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i 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 television like monitor, show results of the computer work. f.. 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.

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.

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 computers 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.

- Modem 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 co­processors, 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. 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. Modem 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 Fl, 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.

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.

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. -v';:--. <?^<.:-,; v;:,, , ,,

*--•-:--' 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.

.-••• 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

Jhigher speeds over fiber-optic phone lines or television cables.

f A scanner is a peripheral unit that uses light and devices that sense light to digitize

photographs and other illustrations-mat 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. -,. 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.

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. 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 O'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.

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. Modem monitors offer millions of color combinations, and can display video and animation.

Plotters use pens to create drawings, diagrams, and graphs on paper or clear plastic.

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.

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