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 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 modem (modulator-demodulator) 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 fibre-optic phone lines or television cables.

Monitors have a screen much like a on 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 colour combinations, and can display video and animation. 

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

     The smallest screens are only about 2.5 to 5.0 centimetres 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.

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. 

    In desktop PCs, 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.

  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.

  Motherboard

Cable-Hub

  Video-Card

Sound Card & Sound System

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