A computer is a device or machine for making calculations or controlling operations that are expressible in numerical or logical terms. Computers are constructed from components that
perform simple well-defined functions. The complex interactions of these components endow computers with the ability to process
information. If correctly configured (usually by programming) a
computer can be made to represent some aspect of a problem or part of a system. If a computer configured in this way is given
appropriate input data, then it can automatically solve the problem or predict the behavior of the system.
The discipline which studies the theory, design, and application of computers is called computer science.
General principles
Computers can work through the movement of mechanical parts, electrons, photons, quantum particles, or any other
well-understood physical phenomenon. Although computers have been built out of many different technologies, nearly all computers
today are electronic.
Computers may directly model the problem being solved, in the sense that the problem being solved is mapped as
closely as possible onto the physical phenomena being exploited. For example, electron flows might be used to model the flow of
water in a dam. Such analog computers were once common in the 1960s but are now rare.
In most computers today, the problem is first translated into mathematical terms by rendering all relevant information into
the binary base-two numeral
system (ones and zeros). Next, all operations on that information are reduced to simple Boolean algebra.
Electronic circuits are then used to represent Boolean operations. Since almost all of mathematics can be reduced to Boolean
operations, a sufficiently fast electronic computer is capable of attacking the majority of mathematical problems (and the
majority of information processing problems that can be translated into mathematical ones). This basic idea, which made modern
digital computers possible, was formally identified and explored by Claude E. Shannon.
Computers cannot solve all mathematical problems. Alan Turing identified
which problems could and could not be solved by computers, and in doing so founded theoretical computer science.
When the computer is finished calculating the problem, the result must be displayed to the user as output through output devices like light bulbs, LEDs, monitors, and printers.
Novice users, especially children, often have difficulty understanding the important idea that the computer is only a machine,
and cannot "think" or "understand" the words it displays. The computer is simply performing a mechanical lookup on preprogrammed
tables of lines and colors, which are then translated into arbitrary patterns of light by the output device. It is the human
brain which recognizes that those patterns form letters and numbers, and attaches meaning to them. From the computer's point of
view, all it "sees," assuming computers could ever be made self-aware, are electrons that are logically equivalent to ones and
zeros. See artificial intelligence.
Etymology
The word was originally used to describe a person who performed arithmetic calculations and this usage is still valid
(although it is becoming quite rare in the United States). The OED2 lists the year 1897 as the first year the word was used to refer to a mechanical calculating device. By 1946 several
qualifiers were introduced by the OED2 to differentiate between the different types of machine. These qualifiers included
analogue, digital and electronic. However, from the context of the citation, it is obvious these terms were
in use prior to 1946. .
See the Wiktionary entry for the word "computer" for definitions, translations
and a detailed etymology)
The exponential progress of computer development
Computing devices have doubled in capacity (instructions processed per second per $1000) every 18 to 24 months since 1900. Gordon E. Moore, co-founder of
Intel, first described this property of computer development in 1965 (see Moore's Law). Hand-in-hand with this increase in
capacity per unit cost has been an equally dramatic process of miniaturization. The first electronic computers, such as the ENIAC (announced in 1946), were huge devices that weighed tons, occupied
entire rooms, and required many operators to function successfully. They were so expensive that only governments and large
research organizations could afford them and were considered so exotic that only a handful would ever be required to satisfy
global demand. By contrast modern computers are orders of magnitude more powerful, less expensive, smaller and have become
ubiquitous.
Classification of computers
The following sections describe different approaches to classifying computers.
Classification by intended use
The colloquial nature of
this classification approach means it is ambiguous. It is usual for only current, commonly available devices to be included. The rapid nature
of computer development means new uses for computers are frequently found and current definitions quickly become outdated. Many
classes of computer that are no longer used, such as differential analyzers, are not commonly included in such lists. Other classification schemes are
required to unambiguously define the term computer.
Classification by implementation technology
A less ambiguous approach for classifying computing machines is by their implementation technology. The earliest computers
were purely mechanical. In the 1930s electro-mechanical components (relays) were introduced from the telecommunications
industry, and in the 1940s the first purely electronic computers were constructed from thermionic
valves (tubes). In the 1950s and 1960s valves
were gradually replaced with transistors and in the late 1960s and early 1970s semiconductor integrated circuits (silicon chips) were adopted and have been the mainstay of computing
technology ever since.
This description of implementation technologies is not exhaustive; it only covers the mainstream of development. Historically
many exotic technologies have been explored and abandoned. For example, economic models have been constructed using water flowing through multiple-constricted channels, and
between 1903 and 1909 Percy E. Ludgate developed a
design for a programmable analytical machine based weaving technologies in which
variables were carried in shuttles.
Efforts are currently underway to develop optical computers that
use light rather than electricity and the possibility that DNA can be
used for computing is being explored. One radical new area of research that could lead to computers with dramatic new
capabilities is the field of quantum computing but this is presently in its early
stages. With the exception of quantum computers the implementation
technology of a computer is not as important for classification purposes as the features that the machine implements.
Classification by design features
Modern computers combine fundamental design features that have been developed by various contributors over many years. These
features are often independent of implementation technology. Modern computers derive their overall capabilities from the way
these features interact. Some of the most important design features are listed below.
Digital versus analog
A fundamental decision in designing a computer is whether it should be digital or
analog. Digital computers process discrete numeric or symbolic values, while analog computers process continuous data signals. Since the 1940s digital computers have become by far the most common, although analog computers are still
used for some specialized purposes such as robotics and cyclotron control. Other approaches, such as pulse
computing and quantum computing are possible but are either
used for special purposes or are still experimental.
Binary versus decimal
A significant design development in digital computing was the introduction of binary as the internal numeral system. This
removed the need for complex carry mechanisms required for computers based on other numeral systems, such as the decimal system. The adoption of binary resulted in simplified designs for implementing
arithmetic functions and logic operations.
Programmability
The ability to program a computer - provide it with a set of
instructions for execution- without physically reconfiguring the machine is a fundamental design feature of most computers. This
feature was significantly extended when machines were developed that could dynamically control the flow of execution of the program. This allowed computers
to control the order in which the program of instructions was executed based on data calculated by the program as it executed.
This major design advance was dramatically simplified by the introduction of binary arithmetic which can be used to represent
various logic operations.
Storage
During the course of a calculation it is often necessary to store intermediate values for use in later calculations. The
performance of many computers is largely dictated by the speed with which they can read and write values to and from this
memory, and the overall capacity of the memory. Originally memory was used only for intermediate values but in the 1940s it was suggested that the program itself could be stored in this way. This advance led to
the development of the first stored-program computers of the type used today.
Classification by capability
Perhaps the best way to classify the various types of computing device is by their intrinsic capabilities rather than their
usage, implementation technology or design features. Computers can be subdivided into three main types based on capability:
Single-Purpose devices that can compute only one function (e.g. The Antikythera Mechanism 87 BC, and Lord Kelvin's Tide predictor 1876),
Special-Purpose devices that can compute a limited range of functions (e.g. Charles Babbage's Difference Engine No 1. 1832 and Vannevar Bush's Differential
analyser 1932), and General-Purpose devices of the type used today. Historically the
word computer has been used to describe all these types of machine but modern colloquial usage usually restricts the term to
general-purpose machines.
General-purpose computers
By definition a general-purpose computer can solve any problem that can be expressed as a program and executed within the practical limits set by: the storage capacity of the computer, the size of program, the speed of program execution, and the reliability
of the machine. In 1934 Alan Turing
proved that, given the right program, any general-purpose computer could emulate the behavior of any other computer. This
mathematical proof was purely theoretical as no general-purpose
computers existed at the time. The implications of this proof are profound, for example, any existing general-purpose computer is
theoretically able to emulate, albeit slowly, any general-purpose computer that may be built in the future.
Computers with general-purpose capabilities are called Turing-complete and this status is often used as the threshold capability that defines modern computers, however, this definition is
problematic. Several
computing devices with simplistic designs have been shown to be Turing-complete. The Z3,
developed by Konrad Zuse in 1941 is the
earliest working computer that has been shown to be Turing-complete, so far (the proof was developed in 1998). While the Z3 and possibly other early devices may be theoretically
Turing-complete they are impractical as general-purpose computers. They lie in what is humorously known as the Turing Tar-Pit (http://catb.org/~esr/jargon/html/T/Turing-tar-pit.html) - "a place where anything is possible
but nothing of interest is practical" (See The Jargon File (http://catb.org/~esr/jargon/)).
Modern computers are more than theoretically general-purpose; they are also practical general-purpose tools. The modern,
digital, electronic, general-purpose computer was developed, by many contributors, over an extended period from the mid 1930s to the late 1940s, during this period many
experimental machines were built that were possibly Turing-complete (ABC, ENIAC, Harvard Mk I, Colossus etc see the History of computing hardware). All these
machines have been claimed, at one time or another, as the first computer, but they all had limited utility as general-purpose
problem-solving devices and their designs have been discarded.
Stored-program computers
During the late 1940s the first design for a Stored-Program Computer was developed and
documented (see The first draft) at the Moore School of Electrical Engineering at The University of Pennsylvania. The approach described by
this document has become known as the Von Neumann
architecture, after its only named author Jon von Neumann
although others at the Moore School essentially invented the design. The Von Neumann architecture solved problems inherent in the design of the ENIAC, which was then under construction, by storing the machines program in its own memory. Von Neumann made
the design available to other researchers shortly after the ENIAC was annouced in 1946. Plans were developed to implemented the
design at the Moore School in a machine called the EDVAC. The EDVAC was not operational until 1953 due to technical difficulties in
implementing a reliable memory. Other research institutes, who had obtained copies of the design, solved the considerable
technical problems of implementing a working memory before the Moore School team and implemented their own stored-program
computers. In order of first successful operation the first 5 stored-program computers, that implemented the main features of the
von Neumann Architecture were:
The Stored Program design defined by the von-Neumann Architecture finally allowed computers to readily exploit their
general-purpose potential. By storing the computer's program in its own memory it became possible to rapidly "jump" from one
instruction to another based on the result of evaluating a condition defined within the program. This condition usually evaluated
data values calculated by the program and allowed programs to become highly dynamic. The design also supported the ability to
automatically re-write the program as it executed - a powerful feature that must be used carefully. These features are
fundamental to the way modern computers work.
To be precise, most modern computers are binary, electronic, stored-program, general-purpose, computing devices.
Special-purpose computers
The special-purpose computers that were popular in the 1930s and early 1940s have not been completely replaced by General-Purpose computers. As the cost and size of
computers has fallen and their capabilities have increased it has become cost effective to use them for special-purpose
applications. Many domestic and industrial devices including; mobile telephones, video recorders, automotive ignition systems,
etc now contain special-purpose computers. In some cases these computers are Turing-complete (Video Games, PDAs) but many are programmed once in the factory and
only seldom, if ever, reprogrammed. The program that these devices execute is often contained in a Read Only Memory (ROM chip) which would need to be replaced to change the
operation of the machine. Computers embedded inside other devices are commonly referred to as microcontrollers or embedded computers.
Single-purpose computers
Single-purpose computers were the earliest form of computing device. Given some inputs they could calculate the result of the
single function that was implemented by their
mechanism. General-Purpose computers have almost completely replaced single-purpose computers and in doing so have created a
completely new field of human endeavor - Software
Development. General-purpose computers must be programmed with a set of instructions specific to the task they are required
to perform and these instructions are collectively know as computer
software. The design of single-purpose computing devices and many special-purpose computing devices is now a conceptual
exercise that consists solely of designing software.
Classification by type of operation
Computers may be classified according to the way they are operated by the users. Two main types exist: batch processing and interactive
processing.
Computer applications
The first electronic digital computers, with their large size and cost, mainly performed scientific calculations, often to
support military objectives. The ENIAC was originally designed to calculate ballistics
firing tables for artillery, but it was also used to calculate neutron
cross-sectional densities to help in the design of the hydrogen bomb.
This calculation, performed in December, 1945
through January, 1946 and involving over a
million punch cards of data, showed the
design then under consideration would fail. (Many of the most powerful supercomputers available today are also used for nuclear weapons simulations.) The CSIR Mk I, the first Australian stored-program computer,
evaluated rainfall patterns for the catchment area of the Snowy Mountains Scheme, a large hydroelectric generation project. Others were used in cryptanalysis, for example the world's first programmable (though not general-purpose) digital electronic
computer, Colossus, built during World War II. Despite this early focus of scientific applications, computers were quickly used in other
areas.
From the beginning, stored program computers were applied to business problems. The LEO, a stored program-computer built by J. Lyons and
Co. in Britain, was operational and being used for inventory management and other
purposes 3 years before IBM built their first commercial stored-program computer. Continual
reductions in the cost and size of computers saw them adopted by ever-smaller organizations. And with the invention of the
microprocessor in the 1970s, it
became possible to produce inexpensive computers. In the 1980s, personal computers became popular for many tasks, including book-keeping, writing and printing documents, calculating forecasts and other
repetitive mathematical tasks involving spreadsheets.
The Internet
In the 1970s, computer engineers at research institutions throughout the US began to link their computers together using
telecommunications technology. This effort was funded by ARPA, and the computer
network that it produced was called the ARPANET. The technologies that made the
Arpanet possible spread and evolved. In time, the network spread beyond academic institutions and became known as the Internet. In the 1990s, the development of World Wide Web technologies enabled non-technical people to use the internet,
and it grew rapidly to become a global
communications medium.
How computers work
While the technologies used in computers have changed dramatically since the first electronic, general-purpose, computers of
the 1940s (see History of computing hardware for more details), most still use the von Neumann architecture.
The von Neumann architecture describes a
computer with four main sections: the Arithmetic and Logic Unit (ALU), the control circuitry, the memory, and the input and output devices (collectively termed I/O). These parts are interconnected by a
bundle of wires (a "bus") and are usually driven by a timer or clock (although other events could drive the control
circuitry).
Memory
In this system, memory is a sequence of numbered cells,
each containing a small piece of information. The information may be an instruction to tell the computer what to do. The cell may contain data
that the computer needs to perform the instruction. Any cell may contain either, and indeed what is at one time data might be
instructions later.
In general, the contents of a memory cell can be changed at any time - it is a scratchpad rather than a stone tablet.
The size of each cell, and the number of cells, varies greatly from computer to computer, and the technologies used to
implement memory have varied greatly - from electromechanical relays, to mercury-filled
tubes (and later springs) in which acoustic pulses were formed, to matrices of permanent magnets, to individual transistors, to integrated circuits with millions of capacitors on a
single chip.
The arithmetic and logical unit, or ALU, is the device that performs elementary operations such as arithmetic operations (addition, subtraction, and so on), logical operations (AND, OR, NOT), and comparison operations (for
example, comparing the contents of two bytes for equality). This unit is where the "real
work" is done.
The control unit keeps track of which bytes in memory contain the
current instruction that the computer is performing, telling the ALU what operation to perform and retrieving the information
(from memory) that it needs to perform it, and transfers the result back to the appropriate memory location. Once that occurs,
the control unit goes to the next instruction (typically located in the next slot (memory address), unless the instruction is a jump
instruction informing the computer that the next instruction is located in another location). When referrring to memory, the
current instruction may use various addressing modes to determine the
relevant address in memory.
Input and output
The I/O allows the computer to obtain information from the
outside world, and send the results of its work back there. There is an broad range of I/O devices, from the familiar keyboards, monitors and floppy disk drives, to the more unusual
such as webcams.
What all input devices have in common is that they encode (convert) information of
some type into data which can further be processed by the digital computer system. Output
devices on the other hand, decode the data into information which can be understood by
the computer user. In this sense, a digital computer system is an example of a data processing system.
Instructions
The machine set of instructions are not the rich instructions of a human language. A computer only has a limited number of
well-defined, simple instructions. Computers can perform two tasks. They can count and they can compare. Typical sorts of
instructions supported by most computers are "copy the contents of cell 123, and place the copy in cell 456", "add the contents
of cell 666 to cell 042, and place the result in cell 013", "if the contents of cell 999 are 0, your next instruction is at cell
345".
Instructions are represented within the computer as binary code - a base two system
of counting. The code for "copy" might be 001, for example. The particular instruction set that a specific computer supports is
known as that computer's machine language. In practice, people do
not normally write the instructions for computers directly in machine language but rather use a "high level" programming language which is then translated into the machine
language automatically by special computer programs (interpreters and compilers). Some programming
languages map very closely to the machine language, such as assembler (low level
languages); at the other end, languages like Prolog are based on abstract principles
far removed from the details of the machine's actual operation (high level languages).
Architecture
Contemporary computers put the ALU and control unit into a single integrated
circuit known as the Central Processing Unit or
CPU. Typically, the computer's memory is located on a few small integrated circuits near the CPU. The overwhelming majority of
the computer's mass is either ancillary systems (for instance, to supply electrical power) or I/O devices.
Some larger computers differ from the above model in one major respect - they have multiple CPUs and control units working
simultaneously. Additionally, a few computers, used mainly for research purposes and scientific computing, have differed
significantly from the above model, but they have found little commercial application, because their programming model has not
yet standardized.
The functioning of a computer is therefore in principle quite straightforward. Typically, on each clock cycle, the computer
fetches instructions and data from its memory. The instructions are executed, the results are stored, and the next instruction is
fetched. This procedure repeats until a halt instruction is encountered.
Programs
Computer programs are simply large lists of instructions for the
computer to execute, perhaps with tables of data. Many computer programs contain millions of instructions, and many of those
instructions are executed repeatedly. A typical modern PC(in the
year 2003) can execute around 2-3 billion instructions per second. Computers do not gain
their extraordinary capabilities through the ability to execute complex instructions. Rather, they do millions of simple
instructions arranged by people known as "programmers." Good programmers
develop sets of instructions to do common tasks (for instance, draw a dot on screen) and then make those sets of instructions
available to other programmers.
Nowadays, most computers appear to execute several programs at the same time. This is usually referred to as multitasking. In reality, the CPU executes instructions from one program, then
after a short period of time, it switches to a second program and executes some of its instructions. This small interval of time
is often referred to as a time slice. This creates the illusion of multiple programs being executed simultaneously by sharing the
CPU's time between the programs. This is similar to how a movie is simply a rapid succession of still frames. The operating system is the program that usually controls this time
sharing.
Operating system
A computer will always need at least one program running at all times to operate. Under normal operation (in the typical
general-purpose computer), this program is the operating system
(OS). The operating system decides which programs are run, when, and what resources (such as memory or input/output - I/O)
they get to use. The operating system also provides a layer of abstraction over the hardware, and gives access by providing
services to other programs, such as code ("drivers") which allow programmers to write programs for a machine without needing to
know the intimate details of all attached electronic devices.
Most operating systems that have hardware abstraction layers also provide a standardized user interface.
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