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Different pieces of hardware the physical devices of a computer system are used to input, process, and output data. Input devices, which range from keyboards and touch-sensitive screens to mice and light pens, translate the information into a series of on and off electrical signals.

Processing is controlled by the central processing unit (CPU), which consists of a control unit that directs the transfer of data and an arithmetic/ logic unit (ALU) that performs mathematical and logical calculations. Once entered, data is moved to memory for processing this is the information that appears on the screen or written to the disk for permanent storage.

The system unit itself houses the processing devices, electronic circuits, and other components. Many of these devices are tiny silicon chips, also known as semiconductors or integrated circuits. These chips include the CPU, RAM (random access memory), and ROM (read-only memory); they are plugged into sockets on a printed circuit board (a board with electrical circuits printed on it). The board that contains the CPU is known as the system board (or motherboard); it provides several expansion slots that allow users to add memory and expansion boards for input and output devices (peripheral devices) such as mice, monitors, modems, and scanners.

Read-only memory contains the computer's built-in instructions; the computer can read but not alter the contents. Random access memory allows the manipulation of data; if the program is ended or the power turned off before the changes have been saved, all new material is lost.

Microcomputers typically include a hard disk and one or two floppy disk drives. The hard disk provides greater storage space and speed than a floppy disk provides; it is usually sealed within the unit (some removable drives are available) and contains the operating system and whatever programs and data you choose to write to it. Because hard disks can malfunction (crash), it is advisable to back up their data onto floppy disks (or tape) to be stored outside the system. Other storage devices include optical discs, where the data has been burned onto the disc by means of a laser, and interactive video.

Input and output devices, connected to the system unit through serial ports, parallel ports, and specialized ports, let the user interact with the system to input data and receive processed results. Common input devices include the keyboard, mouse, scanner, and light pen; common output devices include the monitor, printer, and plotter. The modem functions as both an input and output device. The availability of modems has opened up staggering opportunities for people to link their computers via phone, to access information from remote databases, and communicate in a variety of ways.



Unit 3


Pre-reading activity



Reading activity




EPOS (electronic point of sale) tills used in supermarkets form part of a computer system with various input and output peripheral devices attached to the till, including: electronic scales for weighing produce, barcode reader for looking up prices using barcodes, swipe card reader for reading bank cards, numeric keypad for inputting prices manually, LCD (liquid crystal display) screen for outputting purchase details.

Digital cameras are gradually being developed that are as good as conventional cameras. They have various electronic devices inside, including:

a) LCD (Liquid Crystal Display) screen used as a view-finder and for viewing the pictures after they have been taken.

b) CCD (Charge-Coupled Device) consisting of thousands of photo-transistors (light-sensitive transistors - a transistor is an electronic switch). It creates the pictures as a set of dots or pixels (picture elements).

c) Memory cards e.g. flash cards solid state memory (electronic integrated circuits, i.e. chips, used for storing the pictures).

There is no delay in getting pictures from digital cameras because there is no film requiring chemical processing. They can be attached to a computer to directly transfer pictures for editing using special software and unwanted pictures can be deleted. However, they cost more than conventional cameras and the quality is not quite as good. You also need to buy rechargeable batteries and a photo-quality colour printer with high printing costs for paper, ink, etc. Two important features when buying a digital camera are:

a) picture quality or resolution. The resolution of a camera is measured in pixels and given as two numbers, indicating how many pixels there are across the image and how many going down the image e.g. 1280 by 960 (or 1280x960).

b) the number of pictures the camera can store. The higher the resolution, i.e. the more pixels, the more memory is required to store the pictures. Data can be compressed to allow more pictures to be stored.

Storage devices are used to store data and programs that are not being used by the processor. They usually consist of:

a) storage media in the form of a circular disk or a tape where the data is stored

b) a disk or tape drive that moves the media past a read/write head that reads the data from and writes data to the storage media.


Types of storage devices include:


magnetic devices (that use magnetism) floppy disks (diskettes) and magnetic tape made of a magnetic coated flexible plastic; hard disks made of magnetic coated aluminium disks
optical devices (that use laser light)   CD-ROM compact disk read only memory CD-R recordable compact disk CD-RW re-writable compact disk DVD-ROM digital versatile disk read only memory DVD-RAM digital versatile disk random access memory
magneto-optical devices (that use a combination of magnetism and laser light) CD-MO magneto optical compact disk  


Read only media enable the user to both read data from and write data to the media. Read and write media can only be used for reading data i.e. the stored data cannot be changed in any way.

Removable storage enables the user to change the media and transfer it to another computer.

Fixed storage does not allow the media to be changed or transferred to another computer.

Other factors that vary between storage devices include:

a) the speed at which the drive moves the media past the read/write head and reads or writes data to the storage media

b) the capacity of the media i.e. how much data can be stored on each disk or tape

c) the cost of the drive and the media.

There are various types of printers for out-putting text and graphics to paper.

Some types of printers are mono (print in black and white only) and others can print in colour. The speed, quality and cost of printing varies between different types of printer. Some are designed for printing text and are not really suited to printing graphics.

Data can take many forms and there is a wide variety of input, output, storage and communication peripherals.

Units of measurement used in data storage include:


bit a binary digit i.e. a 1 or a 0
byte 8 bits = 1 character i.e. a letter, numerical digit or a punctuation mark
megabyte (MB) 1,048,576 bytes (approximately one million bytes)
gigabyte (GB) 1,073,741, 824 bytes (approximately one thousand million bytes)
terabit 1,099,5 11, 627,776 bits (approximately one thousand gigabits)
micron one millionth of a metre
angstrom the approximate radius of an atom

Date: 2015-01-02; view: 995

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