A standard fully featured desktop configuration has basically four types of featured devices

1. Input Devices 2. Output Devices 3. Memory 4. Storage Devices



Introduction to CPU


§ The Arithmetic / Logic Unit (ALU)

§ The Control Unit

§ Main Memory

§ External Memory

§ Input / Output Devices

§ The System Bus


The fundamental operation of most CPUs

- To execute a sequence of stored instructions called a program.

1. The program is represented by a series of numbers that are kept in some kind of computer memory.

2. There are four steps that nearly all CPUs use in their operation: fetch, decode, execute, and write back.

3. Fetch:

o Retrieving an instruction from program memory.

o The location in program memory is determined by a program counter (PC)

o After an instruction is fetched, the PC is incremented by the length of the instruction word in terms of memory units.

Decode :

1.The instruction is broken up into parts that have significance to other portions of the CPU.

2.The way in which the numerical instruction value is interpreted is defined by the CPU's instruction set architecture (ISA).

3.Opcode, indicates which operation to perform.

4.The remaining parts of the number usually provide information required for that instruction, such as operands for an addition operation.

5.Such operands may be given as a constant value or as a place to locate a value: a register or a memory address, as determined by some addressing mode.

Execute :

1.During this step, various portions of the CPU are connected so they can perform the desired operation.

2.If, for instance, an addition operation was requested, an arithmetic logic unit (ALU) will be connected to a set of inputs and a set of outputs.

3.The inputs provide the numbers to be added, and the outputs will contain the final sum.

4. If the addition operation produces a result too large for the CPU to handle, an arithmetic overflow flag in a flags register may also be set.

Write back :

1.Simply "writes back" the results of the execute step to some form of memory.

2.Very often the results are written to some internal CPU register for quick access by subsequent instructions.

3.In other cases results may be written to slower, but cheaper and larger, main memory.

Some types of instructions manipulate the program counter rather than directly produce result data.


Anything that feeds the data into the computer. This data can be in alpha-numeric form which needs

to be keyed-in or in its very basic natural form i.e. hear, smell, touch, see; taste & the sixth sense


Typical input devices are:

1. Keyboard 2. Mouse

3. Joystick 4. Digitizing Tablet

5. Touch Sensitive Screen 6. Light Pen

7. Space Mouse 8.Digital Stills Camera

9. Magnetic Ink Character 10.OpticalMarkReader

Recognition (MICR) (OMR)

11. Image Scanner 12. Bar Codes

13. Magnetic Reader 14. Smart Cards

15. Voice Data Entry 16. Sound Capture

17. Video Capture

The Keyboard is the standard data input and operator control device for a computer. It consists of the standard QWERTY layout with a numeric keypad and additional function keys for control purposes.

The Mouse is a popular input device. You move it across the desk and its movement is shown on the screen by a marker known as a 'cursor'. You will need to click the buttons at the top of the mouse to select an option.

Track ball looks like a mouse, as the roller is on the top with selection buttons on the side. It is also a pointing device used to move the cursor and works like a mouse. For moving the cursor in a particular direction, the user spins the ball in that direction. It is sometimes considered better than a mouse, because it requires little arm movement and less desktop space. It is generally used with Portable computers.

Magnetic Ink Character Recognition (MICR) is used to recognize the magnetically charged characters, mainly found on bank cheques. The magnetically charged characters are written by special ink called magnetic ink. MICR device reads the patterns of these characters and compares them with special patterns stored in memory. Using MICR device, a large volume of cheques can be processed in a day. MICR is widely used by the banking industry for the processing of cheques.

The joystick is a rotary lever. Similar to an aircraft's control stick, it enables you to move within the screen's environment, and is widely used in the computer games industry.

A Digitising Tablet is a pointing device that facilitates the accurate input of drawings and designs. A drawing can be placed directly on the tablet, and the user traces outlines or inputs coordinate positions with a hand-held stylus.

A Touch Sensitive Screen is a pointing device that enables the user to interact with the computer by touching the screen. There are three types of Touch Screens: pressure-sensitive, capacitive surface and light beam.

A Light Pen is a pointing device shaped like a pen and is connected to a VDU. The tip of the light pen contains a light-sensitive element which, when placed against the screen, detects the light from the screen enabling the computer to identify the location of the pen on the screen. Light pens have the advantage of 'drawing' directly onto the screen, but this can become uncomfortable, and they are not as accurate as digitising tablets.

The Space mouse is different from a normal mouse as it has an X axis, a Y axis and a Z axis. It can be used for developing and moving around 3-D environments.

Digital Stills Cameras capture an image which is stored in memory within the camera. When the memory is full it can be erased and further images captured. The digital images can then be downloaded from the camera to a computer where they can be displayed, manipulated or printed.

The Optical Mark Reader (OMR) can read information in the form of numbers or letters and put it into the computer. The marks have to be precisely located as in multiple choice test papers.

Scanners allow information such as a photo or text to be input into a computer. Scanners are usually either A4 size (flatbed), or hand-held to scan a much smaller area. If text is to be scanned, you would use an Optical Character Recognition (OCR) program to recognise the printed text and then convert it to a digital text file that can be accessed using a computer.

A Bar Code is a pattern printed in lines of differing thickness. The system gives fast and error-free entry of information into the computer. You might have seen bar codes on goods in supermarkets, in libraries and on magazines. Bar codes provide a quick method of recording the sale of items.

Card Reader: This input device reads a magnetic strip on a card. Handy for security reasons, it provides quick identification of the card's owner. This method is used to run bank cash points or to provide quick identification of people entering buildings.

Smart Card: This input device stores data in a microprocessor embedded in the card. This allows information, which can be updated, to be stored on the card. This method is used in store cards which accumulate points for the purchaser, and to store phone numbers for cellular phones.


Output devices display information in a way that you can you can understand. The most common output device is a monitor. It looks a lot a like a TV and houses the computer screen. The monitor allows you to 'see' what you and the computer are doing together.

Brief of Output Device

Output devices are pieces of equipment that are used to get information or any other response out from computer. These devices display information that has been held or generated within a computer. Output devices display information in a way that you can understand. The most common output device is a monitor.

Types of Output Device

Printing: Plotter, Printer

Sound : Speakers

Visual : Monitor

A Printer is another common part of a computer system. It takes what you see on the computer screen and prints it on paper. There are two types of printers; Impact Printers and Non-Impact Printers.

Speakers are output devices that allow you to hear sound from your computer. Computer speakers are just like stereo speakers. There are usually two of them and they come in various sizes.

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