2.1      Input Devices

An input device lets you communicate with a computer.  They are used to enter information and issue commands to the computer. Commands tell the computer to do something, like save the file. A keyboard, mouse, scanner, digital camera, touch pads and joystick are examples of input devices.

Keyboard Used to type data into the computer. It has special keys for giving the computer commands called command or function keys

Pointing Devices Pointing devices move some object on the screen and can do some action Mouse is a  common pointing device

Scanner allows you to scan documents, pictures, or graphics and view them on the computer.  You can also use software to edit the items you scan. Used to put printed pictures and text into a computer. It Converts an image into dots that the computer can understand .To scan text, optical character recognition (OCR) software is needed

Digital Camera Used to take electronic pictures of an object. The pictures taken by a digital camera can be used directly by a computer

Microphone Used to put sound into a computer. Need sound recording software

Video Capture Card Usually place inside the computer’s case. Use to put video into a computer. Need a video source, either a video camera or video recorder

Voice input device-A computer I/O device in which vocal commands may be entered into a computer system.

Optical character recognition (OCR) is computer software designed to translate images of handwritten or typewritten text (usually captured by a scanner) into machine-editable text, or to translate pictures of characters into a standard encoding scheme representing them (e.g. ASCII or Unicode).

Optical Mark Reader (OMR) A special scanning device that can read carefully placed pencil marks on specially designed documents. OMR is frequently used in forms, questionnaires, and answer-sheets

Magnetic Stripe reader Cards with magnetic stripes are used as credit cards, debit cards, railway tickets, phone cards and many other applications. The magnetic stripe can be encoded with up to 220 characters of data. A magnetic stripe reads the information held in the magnetic stripe.

Magnetic Ink Character Recognition (MICR) In this method, human readable characters are printed on documents such as cheque using special magnetic ink. The cheque can be read using a special input unit, which can recognize magnetic ink characters.

2.2      Output devices

An output device displays information on a screen, creates printed copies or generates  sound.   A  monitor,  printer,  and  speakers are examples  of  output devices.

Monitors and Displays Shows the processed information on a screen. A monitor uses a Picture Tube like a television with the image displayed on the front of the tube, which is called the screen.

Printers produce a hard copy. The information is printed on paper and can be used when the device is off. It is also called a printout. There different types of printers;

Dot-matrix printers (impact printer)

Uses metal pins to strike an inked ribbon to make dots on a piece of paper.

Lowest print quality of all of the printers. Very low in cost per page to use.

Ink jet printers (non-impact printer)

Use drops of magnetic ink to produce dots on a page to produce text or images.

The print quality is almost the same as a laser printer’s. The ink is very expensive

The ink is water soluble and will run if the paper gets wet

Highest cost per page of all the printers

For producing color documents, it has the highest quality at a reasonable price.

Laser printers (non-impact printer)

A laser or LEDs make dots on a light sensitive drum

Toner  (very tiny  particles of plastic) stick  to the  drum where the dots where made

For black and white printouts, very low cost per page

Printout is permanent

Color laser printers are still fairly expensive

Speakers Used to output sound

LCD Projectors Similar to monitors but projects an image on to a screen.

They are mainly used for presentations.

2.3      Central Processing Unit (CPU)/Processor:

It is the main part of a computer system like the brain of a human being. It interprets the instructions in the program and executes one by one. The CPU of a microcomputer is called a microprocessor. Central Processing Unit is implemented in a single piece of silicon device known as a computer chip.

The processor and main memory of a PC are commonly held on a single board called a mother board. The processor has the following functions:

It controls the transmission of data from input devices to memory;   It processes the data held in main memory;

It controls the transmission of information from main memory to output devices.

The processor contains the control unit and the arithmetic/logic unit(ALU).

The control unit coordinates and controls all the operations carried out by the computer. The control unit operates by repeating three operations which are:

Fetch – cause the next instruction to be fetched from memory;

Decode  –  translate  the  program  instruction  into  commands  that  the computer can process

Execute – cause the instruction to be executed

The arithmetic/logic unit(ALU) plays two roles.

Arithmetic   operations   –  these   operations   are   addition,   subtraction, multiplication and division..

Logical operations – it compares two data items to determine whether the first one is smaller than, equal to or greater than the second item.

2.4      Main Memory:

The cycle (input – processing – output) would not be possible without a holding place for the instructions and data that the processors (CPU) can easily reach. This holding place is known as memory also called main storage and is internal to the computer consisting of RAM and possibly ROM.

Random Access Memory (RAM)

Is the basic kind of internal memory that holds data and instructions while the computer is in use.

It can be read from and written to.

It is called random access because the processor or computer can access any location in memory in any order as contrasted with sequential access devices which must be accessed in order.

RAM is volatile; losing the stored information in an event of power loss, and quite expensive.

There are two basic types of RAM.

Static RAM does not need to be refreshed, which makes it faster; but it is also more expensive than dynamic RAM. Dynamic RAM needs to be refreshed thousands of times per second. Both types of RAM are volatile, meaning that they

lose their contents when the power is turned off.

ROM (Read only memory)

Is also random access but only for reads, once data has been written onto a

ROM chip, it cannot be removed and can only be read.

It refers to special memory used to store programs that boot the computer and perform diagnostics. Most personal computers have a small amount of ROM (a few thousand bytes).

Retains its contents even when the computer is turned off and is therefore referred to as being nonvolatile.

2.5      Secondary storage

These are devices which are used to store huge information for future use. This is mostly hard drives and removable media such as floppy disks, optical media (CD ROM) etc.

Hard Drive:

Floppy Disk: Floppy disks allow information to be transported easily from one computer  to  another  they  have  limited  storage capacity,  generally  1.44  MB. Saving and retrieving information from a floppy disk is slower than on a hard drive. They are more susceptible to physical damage and viruses than the hard drive. The size of a hard drive is usually expressed in terms of megabytes and gigabytes.

Compact Disk Read Only Memory (CD ROM): CD ROMs are read only storage medium. Typically, a CD ROM holds up to 650 MB of information. While information retrieval is faster than from a floppy disk, it is still not as fast as from the hard drive.

Compact Disk-Writable (CD-R): A CD-R is highly effective for storing a large amount of data. Can hold up to 700MB of information. A CD-R is a one time recordable compact disc.

Compact Disk-Re-Writable (CD-RW):

A CD-RW allows you to read, write, erase and write again. Writing takes place in a single pass of the focused laser beam. This is sometimes referred to as direct overwriting and can be repeated several thousand times per disc.

The  diagram  above  shows  how  the  units  interact  with  each  other  in  the processing of data. Input devices enter information to be processed by the processor. The processor can read and write into the secondary storage devices.

The processor also stores the instructions being currently executed into the main memory. So can be able to read and write into the main memory (RAM). Once the data has been processed by the processor, the data can be displayed by the output devices. Please note the direction of the arrows as it depicts the flow of the data and instructions.

2.7      How information is stored in computers

Information is stored in computers in the form of bits. A bit is used to represent

information in the computer. They are referred to as binary digits  i.e. the 0‟s and

1‟s with  0 representing an OFF state and 1 representing an ON state.

The stored bits are usually retrieved from computers memory for manipulation by the processor

A single bit alone cannot represent a number, letters or special characters, to represent information; bits are combined into groups of eight. A group of eight bits is called a byte. Each byte can be used to represent a number, letter or special character.

2.8      Size

Byte – a string of 8 bits

Kilobyte – 1,000 bytes Megabyte – 1,000,000 bytes Gigabyte – 1,000,000,000 bytes Terabyte – 1,000,000,000,000 bytes


2.9      Chapter Review Questions

  1. Which are the five basic units of a computer?

(a) Central processing unit, Arithmetic and Logic Unit, Input Unit, Output Unit, Visual Display unit

(b) Central  processing  unit,  Random  Access  Memory,  Input  Unit,  Output Unit, Visual Display unit

(c) Central  processing  unit,  Random  Access  Memory,  Input  Unit,  Output Unit, Visual Display unit

(d) Central processing unit, Main Memory, Input Unit, Output Unit, Backing Storage

  1. Which of the following is not an input device

(a) Mouse

(b) speaker

(c) Scanner

(d) Digital Camera

  1. Which of the following is not an output device

(a) Printer

(b) Scanner

(c) speaker

(d) Monitor

  1. Which of the following is not a task of the Central Processing Unit?

(a) It controls the transmission of information from application programs to output devices

(b) It controls the transmission of data from input devices to memory;

(c) It processes the data held in main memory;

(d) It controls the transmission of information from main memory to output devices

  1. Which of the following is used to store programs and data that are currently being used

(a) Read only Memory

(b) Hard Disk

(c) Random Access Memory

(d) Magnetic Disk


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