Computer is a device that processes information with astonishing speed and accuracy. Computers process information by helping to create it and by displaying it, storing it, reorganizing it, calculating with it, and communicating it to other computers. Computers can process numbers, words, still pictures, moving pictures, and sounds. The most powerful computers can perform tens of billions of calculations per second.

The computer has changed the way we work, learn, communicate, and play. Virtually every kind of organization throughout the world conducts business with computers. Students, teachers, and research scientists use the computer as a learning tool. Millions of individuals and organizations communicate with one another over a network of computers called the Internet. Computer games entertain people of all ages.

Almost all computers are electronic digital computers. They are electronic in their use of electric current (a flow of electric charge) to carry information. They are digital in that they process information as units of electric charge representing numbers. The word digital means having to do with numbers.

To enable a computer to process information that is not numerical--such as words, pictures, or sounds--the computer or some other device must first digitize that information. A device digitizes information by translating it into charges that represent numbers. After the computer processes the digitized information by working with the charges, the computer or a device connected to the computer translates its results back into their original form.

Thus, an artist might use a machine called a scanner to digitize a photograph. The artist would next process the resulting electric charges in a computer to change the photograph--perhaps to add a border. The artist would then use a printer connected to the computer to produce a copy of the altered photo.

Digital computers are one of two general kinds of computers. The other kind are calculating devices called analog computers. An analog computer represents amounts with physical quantities, such as distances along a scale, rather than with numbers. The remainder of this article deals with digital computers.

The technology of computer hardware (the physical parts of computer systems) has advanced tremendously since 1946, when the first electronic digital computer was built. That machine filled a huge room. Today, a single microprocessor, a device the size of a fingernail, can do the same work as that pioneering machine.

The technology of software (programs, or sets of computer instructions and information) is also advancing rapidly. Early users of computers wrote their ownsoftware. Today, most users buy programs created by companies that specialize in writing software. Hundreds of thousands of different programs are available for businesses and individuals.

Because of advances in hardware and software, the price of computing has dropped sharply. As a result, the number of computers in operation has risen rapidly ever since the first commercial digital computers were manufactured in the 1950's. More than 10,000 computers were in operation worldwide by 1961. Ten years later, the number exceeded 100,000. By 1990, about 100 million computers were running. By the mid-1990's, the number had reached about 200 million.

Kinds of computers

Computers vary widely in size, speed, and ability. They may be grouped into four categories: (1) personal computers, (2) mainframes, (3) dedicated computers, and (4) embedded computers.

Personal computers are used by one person at a time. The largest personal computers, or PC's, can fit on a desktop. Some of these machines have more than one microprocessor. Besides the primary processor, which is a general-purpose device, a PC may have one or more co-processors to handle special kinds of work. Some machines, for example, have math co-processors. Others have graphics co-processors to help process photographs and other illustrations. Still others have sound co-processors.

Large and middle-sized businesses commonly use PC's in client-server networks. A network consists of a group of computers connected by telephone lines or other communications cables. In a client-server network, a powerful central computer distributes information to a number of PC's. The central computer is called the server. The PC's are the clients but are usually referred to as workstations or simply PC's. The central computer may be a PC, a machine much like a PC with extra-large storage capacity, or an even larger type of computer.

The server also stores all the network's essential information. In a typical network, individuals operating the workstations obtain copies of information from the server. The workers process this information, then send copies of the processed information back to the server. In most cases, the workers are in the same office as the server. But a large company may have a network that connects its branch offices to the main office. In addition, many employees work on computers at home and send their processed data to the company server. This use of computers is known as telecommuting.

Office workers also use PC's that are not connected to a network. These machines are used for such tasks as word processing, performing financial calculations, and organizing and sorting bodies of information called databases. Home users of computers do some of the same kinds of work on personal data. They use word-processing programs for private correspondence, financial software for household budgets, and database management programs for address lists and recipes. Individuals also use their home computers to play games and to communicate over the Internet.

Small, portable PC's are popular with people who often work away from their desks. The portables include laptop computers, which can be held on the lap; notebook computers, which are about the size of a loose-leaf notebook; and palmtop, or handheld, computers, which can be operated while held in the hand. Laptop and notebook computers have the same power as desktop computers. Palmtop computers have less power but still provide some advanced computing capabilities. For example, they can process household financial data.

Many users of personal computers do not use the term PC to refer to all such machines. Instead, they apply the term to only two groups of machines. The first group consists of machines made by International Business Machines Corporation (IBM). In the second group are IBM clones, similar machines produced by other companies using technology developed by IBM. This usage comes from the name of IBM's first personal computer, the PC, introduced in 1981. The usage distinguishes IBM machines and IBM clones from Macintosh personal computers, made by Apple Computer, Inc. Macintoshes are often referred to as Macs.

Mainframes are the fastest computers, and they use the largest storage systems. As a result, they can solve more complex problems and handle more information than can any other category of computer. Mainframes are also the largest computers. Most of them are housed in several large cabinets.

Some mainframes do a single job, such as copying and storing the information generated by a laboratory experiment. Others perform many different tasks.

Hundreds of people may be logged on (running programs on) a large mainframe at the same time. Such users are said to be time-sharing. In this situation, the mainframe appears to be processing information for all the users every instant. However, the computer is actually switching rapidly from program to program, doing a small amount of work on one, then hurrying to another.

The fastest mainframes are called supercomputers. These machines are used for major projects, such as the design of aircraft, the study of weather systems, and the design and analysis of drug molecules. Supercomputers are few in number because they are extremely expensive. Supercomputer users--mostly scientists and engineers at large scientific installations--sometimes run programs by means of long-distance telephone networks.

The fastest supercomputers are parallel computers. They are fast because they have dozens or even hundreds of microprocessors that operate at the same time. Each processor works on a separate piece of a program. Minicomputers and superminis have many of the capabilities of mainframes, but they are smaller, less expensive, and less powerful.

Dedicated computers are special-purpose machines. They include video game units and word processors. Video game units come in a range of sizes. The smallest are handheld, battery-operated toys. A larger unit for home use sits on the floor or a table and is connected to a television set. A player controls some of these home units through a lever called a joystick. The largest units stand on the floor in game rooms called arcades and other commercial establishments.

Word processors are computers that mainly type, edit, and print letters and other documents. Some of them can also run simple financial programs. Many word processors resemble electronic typewriters, with a small screen built in above the keyboard. Word processors are much less expensive than PC's.

Embedded computers are control units that are built into the devices they control. Virtually all embedded computers are single microprocessors. Such devices as digital wristwatches, telephones, videotape recorders, and the ignition systems of automobiles contain embedded computers.

Commercial airliners and military planes carry embedded computers that help control the aircraft. Embedded computers also control the movements of industrial robots, maintain the orbits of satellites, and guide modern weapons systems, such as missiles, to their targets.

Parts of computers

A computer is made up of many components (parts). There are five main types of components: (1) microprocessors, (2) memory chips, (3) input devices, (4) storage devices, and (5) output devices. The microprocessor, also known as the central processing unit (CPU), does the actual computing. Memory chips hold data and processing instructions for use by the microprocessor. The computer receives data through input devices, such as a keyboard. Storage devices, which include disks and tapes, hold data and instructions for transfer to memory. Output devices, such as a televisionlike monitor, show results of the computer work.

In desktop PC's, certain components are housed in a box often called simply the computer. Because these components include the microprocessor, the box is sometimes referred to as the CPU. The remaining components are separately housed units called peripherals.

Microprocessors control computer systems and process information. The information is encoded as units of electric charge that represent numbers.

The microprocessor consists of thousands, or even millions, of switches called transistors, other electronic devices, and wires. These parts are arranged in circuits and built into a chip, usually made of silicon, that is no larger than a fingernail.

There are two groups of circuits: (1) the control unit and (2) the digital logic unit, sometimes called the arithmetic/logic unit. The control unit directs and coordinates the operations of the entire computer according to instructions stored in the memory. The digital logic unit performs the computations. Almost all microprocessor chips also contain memory devices.

Computer code. All data enter the processor as electric charges that represent numbers. The computer uses only two levels of charge. One level represents the digit (numeral, or number symbol) 0, and the other level represents 1. Thus, the computer uses the binary numeration system. The word binary comes from a Latin word meaning two at a time. In the binary system, a 0 or a 1 by itself is called a bit, which is short for binary digit.

Number symbols in the binary system are much different from those in the decimal system, which we use in almost all other cases. For example, the symbol 10 in the binary system corresponds to 2 in the decimal system.

In the computer, different combinations of bit charges represent numbers, letters, and portions of pictures and sounds--all the data that the computer processes and all the instructions used to process the data. The computer uses combinations of eight bits, called bytes. For example, in a common code known as the American Standard Code for Information Interchange (ASCII), the byte 01000001 represents the letter A. Other bytes in ASCII represent lower-case a and all the other letters, all the decimal digits, and certain punctuation marks and mathematical symbols.

The microprocessor processes the bit charges by switching them from circuit to circuit. Each of a computer's thousands or millions of tiny electronic circuits operates much like an ordinary light switch. When a circuit is off, it corresponds to the binary digit 0. When a circuit is on, it corresponds to the digit 1. Binary digits, like decimal numbers, can be added, subtracted, multiplied, and divided. Thus, a computer can perform all the basic arithmetic operations.

Computer information is measured in multiples of bytes. A kilobyte equals 1,024 bytes; a megabyte, 1,048,576 bytes; and a gigabyte, 1,073,741,824 bytes. For simplicity, 1 kilobyte, 1 megabyte, and 1 gigabyte are often said to equal 1,000, 1 million, and 1 billion (1,000 million) bytes, respectively.

The control unit directs and coordinates computer operations according to instructions stored in the memory. Each set of instructions is expressed as a binary operation code. This code also indicates where data for each processing operation are stored in the memory. The control unit interprets the instructions and relays commands to the logic unit. The control unit also regulates the flow of data between the memory and the digital logic unit and routes processed information to output or storage devices.

The digital logic unit carries out the computer's mathematical and logical processes. In this unit, circuits called registers temporarily store data from the memory. To carry out an arithmetical calculation, a group of bit charges travels from a register through a wire to another circuit. The answer comes out on a wire at the other end of this circuit.

The logic unit has a number of kinds of basic circuits. Various combinations of these circuits perform different mathematical and logical operations. For example, one combination of logic circuits performs addition. Another combination compares two numbers, then acts on the result of the comparison.

After completing an operation, the microprocessor may send the result to the memory until it is needed for another operation. In other cases, the result travels to an output device or a storage device.

Memory chips hold data and instructions inside the computer. Like microprocessors, memory chips consist of transistors, other electronic components, and wires arranged as circuits built into chips no larger than a fingernail.

There are two basic kinds of memory chips: (1) read-only memory (ROM), and (2) random-access memory (RAM). A ROM chip holds its memory even when the computer is turned off. However, the computer user cannot change the memory. ROM chips are used to hold instructions that a computer runs repeatedly.

A RAM chip holds its memory as long as power is on, but the user can change the memory. Random-access memory is sometimes called internal memory or main memory. RAM chips receive information and instructions from the microprocessor, an input device, or a storage device. RAM chips store only the information that is currently needed by the microprocessor.

Modern computers are designed so that a technician can change their capabilities by adding or removing components. In a typical PC, for example, many components are mounted on thin, rigid boards called circuit boards. The primary microprocessor and main memory are on a circuit board called the motherboard. Other components, such as sound and graphics co-processors, come on circuit boards called cards. These cards can be plugged into sockets called expansion slots inside the computer. Peripheral devices, such as printers and monitors, connect by wire or cable to sockets called ports.

Input devices send information and instructions to the computer. Some input devices are hand-operated, but others work automatically once they are turned on. There are five main kinds of hand-operated units: (1) a keyboard, (2) a mouse, (3) a trackball, (4) a light pen, and (5) a touch screen. There are three principal types of automatic units: (1) a modem, (2) a scanner, and (3) a microphone. In addition, all storage devices can function as input devices.

A keyboard is the main input device. Modern computer keyboards are different from typewriter keyboards. For example, along the top of the keyboard of a desktop PC is a row of function keys, which are designated F1, F2, F3, and so on. Function keys perform special tasks, such as removing a passage of text from one part of a document and inserting it in another. Elsewhere on the keyboard are keys that control the movement of an electronic marker called the insertion point that appears on the monitor screen. The insertion point indicates where the next character typed will appear.

Special keys with such names as command, control, and option give other keys extra functions. For example, with some software, pressing the command and p keys at the same time instructs the computer to prepare to print a document. A user can also reprogram the keyboard to change the function of certain keys, or even make them produce letters from a different alphabet.

The keyboard itself is an electronic device. When the user types a character, electronic circuits in the keyboard translate the character into bit charges. The charges travel through wire to a buffer, a temporary storage location in the computer. As soon as the charges appear in the buffer, the microprocessor moves them to RAM. At the same time, the microprocessor instructs the monitor to put the character on the screen.

A mouse is a palm-sized device that the computer operator moves about on a flat surface. The mouse has two functions: (1) to move the insertion point, and (2) to give commands to the computer. Computer users commonly operate the mouse on a smooth plastic rectangle called a mouse pad. Built into the bottom of the mouse is a ball that rotates when the user moves the mouse. On the upper surface are one or more buttons.

When the mouse is moved on the pad, an electronic code representing the movement of the ball travels by cable to the computer box. The microprocessor uses this code to move the an electronic marker called the pointer in the same way as the mouse was moved.

To move the insertion point, the operator moves the mouse to position the pointer in the new location. The operator then presses a mouse button and releases it, an action known as clicking the mouse.

To give a command, the user moves the mouse so that the pointer travels to a tiny picture called an icon. Each icon represents a command. The user then clicks the mouse.

The user can also use the mouse to drag (change the location of) an icon. The user moves the pointer to the icon, then presses a mouse button. Holding the button down, the user moves the mouse. The pointer and icon move together to the new location. The user then releases the button, removing the icon from the control of the mouse. Other uses of the mouse include dragging and changing the size of windows (rectangular frames that appear on the screen). The system of icons, pointer, and mouse is known as a graphical user interface (GUI).

A trackball is commonly used instead of a mouse on laptop and notebook computers. The main part of this device is a ball built into the keyboard. The ball has the same function as the ball built into the bottom of a mouse. Turning the ball by hand moves the pointer.

A light pen enables a computer user to write words or draw pictures directly into a computer. The user touches the point of the pen on the screen of a special pad, then makes writing or drawing motions. Electronic devices in the pen and under the surface of the pad interact to translate the motions into computer code. The microprocessor then uses the code to put an image corresponding to the motions on the screen. Some artists and designers use an electronic pen and pad to produce illustrations.

Words produced in this manner are processed as pictures of words, rather than as individually encoded letters. These words therefore cannot be edited like words entered via a word-processing program. However, a computer equipped with handwriting-recognition software can translate hand-printed characters into editable code. The code, in turn, produces characters that look as if they were typed. Pen-based computers use such software. These portable machines are used to record data in situations where typing would be difficult. For example, people who deliver packages by truck use them.

A touch screen can produce the same results as an electronic pen and pad. A touch screen produces electric current underneath whatever areas of the screen are pressed. A microprocessor uses the current to produce an image. The object pressing the screen can be a pen, as with certain pen-based computers, or a finger, in the case of map kiosks in shopping malls.

A modem is an electronic device that communicates with other computers over telephone lines. Thus, a modem functions as both an input device and an output device. In many of the latest computers, the modem is built-in. Others use a peripheral modem.

A modem translates bit charges into tones, then sends the tones over telephone lines to modems of other computers. The other modems reverse the translation process. The speed at which modems transmit and receive data is measured in bits per second (bps). Typical modems built in the mid-1990's have speeds ranging from 14,400 to 28,800 bps. Data can travel at much higher speeds over fiber-optic phone lines or television cables.

A scanner is a peripheral unit that uses light and devices that sense light to digitize photographs and other illustrations--that is, to translate them into numerical form for processing by a computer. A scanner equipped with optical character recognition (OCR) software translates the scanned image of a page of text into individually coded characters. Publishers are major users of scanners, but scanners are also becoming popular with home computer users.

Laser devices that read bar codes at checkout counters in stores are also scanners. These devices send coded information to a store's main computer.

A microphone translates sound into electric current, which the computer digitizes. The operator of the computer can process the sound by itself, or as part of a different kind of file--a motion-picture file, for example.

A special input device called a voice activator uses a microphone and digitizer to help a computer recognize spoken commands. The person operating the computer first records a series of commands by speaking into the microphone, as instructed by the computer. The computer digitizes the sounds and stores the resulting bit charges. To control the computer, the user then speaks the appropriate command. The computer searches the stored commands until it finds the one matching the command just spoken, then carries out the appropriate instruction.

Storage devices store computer files. A file is a body of processed information. For example, a user might store a business letter as a text file and a photograph as a graphics file.

Storage devices include hard disks, floppy disks, special compact discs called CD-ROMs, and tapes. All these devices can also serve as input devices, and all except the CD-ROMs can serve as output devices.

Hard disks can hold thousands of programs and files. The capacity of a typical PC hard disk built in the mid-1990's is about 500 megabytes, but some hold more than twice that amount. The disk, which actually consists of one or more platters, is built into the computer. The disk receives and transmits data via a machine called a drive, in which a mechanical device spins the disk past an electronic device.

To record information, the electronic device translates bit charges received from the microprocessor or memory into magnetic fields. The fields, in turn, magnetize tiny portions of the surface of a platter passing near the device. Every 0 is represented on the platter by a little magnet--a part of the surface--pointing in a certain direction. Every 1 is represented by a magnet pointing in the opposite direction. To obtain information from the platter, the mechanical device spins the disk, and the electronic device translates the magnetic fields of the little magnets back into code.

Floppy disks are small magnetic disks that can be removed from the computer. Floppy disks are used to store information for later use by the same computer, and to move information from one computer to another. Another use is to deliver software. Typical floppy disks can hold 1 or 2 megabytes.

Magnetic disks called removable storage disks have the same diameter as a floppy disk, but can store up to 1 gigabyte. The disks use a peripheral drive. Computer operators use them to expand storage capacity beyond what is available on the hard drive, to transport information between computers, and to back up (copy) large amounts of information stored on hard disks. If the hard disks were to be damaged or fail, the operator could restore the information by copying it from the removable storage disks.

CD-ROM's are the most common means of distributing software to PC's. The abbreviation stands for Compact Disc Read-Only Memory. The phrase read-only memory means that CD-ROM's are permanently inscribed with their data. The computer cannot insert, change, or delete any of the data.

A CD-ROM is the same size, and works in the same way, as an audio compact disc. On one side of the CD-ROM are tiny pits and flat spaces that represent 0's and 1's. A laser device uses a beam of light to ""read'' the disc, producing bit charges. Most new PC's have a built-in CD-ROM drive to play the discs.

A standard CD-ROM can store about 650 megabytes, roughly equivalent to 325,000 pages of double-spaced typewritten text. Because of their high capacity, CD-ROM's are the primary means of distributing multimedia programs. Such programs combine several forms of information--text, illustration, animation, and sound, for example.

A kind of optical disc called the DVD appeared in 1996. A DVD is the same size as a CD-ROM but can store much more information. A DVD stores data on one or both sides. Capacities range to a total of 17 gigabytes for storage on both sides. The DVD requires a special drive, which can also play CD-ROMs.

Yet another kind of storage disc, called a platter, works like a CD-ROM, but is much larger and can hold many gigabytes of data. Platters are expensive, so most of them are used by businesses, government agencies, and institutions for storing large databases.

Tape drives that are used to store data work in much the same way as audio cassette tapes. Tape drives are much slower than disk drives are. The main use of tape drives for file storage is to back up information stored on hard disks.

Output devices display the work done by the computer. These devices include monitors, printers, plotters, and speakers.

Monitors have a screen much like a television screen. The most common type, the monitor used with a desktop machine, is a cathode-ray tube (CRT), a vacuum tube like a television picture tube. Modern monitors offer millions of color combinations, and can display video and animation.

Laptop and notebook computers use flat panel screens, which can also display color and motion. These devices are not CRT's. Some of them use liquid crystals--molecules that display colors when activated by electric signals. Certain embedded computers, such as those in watches, calculators, and some automobile displays, use light-emitting diodes (LED's), solid crystals that display colors in response to electric signals.

The smallest screens are only about 1 to 2 inches (2.5 to 5.0 centimeters) across, yet can display pictures as well as full-sized monitors. Headsets or goggles used for viewing virtual reality programs use two such screens, one for each eye. Virtual reality programs use graphics and other tools to create an artificial world through which the user can seem to move.

Printers produce output on paper. There are three major kinds of printers: (1) laser, (2) inkjet, and (3) dot matrix.

Laser printers operate by electrically charging a special cylinder in patterns that represent a computer document, including text and illustrations. Dry ink called toner is then dusted onto the cylinder. The toner sticks to only the charged area. When a sheet of paper passes over the cylinder, the toner transfers to the paper and is heated for an instant. The toner melts, then resolidifies, creating a copy of the document on the paper.

Inkjet printers spray ink onto paper, using a tiny nozzle that moves quickly to form text and illustrations. Dot-matrix printers use tiny pins that strike a ribbon located in front of the paper. Each strike of a pin creates a tiny dot. Various combinations of pins striking the ribbon produce letters, numerals, other symbols, and even illustrations.

Plotters use pens to create drawings, diagrams, and graphs on paper or clear plastic. Architects use plotters to prepare large blueprints and plans.

Speakers are used to process and play sound files. Most desktop PC's for home use come with speakers similar to those on small stereophonic units.

Computer software

Computer software consists of instructions that control the operation of the computer. Much software also includes information for the computer to process. There are two general kinds of software: (1) operating system software and (2) applications software.

Operating system software reads and responds to user commands, and coordinates the flow of information among the different input and output devices. It also manages the different programs the user runs. It places these programs and the user's data into memory and makes sure that the microprocessor executes the right commands. Thus, the operating system combines the many separate parts of a computer into a single useful system.

Because operating systems serve as the interface (go-between) for computer users and their programs, much work has gone into operating system design. Early operating systems such as MS-DOS (Microsoft Disk-Operating System) used typed commands that users had to memorize or look up. Gradually, however, icons replaced typed commands, giving rise to graphical user interfaces (GUI's).

Applications software is made up of programs for all the specific uses of computers, including word processing, the management of financial documents, database management, and the processing of pictures and sounds.

Word-processing programs enable people to typewords into a computer to write articles, books, reports, letters, and other documents. Word-processing software greatly simplifies the work of editing a document. The writer can insert, change, or delete letters, words, or groups of words--sentences, paragraphs, or entire sections of a document. The software automatically adjusts the remainder of the document to reflect the changes.

Most word-processing software has dictionary and spelling programs that can check a document automatically, pointing out possible errors and offering corrections. Some software can also check grammar and punctuation. When the document is completed, the user can print it or distribute it by floppy disk or modem.

Spreadsheet programs are the major type of financial software. The word spreadsheet refers to a table used to present information in these programs. The basic structure of the table is a series of vertical columns and horizontal lines. The rectangular areas created by the intersection of the columns and lines are called cells. Each cell represents a specific kind of business information, such as the cost of a product or the number of units sold. The user enters numbers and other data into cells.

Mathematical equations link certain cells so that a change in one cell automatically changes all related cells. For example, if a student entered the amount $10 in a cell representing dues paid to a school club, the computer would automatically also add $10 to the cell representing the club's total income. Business and financial professionals can create large spreadsheets with thousands or tens of thousands of cells.

A technique called spreadsheet modeling has become a major tool of business. A corporate accountant, for example, might create a model (mathematical description) of the corporation's financial position. The accountant would then change the numbers in certain cells to represent a decision that the corporation had been considering--building a warehouse in a certain sales region, for instance. The computer would automatically change the related cells, and the accountant would examine the results. The accountant might then go back to the original model and change cells to represent an expansion of a company warehouse in a neighboring region. The accountant could use the model in this way to examine results of several potential decisions. The various possibilities examined by such modeling are sometimes called what-if scenarios.

The first spreadsheet program was VisiCalc, introduced in the late 1970's for the Apple II personal computer. Many computer experts consider VisiCalc to be the single program that persuaded many business people to use PC's. VisiCalc could perform only simple calculations over a limited number of cells. Today's spreadsheets for PC's offer hundreds of functions. In addition, numerous spreadsheets representing different aspects of a business can be linked together. Changes in one cell of one spreadsheet are reflected in related cells in all linked spreadsheets.

Most programs also incorporate powerful graphics tools, which can convert spreadsheet data into charts, graphs, and other illustrations. Some software provides presentation tools, computer instructions for generating posters or photographic slides.

Database management programs enable users to store large bodies of information and to search these databases in several ways. For example, a government revenue official might look up tax records alphabetically, by ZIP code, or by Social Security number.

The efficiency with which computers store and retrieve information makes database management a major function in a wide variety of professions. Scientists store the results of experiments and compare their results with those of other scientists. Libraries use computerized catalogs to hold information about their collections. Hospitals use computers to maintain patient records. Governments store election returns and census information in computers. Businesses rely on computers to keep information about their employees, customers, and products. Computers also enable traders of stocks, bonds, currencies, and commodities to track swiftly changing prices throughout the world.

Computer graphics programs enable computer users to create, change, and display pictures. The term computer graphics is also used to mean the pictures produced with these programs. The computer operator can create the original image on the computer or can use a previously created photograph or other picture that has been digitized. A digitized photo can be changed in a variety of ways. The user can change its dimensions or its colors, for example, or eliminate a part of it. Images in a photo can be moved or copied. Images from many separate photos can be combined. The finished picture can be displayed, printed, sent to other computers via modem, or saved on a disk.

A photograph takes much more disk space than text does. For example, a high-quality photo the size of a sheet of typewriter paper takes about 40 megabytes of space, roughly equal to 20,000 pages of double-spaced typewritten text.

Some computer graphics software works with motion pictures. These programs treat a motion picture as a series of still pictures called frames. Many of the spectacular special effects in today's movies are products of computer graphics. In 1995, Toy Story became the first motion picture created entirely on computers.

Computer graphics plays a vital role in the publishing industry. Most books, magazines, and newspapers are prepared on computers, using a process called desktop publishing. Writers, editors, and production specialists prepare the text on computers. Designers lay out the pages on computers, and artists and illustrators manage the graphics the same way. In many cases, every bit of text and every illustration is placed on the page on computers. Several publishers also produce magazines and other publications that appear only on computers.

Desktop publishing software is also available to home computer users. Individuals can use these programs to produce newsletters and other documents on their PC's. Users can choose from a variety of typefaces, arrange text in columns, and insert photos and other illustrations.

Presentation software is another major type of graphics program. This software enables users to create graphics to project onto motion-picture screens at meetings. At a sales meeting, for example, a manager might project sales charts and graphs, along with posters suggesting how to improve sales.

Graphics created with presentation software are transferred to slides or, with special hardware, projected onto the screen from the computer itself. Most presentation software can also produce special effects, such as images that fade away or are transformed into other images, and even sound effects.

Games software combines graphics, animation, sound, and music with clever design to produce exciting adventures and puzzles. Computer games are played on PC's or dedicated computers called video game units. These special units generally display their pictures on television screens. Some computer games can be played on two or more computers connected by modems. Using a keyboard, a joystick, or a mouse, players can interact with the games, controlling the movement of one or more characters or other elements.

Virtual reality (VR) software uses graphics, sound, and other tools to create an artificial world through which a user can seem to move. Virtual reality systems generally include a headset that has two tiny display screens, one for each eye. The images on the screens differ in a way that creates the illusion of three dimensions when viewed together. That illusion is enhanced when the user's head moves. The computer adjusts the images to the new perspective resulting from the movement.

Virtual worlds are filled with objects that can be "handled" by users wearing special sensor-lined gloves. The sensors tell the computer when the wearer moves a gloved hand. Suppose the user of a VR system closes the fingers of a gloved hand so that the corresponding virtual hand closes around a virtual ball. Then suppose the user raises the gloved hand. The computer will display the image of the ball being picked up.

Virtual reality images do not have nearly the detail of what is seen in the actual world, or even in nonvirtual video games. However, the images are realistic enough for video games.

Computer-aided design (CAD) programs are essential to many professions, especially engineering and architecture. For example, an architect can create pictures that show every aspect of a building's wiring, plumbing, and construction components. Other uses for CAD programs include the design of aircraft and automobiles, electronic machinery, and even clothing and fabrics.

One of the most advanced airplanes in the world, the Boeing 777, was completely planned with the aid of a CAD technique called paperless design. Powerful graphics and mathematics software enabled the aircraft's creators to simulate (imitate) every part of the plane in computers, ensuring that all the parts fit together perfectly. The designers even simulated tests of the aircraft's flight capabilities under all conditions the craft was likely to encounter. Only after the simulated airplane was thoroughly tested was construction begun, and the actual airplane went into service in 1995.

Scientific visualization software is used in virtually every branch of science. One use of this software is to develop and test theories. Astronomers, for example, use mathematical models (sets of equations) to develop theories of how groups of stars form galaxies of various shapes. First, an astronomer enters a model of a group of imaginary stars into a computer. The computer then solves the equations--or shows that the model is incorrect. A successful model will predict the shape of the galaxy that the stars would form. To test the theory, the astronomer can observe a real galaxy to see whether it has the predicted shape.

Scientists working in other fields use similar techniques. Biochemists, for example, model drug molecules. Meteorologists create models of weather systems to study how storms develop.

Medical visualization software enables physicians and computer specialists to generate detailed images of a patient's internal organs, including the brain, without operating on the patient. This software is used with such techniques as computerized tomography (CT), magnetic resonance imaging (MRI), and ultrasound imaging. The pictures that are produced help the physician make diagnoses and detect problems. These images can also be sent by modem to other doctors in distant locations, making consultations with specialists easier and less expensive.

Artificial intelligence (AI) software can help solve complex problems that do not involve numerical equations. This software enables a computer to imitate techniques that people use to solve such problems. For example, a physician investigating an illness applies his or her education, training, and experience to the problem. The physician does not solve the problem by performing calculations.

One type of AI software, an expert system, enables a computer to respond to information entered by the computer user. In making its responses, the computer draws upon vast amounts of data that human experts supplied to the writers of the software.

The computer responds to information entered by the user by asking questions and providing information of its own. In this manner, the computer constantly narrows the field of inquiry until a solution is achieved.

A doctor might use an expert system to evaluate a patient's symptoms. The computer would compare the combination of systems described by the doctor with all the descriptions in its database, then suggest diagnoses and treatments.

Educational software helps students learn. Different types of educational software may display lessons, ask questions, or provide reference resources. Some educational programs create simulations, which are computer models of such experiences as dissecting a frog or flying an airplane. Educational software appeals to students because it is interactive--that is, the student's response determines what happens next.

There is educational software for every level of instruction, from elementary school through university and professional study. Educational software is especially useful for learning such subjects as mathematics or foreign languages, in which drill and practice reinforce the lesson. Foreign language software uses audio so that students can hear words pronounced properly.

Software suites. One of the most popular software packages for PC's consists of several types of software combined into a suite. A typical suite contains a word processor, spreadsheet, database, graphics package, communications software for use with a modem, and an electronic calendar and address book. The programs are designed to work together, enabling the user to transfer information from, for example, a spreadsheet to a word-processing document by pressing a few keys.

Computer networks

The communication of data over telephone lines and by radio is one of the most important and influential uses of computers. Using modems, people can send text and graphics files, exchange messages, and search databases over worldwide computer networks.

As more of the world's information is digitized, more people seek access to the global "digital library." The combination of computers, modems, databases, and communications lines has become known as the information superhighway.

Businesses establish much smaller networks for their own use. One type, a local area network (LAN), connects a company's workstations within the same building or in neighboring buildings. A wide area network (WAN) links workstations over larger areas. Both LAN's and WAN's enable co-workers to exchange information rapidly. They also enable computers to share printers and storage devices.

The Internet. By the mid-1990's, experts estimated that more than 50 million computers were linked to the information superhighway by way of a network called the Internet. This network began in the late 1960's as ARPAnet, a connection of military and other government computers in the United States. The U.S. Department of Defense created ARPAnet to keep these computers secure in the event of war or natural disaster. Soon after ARPAnet was begun, universities and other institutions created their own networks. These networks eventually merged with ARPAnet to form the Internet.

During the 1970's and 1980's, large numbers of businesses connected their computers to the Internet. The explosive growth of PC's in the late 1980's and early 1990's enabled many home computer users to gain access to the Internet.

On the Internet are databases containing information on virtually every branch of human knowledge and enterprise--from the most serious scientific topics to catalogs of jokes. Due to advances in the worldwide telecommunications system, the Internet has become a global network. Universities, businesses, and individual users in virtually every nation are "on the Net."

Not all the information on the Internet is organized in formal databases. Computerized bulletin boards let people post their own messages, bringing a flood of opinion, commentary, and debate to almost every subject. There are also newsgroups on a host of topics. Participants in these groups can share information quickly, despite geographical separation. Computer games in which people assume different, often fantastic, personas (identities) are also popular on the Internet.

An increasingly important function of the Internet is the distribution of software. Programs can be downloaded (received) over the Internet, then run on the computers that receive them.

The World Wide Web. Until the early 1990's, most information on the Internet consisted only of text. The introduction to the Internet of a feature called the World Wide Web in 1991 made it possible to include graphics, animation, video, and sound. With the arrival of these multimedia tools, tremendous numbers of people began to use the Internet.

The World Wide Web consists of tens of millions of documents, databases, bulletin boards, and electronic publications, such as newspapers, books, and magazines. Much of the information has been converted from print into digital form, but even more has been created specifically for the Web.

Every site on the Internet--whether a World Wide Web site or not--has a computer address that is registered in a central location. This address is known as the site's uniform resource locator (URL). Using software that connects to the Internet--called navigation or browser software--a computer operator can select a URL that contains information the operator wishes to examine. The computer then contacts that address, making the information available to the operator.

With millions of separate URL's, indexing is an important Internet function. Indexing services--located on the Internet itself--enable users to search for specific information by entering the topic that interests them. The indexing service then searches the Internet for sites that contain information on those topics.

Local access providers. Computers can connect to the Internet through local access providers, computer services that operate in most communities. The user pays the provider a monthly fee. To gain access to the Internet, the user contacts the provider via modem. The provider then connects the user to the Internet.

On-line services are commercial computer networks to which subscribers pay monthly or hourly fees. The largest on-line services have millions of subscribers. Most services provide news, bulletin boards, databases, games, software libraries, and other features, often including Internet access.

Electronic mail (e-mail) is one of the most-used features available on the Internet and through on-line services. Just as every Web site or Internet location has an electronic address, so does every individual computer connected to a local access provider or on-line service. Individuals and businesses use these addresses to send one another messages known as electronic mail (e-mail).

The user can write an e-mail message in a word-processing program, then transfer it to a communications program. The user can also write the message in an e-mail form, a box displayed on the monitor. Most communications software and on-line services provide e-mail forms. After completing the message, the user can attach nontext material, such as graphics files, to it.

The user then addresses the message. The user can send the message to several addresses without rewriting it. The user merely enters all the addresses as instructed by the computer. The computer automatically enters the sender's electronic address, making response easy. An "electronic mailbox" at each computer address stores the mail.

Chat rooms. Another type of electronic communication, called chat, is like a conversation among several people. Computer users connect their computers over the Internet or through an on-line service and type messages to one another. The individual networks are known as chat rooms.

Programming a computer

Computer programming is the preparation and writing of detailed instructions for a computer. These instructions tell the computer what data to use and what sequence of operations to perform with the data.

Computer scientists and other computer specialists called programmers write most instructions for computers. These people use programming languages that consist of words, individual letters, numerals, and other symbols, as well as rules for combining those elements.

A computer cannot work directly with a program written in a programming language. The instructions must be translated into a machine language composed of binary digits. These digits represent operating codes, memory addresses, and various symbols, such as plus and minus signs. Machine language is also known as low-level language.

Special programs called compilers and assemblers translate programming languages into machine language. Compiled programs are permanently translated into machine language. Other programs are translated each time the program is run, requiring another type of program, called an interpreter.

Compilers, assemblers, and operating systems may be viewed as "smart programs" because they enable a computer to carry out complicated instructions. The user communicates with the smart program, and the smart program communicates with the computer.

Preparing a program begins with a complete description of the job that the computer is to perform. This description explains what input data are needed, what computing must be done, and what the output should be. Computer programmers use the description to prepare diagrams and other pictorial aids that represent the steps needed to complete the task. The programmers may produce a diagram called a systems flow chart that shows how all the major parts of the job fit together. After a computer program is written, it is debugged--that is, tested on the computer for mistakes, then corrected.

Using programming languages. Computers appear to work directly with programs written in programming languages. But actually, the smart program first translates the written program into machine language. The smart program next enters the translated version into the computer's memory. The microprocessor then reads and executes each translated instruction.

There are many high-level programming languages. Some of them closely resemble the language of mathematics. Others enable programmers to use symbols and various everyday expressions, such as "READ," "PRINT," and "STOP."

The language that a programmer uses depends largely on the job to be done. For example, if a task involves processing business data, the programmer would most likely use COBOL (COmmon Business Oriented Language). Programming a computer to solve complicated scientific problems might require the use of a mathematically oriented language, such as FORTRAN (FORmula TRANslation).

Another commonly used programming language is BASIC (Beginner's All-purpose Symbolic Instruction Code). This programming language is well suited for writing relatively simple programs for PC's. Numerous elementary schools and high schools that offer a course in programming teach BASIC. Pascal, named for the French mathematician, scientist, and philosopher Blaise Pascal, also is taught in some schools.

Some computer programs may be written in an assembly language. This kind of language is harder to use than a high-level language. The programmer must state each instruction with much more detail than is needed when using a high-level language.

As computers have grown more powerful and their functions more various, the size of computer programs has grown dramatically. Some modern applications programs contain tens of millions of lines of programming instructions. Because of their large size, many modern programs are distributed on CD-ROM's.

New languages are constantly developed for computers. Some, such as Visual BASIC, build upon earlier versions of a language. Others are developed for new computer functions. One such language is HTML (Hyper-Text Markup Language), which is used to create interactive pages for the World Wide Web.

Many programmers use programming objects. An object is an independent section of code that performs a particular function. Object-Oriented Programming (OOP) uses objects that can work together to create a whole program. OOP relieves programmers of the need to re-create sections of code in long programs. The same object can be used again and again.

The computer industry

The manufacture, development, sales, and servicing of computer hardware and software make up one of the largest industries in the world. The United States has the worlds largest computer industry, employing more than 2 million people. The United States also has more computers than any other country. Japan ranks second in number of computers.

Manufacturing. From a few dozen companies in the early 1960's, the computer industry has grown to more than 10,000 firms throughout the world. These companies manufacture computers and such peripheral equipment as modems and printers. They also develop and publish software and provide computer supplies, such as magnetic disks. Many companies make computer components, including processors. Others make the boards and cables used to create networks.

In the mid-1990's, total annual sales of computer hardware and software in the United States exceeded $200 billion. The largest computer manufacturer in the United States--and the world--is International Business Machines Corporation (IBM). The Hewlett-Packard Company ranks second. Other leading U.S. computer makers include Digital Equipment Corporation, Dell Computer, Apple Computer, Inc., Compaq Computer Corporation, and Sun Microsystems Incorporated.

The largest computer manufacturers outside the United States are Fujitsu and NEC Corporation, both of Japan. The leading computer companies in Europe include Groupe Bull of France and Siemens AG of Germany. The largest software company in the United States and the world is Microsoft Corporation.

Research and development. The constant increase in computer power is a major reason for the computer industry's success. This increase is a result of research and development at businesses and universities throughout the world. Current research goals include even faster processors, faster means of communications, disks that can store more information, and better ways to compress information. Data compression is the use of special coding techniques to reduce the number of bytes needed to carry information.

Artificial intelligence (AI) is an exciting area of software research. Experts in this field design computer systems to perform tasks that appear to require intelligence, such as reasoning and learning. Artificial intelligence experts hope to increase the ability of computers to respond to problems in a "human" manner.

Sales. Computers are sold in a variety of ways. Large manufacturers of computers have teams of sales professionals. These teams call on corporations and institutions to sell combinations of hardware and software.

Another method of selling computers involves a value-added reseller (VAR). A VAR buys computer systems and components from a variety of sources. It then sells the finished products to computer users. Retail outlets play an increasingly important role in the sale of personal computers. Computer specialty stores, mail-order houses, and general merchandise stores also sell many computers. Much software and hardware is sold over the Internet.

Service and repair. Many computer manufacturers offer service contracts that provide for regular maintenance and prompt repair of their computers. Some large businesses and institutions have their own computer maintenance staffs.

Careers. There are many career opportunities in the computer industry. Computer engineers are probably the most technically specialized computer experts. Hardware engineers design the circuits that are engraved on chips, and they develop the wiring that lets information flow smoothly through the computer. Engineers also design the technical aspects of memory, storage, and peripheral equipment. Computer programmers write the instructions that make computers operate properly. Systems analysts determine the most efficient use of computers for particular situations. Software publishers issue programs, write and edit instruction manuals, and provide technical services for customers.

Many career opportunities exist outside the computer industry itself. Data processors who work for companies that use computers enter information into those computers. Workers in many factories oversee computers that control machines. Computer training is also an important industry.

Some of the industry's most successful individuals are self-taught. But most computer careers call for a college degree. College courses that help prepare students for careers in computers include programming, electrical engineering, systems analysis, and data processing.

Computer history

The ideas and inventions of many engineers, mathematicians, and scientists led to the development of the computer. The first true calculating machines were developed in the 1600's. In 1642, the French mathematician, scientist, and philosopher Blaise Pascal invented the first automatic calculator. The device performed addition and subtraction by means of a set of wheels linked to each other by gears. The first wheel represented the numbers 1 to 10, the second wheel represented 10's, the third stood for 100's, and so on. When the first wheel was turned 10 notches, a gear moved the second wheel forward a single notch. The other wheels became engaged in a similar manner.

During the early 1670's, the German mathematician Gottfried Wilhelm von Leibniz improved Pascal's calculator. Leibniz added gear and wheel arrangements that made multiplication and division possible. Leibniz also sought a counting system that would be easier for a machine to handle than the decimal system. He developed the binary numeration system.

An important contribution to the development of binary mathematics was made in the mid-1800's by George Boole, an English logician and mathematician. Boole used the binary system to invent a new type of mathematics. Boolean algebra and Boolean logic perform complex mathematical and logical operations on the symbols 0 and 1. Thus, a mechanical representation of binary mathematics would require the representation of only those two digits. This advance shaped the development of computer logic and computer languages.

Early punched-card computing devices. Joseph Marie Jacquard, a French weaver, made the next great contribution to the development of the computer. In the weaving process, needles directed thread to produce patterns. In 1801, Jacquard invented the Jacquard loom, which used punched cards to automate this process for the first time. The cards had patterns of holes punched in them and were placed between the rising needles and the thread. The presence or absence of a hole could be compared to the two digits of the binary system. Where there were holes, the needles rose and met the thread. Where there were no holes, the needles were blocked. By changing cards and alternating the patterns of punched holes, it became possible to mechanically create complex woven patterns.

The punched cards of the Jacquard loom inspired the English mathematician Charles Babbage. During the 1830's, Babbage developed the idea of a mechanical computer that he called an analytical engine. He worked on the machine for almost 40 years. When performing complex computations or a series of calculations, the analytical engine would store completed sets of punched cards for use in later operations. Babbage's analytical engine contained all the basic elements of an automatic computer--storage, working memory, a system for moving between the two, and an input device. But Babbage lacked funding to build the machine.

The first successful computer. In 1888, American inventor and businessman Herman Hollerith devised a punched card system, including the punching equipment, for tabulating the results of the United States census. Hollerith's machines used electrically charged nails that, when passed through a hole punched in a card, created a circuit. The circuits registered on another part of the machine, where they were read and recorded. Hollerith's machines tabulated the results of the 1890 census, making it the fastest and most economical census up to that date. In a single day, 56 of these machines could tabulate census information about more than 6 million people.

Governments, institutions, and industries found uses for Hollerith's machine. In 1896, Hollerith founded the Tabulating Machine Company. He continued to improve his machines. In 1911, he sold his share of the company. Its name was changed to the Computing-Tabulating-Recording Company (C-T-R). In 1924, the name was changed to International Business Machines Corporation (IBM).

The first electronic computers. The first special-purpose electronic digital computer was constructed in 1939 by John V. Atanasoff, an American mathematician and physicist. In 1944, Howard Aiken, a Harvard University professor, built another digital computer, the Mark 1. The operations of this machine were controlled chiefly by electromechanical relays (switching devices).

In 1946, two engineers at the University of Pennsylvania, J. Presper Eckert, Jr., and John William Mauchly, built the first general-purpose electronic digital computer. They called it ENIAC (Electronic Numerical Integrator And Computer). ENIAC contained about 18,000 vacuum tubes instead of relays. The machine occupied more than 1,500 square feet (140 square meters) of floor space and consumed 150 kilowatts of electric power during operation. ENIAC operated about 1,000 times as fast as the Mark 1. It could perform about 5,000 additions and 1,000 multiplications per second. ENIAC also could store parts of its programming.

Although ENIAC worked rapidly, programming it took a great deal of time. Eckert and Mauchly next worked on developing a computer that could store even more of its programming. They worked with John von Neumann, a Hungarian-born American mathematician. Von Neumann helped assemble all available knowledge of how the logic of computers should operate. He also helped outline how stored programming would improve performance. In 1951, a computer based on the work of the three men became operational. It was called EDVAC (Electronic Discrete Variable Automatic Computer). EDVAC strongly influenced the design of later computers.

Also in 1951, Eckert and Mauchly invented a more advanced computer called UNIVAC 1 (UNIVersal Automatic Computer). Within a few years, UNIVAC 1 became the first commercially available computer. Unlike earlier computers, UNIVAC 1 handled numbers and alphabetical characters equally well. It also was the first computer system in which the operations of the input and output devices were separated from those of the computing unit. Like ENIAC, UNIVAC 1 used vacuum tubes.

The first UNIVAC 1 was installed at the U.S. Bureau of the Census in June 1951. The following year, another UNIVAC 1 was used to tabulate the results of the United States presidential election. Based on available data, UNIVAC 1 accurately predicted the election of President Dwight D. Eisenhower less than 45 minutes after the polls closed.

Miniaturization. The invention of the transistor in 1947 led to the production of faster and more reliable electronic computers. Transistors soon replaced the bulkier, less reliable vacuum tubes. In 1958, Control Data Corporation introduced the first fully transistorized computer, designed by American engineer Seymour Cray. IBM introduced its first transistorized computers in 1959.

Miniaturization continued with the development of the integrated circuit (a complete circuit on a single chip) in the early 1960's. This device enabled engineers to design both minicomputers and high-speed mainframes with huge memories.

By the late 1960's, many large businesses relied on computers. Many companies linked their computers together into networks, enabling different offices to share information.

Computer technology improved rapidly during the 1960's. By the early 1970's, the entire workings of a computer could be placed on a handful of chips. As a result, computers became smaller.

The personal computer. The first personal computer, the Altair, was introduced in 1975. Only electronics hobbyists bought these computers.

In 1977, two young American computer enthusiasts, Steven P. Jobs and Stephen G. Wozniak, founded Apple Computer, Inc., and introduced the Apple II personal computer. The Apple II was much less expensive than mainframes. As a result, computers became available to people other than computer specialists and technicians. Personal computers were purchased by small and medium-sized businesses that could not afford mainframes or did not need the immense computing power the mainframes provided. Millions of individuals, families, and schools also bought personal computers.

In 1975, college students Bill Gates and Paul Allen founded Microsoft Corporation to develop programs for the Altair. In 1981, IBM entered the personal computer market with its PC. The machine was even more successful than the Apple II. Microsoft soon was developing programs for the PC. Gates and Allen went on to become two of the world's wealthiest men. Apple scored another success in 1984 with the introduction of its Macintosh, a powerful, easy-to-use desktop computer.

As computer power increased, so did computer speed. These increases were accompanied by a steady reduction in both size and cost. By the mid-1990's, more than one-third of all U.S. households had personal computers.

Computer security

Many people fear that their right to privacy is threatened by the possible misuse or unauthorized disclosure of information in computer databases. Certain databases hold private and personal information, such as medical, banking, or tax records. Others contain business plans or inventions that a company wishes to conceal from competitors. Still others store top-secret military information and other kinds of data important to a nation's security.

Laws limit the disclosure of information in databases, and operating systems are designed to prevent unauthorized entry into a computer. Many computers require a user to enter a secret password. In addition, some computer systems automatically scramble information so that the information can be decoded only by authorized personnel. Nevertheless, computer crimes sometimes occur. Industrial spies and thieves often use telephone lines to gain access to computers. Some of these criminals steal or change the information in a computer database. Others steal money by transferring funds electronically.

In the late 1980's, computer experts became aware of a dangerous type of program called a computer virus. A computer virus is designed to do mischief, sometimes by deleting or changing information and sometimes by simply inserting a message. A virus can enter a computer's operating system via a modem, a floppy disk, or a CD-ROM. A virus can spread through a network by rapidly making copies of itself in computers connected to the network. Antivirus programs are available to prevent viruses.