device capable of performing a series of arithmetic or logical operations. A computer is distinguished from a calculating machine, such as an electronic calculator, by being able to store a computer program (so that it can repeat its operations and make logical decisions), by the number and complexity of the operations it can perform, and by its ability to process, store, and retrieve data without human intervention. Computers developed along two separate engineering paths, producing two distinct types of computer—analog and digital. An analog computer operates on continuously varying data; a digital computer performs operations on discrete data.

Computers are categorized by both size and the number of people who can use them concurrently. Supercomputers are sophisticated machines designed to perform complex calculations at maximum speed; they are used to model very large dynamic systems, such as weather patterns. Mainframes, the largest and most powerful general-purpose systems, are designed to meet the computing needs of a large organization by serving hundreds of computer terminals at the same time. Minicomputers, though somewhat smaller, also are multiuser computers, intended to meet the needs of a small company by serving up to a hundred terminals. Microcomputers, computers powered by a microprocessor, are subdivided into personal computers and workstations, the latter typically incorporating RISC processors. Although microcomputers were originally single-user computers, the distinction between them and minicomputers has blurred as microprocessors have become more powerful. Linking multiple microcomputers together through a local area network or by joining multiple microprocessors together in a parallel-processing system has enabled smaller systems to perform tasks once reserved for mainframes, and the techniques of grid computing have enabled computer scientists to utilize the unemployed processing power of connected computers.

Advances in the technology of integrated circuits have spurred the development of smaller and more powerful general-purpose digital computers. Not only has this reduced the size of the large, multi-user mainframe computers—which in their early years were large enough to walk through—to that of large pieces of furniture, but it has also made possible powerful, single-user personal computers and workstations that can sit on a desktop. These, because of their relatively low cost and versatility, have largely replaced typewriters in the workplace and rendered the analog computer inefficient.

Analog Computers

An analog computer represents data as physical quantities and operates on the data by manipulating the quantities. It is designed to process data in which the variable quantities vary continuously (see analog circuit); it translates the relationships between the variables of a problem into analogous relationships between electrical quantities, such as current and voltage, and solves the original problem by solving the equivalent problem, or analog, that is set up in its electrical circuits. Because of this feature, analog computers were especially useful in the simulation and evaluation of dynamic situations, such as the flight of a space capsule or the changing weather patterns over a certain area. The key component of the analog computer is the operational amplifier, and the computer's capacity is determined by the number of amplifiers it contains (often over 100). Although analog computers are commonly found in such forms as speedometers and watt-hour meters, they largely have been made obsolete for general-purpose mathematical computations and data storage by digital computers.

Digital Computers

A digital computer is designed to process data in numerical form (see digital circuit); its circuits perform directly the mathematical operations of addition, subtraction, multiplication, and division. The numbers operated on by a digital computer are expressed in the binary system; binary digits, or bits, are 0 and 1, so that 0, 1, 10, 11, 100, 101, etc., correspond to 0, 1, 2, 3, 4, 5, etc. Binary digits are easily expressed in the computer circuitry by the presence (1) or absence (0) of a current or voltage. A series of eight consecutive bits is called a “byte”; the eight-bit byte permits 256 different “on-off” combinations. Each byte can thus represent one of up to 256 alphanumeric characters, and such an arrangement is called a “single-byte character set” (SBCS); the de facto standard for this representation is the extended ASCII character set. Some languages, such as Japanese, Chinese, and Korean, require more than 256 unique symbols. The use of two bytes, or 16 bits, for each symbol, however, permits the representation of up to 65,536 characters or ideographs. Such an arrangement is called a “double-byte character set” (DBCS); Unicode is the international standard for such a character set. One or more bytes, depending on the computer's architecture, is sometimes called a digital word; it may specify not only the magnitude of the number in question, but also its sign (positive or negative), and may also contain redundant bits that allow automatic detection, and in some cases correction, of certain errors (see code; information theory). A digital computer can store the results of its calculations for later use, can compare results with other data, and on the basis of such comparisons can change the series of operations it performs. Digital computers are used for reservations systems, scientific investigation, data-processing and word-processing applications, desktop publishing, electronic games, and many other purposes.

Processing of Data

The operations of a digital computer are carried out by logic circuits, which are digital circuits whose single output is determined by the conditions of the inputs, usually two or more. The various circuits processing data in the computer's interior must operate in a highly synchronized manner; this is accomplished by controlling them with a very stable oscillator, which acts as the computer's “clock.” Typical computer clock rates range from several million cycles per second to several hundred million, with some of the fastest computers having clock rates of about a billion cycles per second. Operating at these speeds, digital computer circuits are capable of performing thousands to trillions of arithmetic or logic operations per second, thus permitting the rapid solution of problems that would be impossible for a human to solve by hand. In addition to the arithmetic and logic circuitry and a small number of registers (storage locations that can be accessed faster than main storage and are used to hold the intermediate results of calculations), the heart of the computer—called the central processing unit, or CPU—contains the circuitry that decodes the set of instructions, or program, and causes it to be executed.

Storage and Retrieval of Data

Associated with the central processing unit is the storage unit, or memory, where results or other data are stored for periods of time ranging from a small fraction of a second to days or weeks before being retrieved for further processing. Once made up of vacuum tubes and later of small doughnut-shaped ferromagnetic cores strung on a wire matrix, main storage now consists of integrated circuits, each of which contains thousands of semiconductor devices. Where each vacuum tube or core represented one bit and the total memory of the computer was measured in thousands of bytes (or kilobytes, KB), each semiconductor device now represents millions of bytes (or megabytes, MB) and the total memory of mainframe computers is measured in billions of bytes (or gigabytes, GB). Random-access memory (RAM), which both can be read from and written to, is lost each time the computer is turned off. Read-only memory (ROM), which cannot be written to, maintains its content at all times and is used to store the computer's control information.

Programs and data that are not currently being used in main storage can be saved on auxiliary storage, or external storage. Although punched paper tape and punched cards once served this purpose, the major materials used today are magnetic tape and magnetic disks, which can be read from and written to, and two types of optical disks, the compact disc (CD) and its successor the digital versatile disc (DVD). DVD is an improved optical storage technology capable of storing vastly greater amounts of data than the CD technology. CD–Read-Only Memory (CD-ROM) and DVD–Read-Only Memory (DVD-ROM) disks can only be read—the disks are impressed with data at the factory but once written cannot be erased and rewritten with new data. The latter part of the 1990s saw the introduction of new optical storage technologies: CD-Recordable (CD-R) and DVD-Recordable (DVD-R), optical disks that can be written to by the computer to create a CD-ROM or DVD-ROM, but can be written to only once; and CD-ReWritable (CD-RW), DVD-ReWritable (DVD-RW and DVD+RW), and DVD–Random Access Memory (DVD-RAM), disks that can be written to multiple times.

When compared to semiconductor memory, magnetic and optical storage is less expensive, is not volatile (i.e., data is not lost when the power to the computer is shut off), and provides a convenient way to transfer data from one computer to another. Thus operating instructions or data output from one computer can be stored away from the computer and then retrieved either by the same computer or another. In a system using magnetic tape the information is stored by a specially designed tape recorder somewhat similar to one used for recording sound. In magnetic and optical disk systems the principle is the same except that the magnetic or optical medium lies in a path, or track, on the surface of a disk. The disk drive also contains a motor to spin the disk and a magnetic or optical head or heads to read and write the data to the disk. Drives take several forms, the most significant difference being whether the disk can be removed from the drive assembly.

Removable magnetic disks are most commonly made of mylar enclosed in a paper or plastic holder. These floppy disks have varying capacities, with very high density disks holding 250 MB—more than enough to contain a dozen books the size of Tolstoy's Anna Karenina. Compact discs can hold many hundreds of megabytes, and are used, for example, to store the information contained in an entire multivolume encyclopedia or set of reference works, and DVD disks can hold ten times as much as that. Nonremovable disks are made of metal and arranged in spaced layers. They can hold more data and can read and write data much faster than floppies.

Data are entered into the computer and the processed data made available via input/output devices. All auxiliary storage devices are used as input/output devices. For many years, the most popular input/output medium was the punched card. Although this is still used, the most popular input device is now the computer terminal and the most popular output device is the high-speed printer. Human beings can directly communicate with the computer through computer terminals, entering instructions and data by means of keyboards much like the ones on typewriters, by using a pointing device such as a mouse, trackball, or touchpad, or by speaking into a microphone that is connected to computer running voice-recognition software. Responses may be displayed on a cathode-ray tube, liquid-crystal display, or printer. The CPU, main storage, auxiliary storage, and input/output devices collectively make up a system.

Sharing the Computer's Resources

Generally, the slowest operations that a computer must perform are those of transferring data, particularly when data is received from or delivered to a human being. The computer's central processor is idle for much of this period, and so two similar techniques are used to use its power more fully.

Time sharing, used on large computers, allows several users at different terminals to use a single computer at the same time. The computer performs part of a task for one user, then suspends that task to do part of another for another user, and so on. Each user only has the computer's use for a fraction of the time, but the task switching is so rapid that most users are not aware of it. Most of the tens of millions of computers in the world are stand-alone, single-user devices known variously as personal computers or workstations. For them, multitasking involves the same type of switching, but for a single user. This permits a user, for example, to have one file printed and another sorted while editing a third in a word-processing session. Such personal computers can also be linked together in a network, where each computer is connected to others, usually by wires or coaxial cables, permitting all to share resources such as printers, modems, and hard-disk storage devices.

Computer Programs and Programming Languages

Before a computer can be used to solve a given problem, it must first be programmed, that is, prepared for solving the problem by being given a set of instructions, or program. The various programs by which a computer controls aspects of its operations, such as those for translating data from one form to another, are known as software, as contrasted with hardware, which is the physical equipment comprising the installation. In most computers the moment-to-moment control of the machine resides in a special software program called an operating system, or supervisor. Other forms of software include assemblers and compilers for programming languages and applications for business and home use (see computer program). Software is of great importance; the usefulness of a highly sophisticated array of hardware can be severely compromised by the lack of adequate software.

Each instruction in the program may be a simple, single step, telling the computer to perform some arithmetic operation, to read the data from some given location in the memory, to compare two numbers, or to take some other action. The program is entered into the computer's memory exactly as if it were data, and on activation, the machine is directed to treat this material in the memory as instructions. Other data may then be read in and the computer can carry out the program to solve the particular problem.

Since computers are designed to operate with binary numbers, all data and instructions must be represented in this form; the machine language, in which the computer operates internally, consists of the various binary codes that define instructions together with the formats in which the instructions are written. Since it is time-consuming and tedious for a programmer to work in actual machine language, a programming language, or high-level language, designed for the programmer's convenience, is used for the writing of most programs. The computer is programmed to translate this high-level language into machine language and then solve the original problem for which the program was written. Certain high-level programming languages are universal, varying little from machine to machine.

Development of Computers

Although the development of digital computers is rooted in the abacus and early mechanical calculating devices, Charles Babbage is credited with the design of the first modern computer, the “analytical engine,” during the 1830s. American scientist Vannevar Bush built a mechanically operated device, called a differential analyzer, in 1930; it was the first general-purpose analog computer. John Atanassoff constructed the first semielectronic digital computing device in 1939.

The first fully automatic calculator was the Mark I, or Automatic Sequence Controlled Calculator, begun in 1939 at Harvard by Howard Aiken, while the first all-purpose electronic digital computer, ENIAC (Electronic Numerical Integrator And Calculator), which used thousands of vacuum tubes, was completed in 1946 at the Univ. of Pennsylvania. UNIVAC (UNIVersal Automatic Computer) became (1951) the first computer to handle both numeric and alphabetic data with equal facility; this was the first commercially available computer.

First-generation computers were supplanted by the transistorized computers (see transistor) of the late 1950s and early 60s, second-generation machines that were smaller, used less power, and could perform a million operations per second. They, in turn, were replaced by the third-generation integrated-circuit machines of the mid-1960s and 1970s that were even smaller and were far more reliable. The 1980s and 90s were characterized by the development of the microprocessor and the evolution of increasingly smaller but powerful computers, such as the personal computer and personal digital assistant, which ushered in a period of rapid growth in the computer industry.

Bibliography

See S. G. Nash, A History of Scientific Computing (1990); D. I. A. Cohen, Introduction to Computer Theory (2d ed. 1996); P. Norton, Peter Norton's Introduction to Computers (2d ed. 1996); A. W. Biermann, Great Ideas in Computer Science: A Gentle Introduction (2d ed. 1997); R. L. Oakman, The Computer Triangle: Hardware, Software, People (2d ed. 1997); R. Maran, Computers Simplified (4th ed. 1998); A. S. Tanenbaum and J. R. Goodman. Structured Computer Organization (4th ed. 1998).

A computer is a machine which manipulates data according to a list of instructions.

Computers take numerous physical forms. The first devices that resemble modern computers date to the mid-20th century (around 1940 - 1941), although the computer concept and various machines similar to computers existed prior. Early electronic computers were the size of a large room, consuming as much power as several hundred modern personal computers.^[1] Modern computers are based on comparatively tiny integrated circuits and are millions to billions of times more capable while occupying a fraction of the space. ^[2] Today, simple computers may be made small enough to fit into a wrist watch and be powered from a watch battery. Personal computers in various forms are icons of the information age and are what most people think of as "a computer". However, the most common form of computer in use today is by far the embedded computer. Embedded computers are small, simple devices that are often used to control other devices — for example, they may be found in machines ranging from fighter aircraft to industrial robots, digital cameras, and even children's toys.

The ability to store and execute lists of instructions called programs makes computers extremely versatile and distinguishes them from calculators. The Church – Turing thesis is a mathematical statement of this versatility: Any computer with a certain minimum capability is, in principle, capable of performing the same tasks that any other computer can perform. Therefore, computers with capability and complexity ranging from that of a personal digital assistant to a supercomputer are all able to perform the same computational tasks given enough time and storage capacity.

History of computing

Main article: History of computing

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

Originally, the term "computer" referred to a person who performed numerical calculations (a human computer), often with the aid of a mechanical calculating device. Examples of early mechanical computing devices included the abacus, the slide rule and arguably the astrolabe and the Antikythera mechanism (which dates from about 150-100 BC). The end of the Middle Ages saw a re-invigoration of European mathematics and engineering, and Wilhelm Schickard's 1623 device was the first of a number of mechanical calculators constructed by European engineers.

However, none of those devices fit the modern definition of a computer because they could not be programmed. In 1801, Joseph Marie Jacquard made an improvement to the textile loom that used a series of punched paper cards as a template to allow his loom to weave intricate patterns automatically. The resulting Jacquard loom was an important step in the development of computers because the use of punched cards to define woven patterns can be viewed as an early, albeit limited, form of programmability.

In 1837, Charles Babbage was the first to conceptualize and design a fully programmable mechanical computer that he called "The Analytical Engine".^[3] Due to limited finance, and an inability to resist tinkering with the design, Babbage never actually built his Analytical Engine.

Large-scale automated data processing of punched cards was performed for the U.S. Census in 1890 by tabulating machines designed by Herman Hollerith and manufactured by the Computing Tabulating Recording Corporation, which later became IBM. By the end of the 19th century a number of technologies that would later prove useful in the realization of practical computers had begun to appear: the punched card, Boolean algebra, the vacuum tube (thermionic valve) and the teleprinter.

During the first half of the 20th century, many scientific computing needs were met by increasingly sophisticated analog computers, which used a direct mechanical or electrical model of the problem as a basis for computation. However, these were not programmable and generally lacked the versatility and accuracy of modern digital computers.

Defining characteristics of five first operative digital computers

Computer	Shown working	Binary	Electronic	Programmable	Turing complete
Zuse Z3	May 1941	Yes	No	By punched film stock	Yes (1998)
Atanasoff–Berry Computer	Summer 1941	Yes	Yes	No	No
Colossus	December 1943 / January 1944	Yes	Yes	Partially, by rewiring	No
Harvard Mark I – IBM ASCC	1944	No	No	By punched paper tape	Yes (1998)
ENIAC	1944	No	Yes	Partially, by rewiring	Yes
	1948	No	Yes	By Function Table ROM	Yes

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 [#wp-endnote_shannon1940_a (Shannon 1940)]. Notable achievements include:

• Konrad Zuse's electromechanical "Z machines". The Z3 (1941) was the first working machine featuring binary arithmetic, including floating point arithmetic and a measure of programmability. In 1998 the Z3 was proved to be Turing complete, therefore being the world's first operational computer.
• The non-programmable Atanasoff – Berry Computer (1941) which used vacuum tube based computation, binary numbers, and regenerative capacitor memory.
• The secret British Colossus computer (1944), which had limited programmability but demonstrated that a device using thousands of tubes could be reasonably reliable and electronically reprogrammable. It was used for breaking German wartime codes.
• The Harvard Mark I (1944), a large-scale electromechanical computer with limited programmability.
• The U.S. Army's Ballistics Research Laboratory ENIAC (1946), which used decimal arithmetic and is sometimes called the first general purpose electronic computer (since Konrad Zuse's Z3 of 1941 used electromagnets instead of electronics). Initially, however, ENIAC had an inflexible architecture which essentially required rewiring to change its programming.

Several developers of ENIAC, recognizing its flaws, came up with a far more flexible and elegant design, which came to be known as the stored program architecture or von Neumann architecture. This design was first formally described by John von Neumann in the paper "First Draft of a Report on the EDVAC", published in 1945. A number of projects to develop computers based on the stored program architecture commenced around this time, the first of these being completed in Great Britain. The first to be demonstrated working was the Manchester Small-Scale Experimental Machine (SSEM) or "Baby". However, the EDSAC, completed a year after SSEM, was perhaps the first practical implementation of the stored program design. Shortly thereafter, the machine originally described by von Neumann's paper — EDVAC — was completed but did not see full-time use for an additional two years.

Nearly all modern computers implement some form of the stored program architecture, making it the single trait by which the word "computer" is now defined. By this standard, many earlier devices would no longer be called computers by today's definition, but are usually referred to as such in their historical context. While the technologies used in computers have changed dramatically since the first electronic, general-purpose computers of the 1940s, most still use the von Neumann architecture. The design made the universal computer a practical reality.

Vacuum tube-based computers were in use throughout the 1950s, but were largely replaced in the 1960s by transistor-based devices, which were smaller, faster, cheaper, used less power and were more reliable. These factors allowed computers to be produced on an unprecedented commercial scale. By the 1970s, the adoption of integrated circuit technology and the subsequent creation of microprocessors such as the Intel 4004 caused another leap in size, speed, cost and reliability. By the 1980s, computers had become sufficiently small and cheap to replace simple mechanical controls in domestic appliances such as washing machines. Around the same time, computers became widely accessible for personal use by individuals in the form of home computers and the now ubiquitous personal computer. In conjunction with the widespread growth of the Internet since the 1990s, personal computers are becoming as common as the television and the telephone and almost all modern electronic devices contain a computer of some kind.

Stored program architecture

Main articles: Computer program and Computer programming

The defining feature of modern computers which distinguishes them from all other machines is that they can be programmed. That is to say that a list of instructions (the program) can be given to the computer and it will store them and carry them out at some time in the future.

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      #0,sum     ; set sum to 0
        mov      #1,num     ; set num to 1
loop:   add      num,sum    ; add num to sum
        add      #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.^[4]

However, computers cannot "think" for themselves in the sense that they only solve problems in exactly the way they are programmed to. An intelligent human faced with the above addition task might soon realize that instead of actually adding up all the numbers one can simply use the equation

and arrive at the correct answer (500,500) with little work.^[5] In other words, a computer programmed to add up the numbers one by one as in the example above would do exactly that without regard to efficiency or alternative solutions.

Programs

In practical terms, a computer program might include anywhere from a dozen instructions to many millions of instructions for something like a word processor or a web browser. A typical modern computer can execute billions of instructions every second and nearly never make a mistake over years of operation.

Large computer programs may take teams of computer programmers years to write and the probability of the entire program having been written completely in the manner intended is unlikely. Errors in computer programs are called bugs. Sometimes bugs are benign and do not affect the usefulness of the program, in other cases they might cause the program to completely fail (crash), in yet other cases there may be subtle problems. Sometimes otherwise benign bugs may be used for malicious intent, creating a security exploit. 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.^[6]

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 instructions) can be represented as lists of numbers and can themselves be manipulated inside the computer just as if they were 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 this technique was used with many early computers,^[7] it is extremely tedious 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. 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 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.^[8]

Though considerably easier than in machine language, writing long programs in assembly language is often difficult and error prone. Therefore, most complicated programs are written in more abstract high-level programming languages that are able to express the needs of the computer 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.^[9] Since high level languages are more abstract than assembly language, it is 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.

The task of developing large software systems is an immense intellectual effort. It has proven, historically, to be very difficult to produce software with an acceptably high reliability, on a predictable schedule and budget. The academic and professional discipline of software engineering concentrates specifically on this problem.

Example

Suppose a computer is being employed to drive a traffic light. A simple stored program might say:

1. Turn off all of the lights
2. Turn on the red light
3. Wait for sixty seconds
4. Turn off the red light
5. Turn on the green light
6. Wait for sixty seconds
7. Turn off the green light
8. Turn on the yellow light
9. Wait for two seconds
10. Turn off the yellow light
11. Jump to instruction number (2)

With this set of instructions, the computer would cycle the light continually through red, green, yellow and back to red again until told to stop running the program.

However, suppose there is a simple on/off switch connected to the computer that is intended be used to make the light flash red while some maintenance operation is being performed. The program might then instruct the computer to:

1. Turn off all of the lights
2. Turn on the red light
3. Wait for sixty seconds
4. Turn off the red light
5. Turn on the green light
6. Wait for sixty seconds
7. Turn off the green light
8. Turn on the yellow light
9. Wait for two seconds
10. Turn off the yellow light
11. If the maintenance switch is NOT turned on then jump to instruction number 2
12. Turn on the red light
13. Wait for one second
14. Turn off the red light
15. Wait for one second
16. Jump to instruction number 11

In this manner, the computer is either running the instructions from number (2) to (11) over and over or its running the instructions from (11) down to (16) over and over, depending on the position of the switch.^[10]

How computers work

Main articles: Central processing unit and Microprocessor

A general purpose computer has four main sections: the arithmetic and 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.

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.

Control unit

Main articles: CPU design and Control unit

The control unit (often called a control system or central controller) directs the various components of a computer. It reads and interprets (decodes) instructions in the program one by one. The control system decodes each instruction and turns it into a series of control signals that operate the other parts of the computer.^[11] 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.^[12]

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.
2. Decode the numerical code for the instruction into a set of commands or signals for each of the other systems.
3. Increment the program counter so it points to the next instruction.
4. 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.
5. Provide the necessary data to an ALU or register.
6. If the instruction requires an ALU or specialized hardware to complete, instruct the hardware to perform the requested operation.
7. Write the result from the ALU back to a memory location or to a register or perhaps an output device.
8. 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).

It is noticeable that 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 that runs a microcode program that causes all of these events to happen.

Arithmetic/logic unit (ALU)

Main article: Arithmetic logic unit

The ALU is capable of performing two classes of operations: arithmetic and logic.

The set of arithmetic operations that a particular ALU supports may be limited to adding and subtracting or might include multiplying or dividing, trigonometry functions (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 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 both for creating complicated conditional statements and processing boolean logic.

Superscalar computers contain multiple ALUs so that they can process several instructions at the same time. Graphics processors and computers with SIMD and MIMD features often provide ALUs that can perform arithmetic on vectors and matrices.

Memory

Main article: Computer 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 up to the software 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; 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 as long as it can be somehow represented in numerical form. 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. Since 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 memory or 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, so 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 is erased when the power to the computer is turned off while 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 software required to perform the task may be stored in ROM. Software that is 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 by retaining data when turned off but being rewritable like RAM. However, flash memory is typically much slower than conventional ROM and RAM so its use is restricted to applications where high speeds are not required.^[13]

In more sophisticated computers there may be one or more RAM cache memories which are slower than registers but fa