Monday, August 9, 2010


PCs consist of electronic circuitry called a microprocessor, such as the central processing unit (CPU), that directs logical and arithmetical functions and executes computer programs. A PC also has electronic memory known as random access memory (RAM) to temporarily store programs and data, as well as external storage devices—such as hard, floppy, compact disc (CD-ROM), and digital video disc (DVD) drives—to permanently store programs and data. A PC user enters information and commands via a keyboard or a pointing device called a mouse. Information from the PC is displayed on a video monitor or on a liquid crystal display (LCD) video screen, or it can be printed on laser, dot-matrix, or inkjet printers (see Hardware). PCs are run by software called the operating system.


Storage hardware provides permanent storage of information and programs for retrieval by the computer. The two main types of storage devices are disk drives and memory. There are several types of disk drives: hard, floppy, magneto-optical, and compact. Hard disk drives store information in magnetic particles embedded in a disk. Usually a permanent part of the computer, hard disk drives can store large amounts of information and retrieve that information very quickly. Floppy disk drives also store information in magnetic particles embedded in removable disks that may be floppy or rigid. Floppy disks store less information than a hard disk drive and retrieve the information at a much slower rate. Magneto-optical disc drives store information on removable discs that are sensitive to both laser light and magnetic fields. They can typically store as much information as hard disks, but they have slightly slower retrieval speeds. Compact disc drives store information on pits burned into the surface of a disc of reflective material (see CD-ROM). CD-ROMs can store about as much information as a hard drive but have a slower rate of information retrieval. A digital video disc (DVD) looks and works like a CD-ROM but can store more than 15 times as much information.
Memory refers to the computer chips that store information for quick retrieval by the CPU. Random access memory (RAM) is used to store the information and instructions that operate the computer's programs. Typically, programs are transferred from storage on a disk drive to RAM. RAM is also known as volatile memory because the information within the computer chips is lost when power to the computer is turned off. Read-only memory (ROM) contains critical information and software that must be permanently available for computer operation, such as the operating system that directs the computer's actions from start up to shut down. ROM is called nonvolatile memory because the memory chips do not lose their information when power to the computer is turned off.
Some devices serve more than one purpose. For example, floppy disks may also be used as input devices if they contain information to be used and processed by the computer user. In addition, they can be used as output devices if the user wants to store the results of computations on them.


 Output hardware consists of external devices that transfer information from the computer's CPU to the computer user. A video display, or screen, converts information generated by the computer into visual information. Displays commonly take one of two forms: a video screen with a cathode ray tube (CRT) or a video screen with a liquid crystal display (LCD). A CRT-based screen, or monitor, looks similar to a television set. Information from the CPU is displayed using a beam of electrons that scans a phosphorescent surface that emits light and creates images. An LCD-based screen displays visual information on a flatter and smaller screen than a CRT-based video monitor. LCDs are frequently used in laptop computers.
Printers take text and image from a computer and print them on paper. Dot-matrix printers use tiny wires to impact upon an inked ribbon to form characters. Laser printers employ beams of light to draw images on a drum that then picks up fine black particles called toner. The toner is fused to a page to produce an image. Inkjet printers fire droplets of ink onto a page to form characters and pictures.


In the mid-1940s, early computers were the size of houses and as expensive as battleships, but they had none of the computational power or ease of use that are common in modern PCs. The miniaturization of electronic circuitry and the invention of integrated circuits and microprocessors enabled computer makers to combine the essential elements of a computer onto tiny silicon computer chips, thereby increasing computer performance and decreasing cost. The first microprocessor, the Intel 4004, created in 1971 by Intel Corporation, was originally designed to be the computing and logical processor of calculators and watches. From its simple design modern microprocessors evolved.
The Altair 8800, developed in 1975 by Micro Instrumentation Telemetry Systems (MITS), is considered to be the first PC. The Altair was built from a kit and programmed by using switches. Information from the computer was displayed by light-emitting diodes on the front panel of the machine. The Altair appeared on the cover of Popular Electronics magazine in 1975 and inspired many computer enthusiasts who would later establish companies to produce computer hardware and software.
American computer designers Steven Jobs and Stephen Wozniak created the Apple II in 1977. The Apple II was one of the first PCs to incorporate a color video display and a keyboard that made the computer easy to use. Jobs and Wozniak incorporated Apple Computer, Inc., the same year.
In 1981 International Business Machines Corporation (IBM) introduced the IBM PC. It was designed with an open architecture that enabled other computer manufacturers to create similar machines, or clones, that could also run software designed for the IBM PC. The design of the IBM PC and its clones soon became the PC standard, and an operating system developed by Microsoft Corporation became the dominant software running PCs.
A graphical user interface (GUI)—a visually appealing way to represent computer commands and data on the screen—was first developed in 1983 when Apple introduced the Lisa, but the new user interface did not gain widespread notice until 1984 with the introduction of the Apple Macintosh. The Macintosh GUI combined icons (pictures that represent files or programs) with windows (boxes that each contain an open file or program). A pointing device known as a mouse controlled information on the screen. Inspired by earlier work of computer scientists at Xerox Corporation, the Macintosh user interface made computers easy and fun to use and eliminated the need to type in complex commands (see User Interface). Today, software available for IBM PCs and clones, as well as most other popular computer platforms, also feature a GUI.
Since the early 1970s, computing power has doubled about every 18 months due to the creation of faster microprocessors, the incorporation of multiple microprocessor designs, and the development of new storage technologies. Ongoing research is focused on creating computers that use light and biological molecules instead of—or in combination with—conventional electronic computer circuitry. These technological advances, coupled with new methods for interconnecting computers, such as the proposed Internet2, an advanced Internet under development by universities, industry, and the government, promise to make PCs even more powerful and useful.


Input hardware consists of external devices—that is, components outside of the computer's CPU—that provide information and instructions to the computer. A light pen is a stylus with a light sensitive tip that is used to draw directly on a computer's video screen or to select information on the screen by pressing a clip in the light pen or by pressing the light pen against the surface of the screen. The pen contains light sensors that identify which portion of the screen it is passed over. A mouse is a pointing device designed to be gripped by one hand. It has a detection device (usually a ball) on the bottom that enables the user to control the motion of an on-screen pointer, or cursor, by moving the mouse on a flat surface. As the device moves across the surface, the cursor moves across the screen. To select items or choose commands on the screen, the user presses a button on the mouse. A joystick is a pointing device composed of a lever that moves in multiple directions to navigate a cursor or other graphical object on a computer screen. A keyboard is a typewriter-like device that allows the user to type in text and commands to the computer. Some keyboards have special function keys or integrated pointing devices, such as a trackball or touch-sensitive regions that let the user's finger motions move an on-screen cursor.


Self-duplicating computer program that interferes with a computer's hardware or operating system (the basic software that runs the computer). Viruses are designed to duplicate or replicate themselves and to avoid detection. Like any other computer program, a virus must be executed for it to function—that is, it must be located in the computer's memory, and the computer must then follow the virus's instructions. These instructions are called the payload of the virus. The payload may disrupt or change data files, display an irrelevant or unwanted message, or cause the operating system to malfunction.

Hardware (computer)

Equipment involved in the function of a computer. Computer hardware consists of the components that can be physically handled. The function of these components is typically divided into three main categories: input, output, and storage. Components in these categories connect to microprocessors, specifically, the computer's central processing unit (CPU), the electronic circuitry that provides the computational ability and control of the computer, via wires or circuitry called a bus.
Software, on the other hand, is the set of instructions a computer uses to manipulate data, such as a word-processing program or a video game. These programs are usually stored and transferred via the computer's hardware to and from the CPU. Software also governs how the hardware is utilized; for example, how information is retrieved from a storage device. The interaction between the input and output hardware is controlled by software called the Basic Input Output System software (BIOS).