologyArtificial intelligenceHardware