Computer System Organization

Computer System Organization

System

          A system is a group of integrated parts that have the common purpose for achieving some objectives. So the following are the three characteristics for a system:

Ø Organisation               àArrangements of components.

Ø Interaction                  à Connection of each component.

Ø Interdependence      à Depending of one component to another

Ø Integration             àMore than one components working together.

Ø Central objective      àMain/Common goal with highest priority.

So human body, computer, school, bank are examples for a system.

Functional Units of Computer (Block Diagram of Computer)

The components of computer system are:

1) Input Unit.  2) Central Processing Unit.  3) Output Unit.  4) Memory Unit.

1. Input Unit

The input device accepts data and information from the user, and then it converts to computer acceptable form.

They are:

a.   Keyboard

b.   Mouse

c.    Optical Character Reader (OCR)

d.   Optical Mark Reader (OMR)

e.    Magnetic Ink Character Reader (MICR)

f.       Scanner

g.   Digital Camera

h.    Web Camera

i.        Bar Code Reader

j.        Microphone

k.     Joystick

l.        Track Ball

m. Light Pen

Keyboard

It is used to inputting text data such as numbers, characters, symbols etc. Usually it consists of 101 to 105 keys. There is a special key for cursor control, insertion, deletion, scrolling and also some function keys. A numeric pad is at the right side to input numeric values.

Working of keyboard

When a key is pressed the processor inside the keyboard detects pressed key. Then a number corresponding to the key is passed to the motherboard then to the microprocessor. It converts the number to the corresponding character.

Mouse

It is a pointing device to point and select objects from the screen and also draws pictures.

They are

                             1. Serial and parallel mouse

                             2. Optical mouse

Working of Mouse

Two light resources with photo detectors in optical mouse emit light. It reflects back to small plastic lenses. When mouse move a grid line detect the light. The mouse returns it as electric pulse to the processor.

Optical Character Reader (OCR)

An OCR is a device that can read characters printed with a predefined font. Characters scanned with optical scanner. Scanned image converted to ASCII code and compare with original. It is faster input method with fewer errors. But only limited characters are used. For proper reading high quality printing is needed.

Optical Mark Reader (OMR)

They detect marks made by a dark pencil or pen on special pre printed form. The OMR scan the document and transform it in to electric pulses. They Used in,

1.    Objective type exam

2.    Surveys.

3.    Order forms

Advantages:

Reliable

Speed

Accurate

Magnetic Ink Character Reader (MICR)

It can recognize human readable characters printed on documents with magnetic ink and special font. They mostly used in bank cheques to print cheque number. So speedily manipulate the cheques with high accuracy.

Scanner

They are used to scan and digitalize images, documents etc. Here a light source moves to and fro to read the document. The detectors convert the document to electric pulse.

Digital Camera

A digital camera allows taking digitized photographs or videos without using films. Without using any other studio equipments we can directly transfer the image to the computer. To store images memory card is used.

Web Camera

Mainly used while we chat or video conferencing to others through internet. The images are less quality than digital camera.

Bar Code Reader

It converts a pattern of printed bars (a sequence of black and white lines of different widths) in to a number. A bar code reader emits light to the code and it reflects back to the device. They used to identify products, books, certificates etc.

Microphone

They help us to input sound to the computer. It translates the vibration in the air in to electric pulse. The sound can store and reuse for voice recognition applications.

Joystick

It is a pointing device used to select and move objects on the screen. They are mainly used to play simulation games. It place in a base, it cal move any direction. It has two control buttons.

Track Ball

It is also a pointing device like mouse. It has a ball, can rotate to control the cursor movements. They are especially used in laptops.

Light Pen

Light pen is a pointing device. To draw pictures, design objects and also put digital signatures directly on the screen. Here the photocell inside the light pen responses the picture element on the screen.

2. Central Processing Unit (CPU)

It is the brain of the computer. It controls, operates and co-ordinates all operations inside the computer. It is also called microprocessor which is a single chip. It has two parts,

1. Control Unit (CU)

          It co-ordinates and controls all activities of computers

2. Arithmetic and Logic Unit (ALU)

          It perform all arithmetic and logical operations

3. Output Unit

It communicates the result of processing to the external world. They translate machine representation in to human readable form. So it is an electro mechanical device.

They are classified in to three,

1.    Visual Display Unit (VDU)

2.    Printers

3.    Plotters

Visual Display Unit (VDU)

It is a standard and soft copy output device of the computer.

They are classified into two,

1. Monochrome Monitors and Colour Monitors with CRT (Cathode Ray Tube)

2. LCD (Liquid Crystal Display) Monitor

Monochrome Monitors and Colour Monitors with CRT (Cathode Ray Tube)

CRT is a vacuum tube. The rectangular part is coated with phosphor. An electron gun (In colour Monitor 3 electron guns for red, green and blue) emits electrons towards the screen. It creates and glow tiny dots called pixels on the screen. The number of times a monitor scans the entire screen in each second is called refresh rate.

Disadvantages of CRT Monitors

They are heavy

Bulky in size

High power consuming

Not portable

Make eyestrain

LCD (Liquid Crystal Display)

They are made up of special kind of liquid crystals, placed between two plates. When electricity passed through it, crystal liquid become solid and blocks the light.

Advantages of LCD Monitors

They are weightless

Slim size

Low power consuming

Portable

Not make eyestrain

Printers

They are used print the data on a paper. Printed output is the most useful and convenient form of information for users. So it is a hard copy device.

They are classified in to two:

1.    Impact Printers

2.    Non Impact Printers.

Impact Printers

In impact printers there is physical contact between with print head and paper. It produces noise. Slow printing speed. It produces poor quality prints.

They are again classified into two,

1.    Line printers

2.    Character Printers

1. Line printers

They print entire line of text at a time. So it is used to print large volume of data.

Examples:
1. Drum printer

2. Chain Printer

1.a) Drum printer

It has a rotating cylindrical drum with characters are embossed on it. Printing hammer is used to strike to print the lines from the print buffer. A carbon ribbon is placed between paper and drum. But here we cannot change the font settings.

1.b)Chain Printer

It has a rotating steel band or chain on which characters are embossed. Each character has print hammers. A carbon ribbon is used between hammer and chain.

2. Character Printers

They are slow and print a single character at a time.

Example:

2.a) Dot Matrix Printer

Characters are formed by the impact of pin sets. When print head moves pins are strike on the paper. They versatile (can print both text and graphics), Print cost is low but low speed, low quality and noisy. But we can change the font settings.

Non Impact Printers

There is no physical contact between print head and paper. Printing speed is high. It produces high quality with colour printing. They do not produce noise.

Example:

1. Inkjet Printers

2. Laser Printers

1. Inkjet Printers

Here ink is used as carbon ribbon. Printing is done by spraying ink to the paper through a nozzle. There is no physical contact between print head and paper. Printing speed is high. It gives high quality with colour printing and it does not produce noise.

2. Laser Printers

Here carbon powder (toner) acts as carbon ribbon. Printing is done by electrostatic process. There is no physical contact between print head and paper. Printing speed is high. It gives high quality with colour printing and it does not produce noise. It has 180-300 CPS speed.

Plotters

They are used to produce large drawings or images. Used to print building plans, machine designing, blue prints etc. It has arms with coloured pens. Useful in CAD (Computer Aided Design)

They are two types:        1) Drum plotter

                                       2) Flat bed plotter

Memory Unit

They are used to store any type of data and information and also can retrieve when the user or other devices are needed.

They are classified in to two,

1)   Primary Memory (Main Memory)

2)   Secondary Memory (Auxiliary Memory)

Primary Memory (Main Memory)

They are used to store the data temporarily on the computer system.

They are classified in to two

1.    Random Access Memory (RAM)

2.    Read Only Memory (ROM)

Random Access Memory (RAM)

It is used for storing programs for execution by the CPU and also data accessed and processed by the CPU.

They are classified into three,

1. Dynamic RAM (DRAM)

          It frequently refreshes its contents.

2. Static RAM (SRAM)

          It does not refreshing its contents frequently.

3. Cache Memory

          It is a high speed buffer in between RAM and CPU

Read Only Memory (ROM)

The data on ROM only to read the contents cannot modify or edit it. It is used to store permanent programs like Boot strap loader to boot up the computer.

They are classified into three,

1)   PROM (Programmable ROM)

2)   EPROM (Erasable PROM)

3)   EEPROM (Electrically EPROM)

Secondary Memory (Auxiliary Memory)

They are used to store the data permanently on the computer system.

They are classified in to two

1. Sequential Access

Here the data accessed in a sequential manner i.e. one after another

Example: Magnetic Type

2. Direct / Random Access

Here the data accessed from any location randomly.

It is classified in to two:

Magnetic Disk and Optical Disk

Magnetic Disk

They are classified in to two

1.    Floppy Disk

2.    Hard Disk

Floppy Disk

It is made up of plastic coated with magnetic film. Its capacity is 1.44 MB. To rotate this it has a hub at its centre. A metallic slide protects the read write opening. There is a read write protection notch.

A floppy drive is used to read the data from a floppy disk. Data will lose when it kept inside a magnetic field or other chemicals, because it coated with magnetic substances.

Hard Disk

It contains a group of metallic disks, coated with magnetic material in a dust proof case. Each plate has read write head. It has huge capacity from 10 GB to 250 GB. The average delay required to access data from the disk is called access time. The recordable surface of a disk is divided into number or invisible concentric circles called Tracks. Each track again divided into pie shaped segments called sectors.

Optical Disk

They are classified into two,

Compact Disk (CD)

It is made up of a layer of aluminium in between two plastic plates. Its capacity is 700 MB. It may be CDROM or WORM (write ones read many) or CD R/W. To read and write high beam of laser light is used.

Digital Versatile Disc (DVD)

It is faster and has more storage capacity than CD. Its capacity is 4.7 GB.

CPU Registers

They are used to retrieve and store data at an extreme speed when manipulations are done by the CPU on a temporary basis.

They are classified into six,

MAR (Memory Address Register)

MBR (Memory Buffer Register)

PC (Program Control Register)

A (Accumulator Register)

I (Instruction Register)

I/O (Input Output Register)

Performance of disk memories

It means the read write speed of data to and from the memory.

They are,

a)  Access time: The total time taken to read and write data to and from a disk is called access time.

b)  Seek time: Time taken to locate the head on the track on which data to be read or write on a disk.

c)   Latency time: Time taken for the rotation of the disk to position the head on the right sector for read and write.

d)  Transfer time: Time taken to transfer data from RAM to disk and vice versa.

e)   Hit ratio: It is the percentage of records updated on every run.

Software

                  It is a set of instructions to perform a desired and useful work done by the help of computers. They are classified in to two. They are 1) System Software 2) Application Software.

System Software

                  It is a collection of programs that directly control the computer’s internal operations and also that help ordinary users to make use of computer effectively and efficiently. It is also known as system package.

                  They are again classified in to two. They are 1) Operating System, 2) Language Processors.

Operating System

                  It acts as an interface between the user and the hardware. It is a set of programs that control, co-ordinate the operations of a computer and help to make efficient use of resources. It has the following functions.

Process management, Memory management, File management, Security and Command interpretation. Example: MS Windows XP, Vista, 7, Linux, DOS

Language processors

                  They are used to translate the assembly or high level language programs or instructions in to equivalent machine language instruction. They are classified in to three. They are 1) Assembler 2) Compiler and 3) Interpreter.

Assembler

          It converts machine language or assembly language codes. Which is uses numbers and memory locations for representing instructions.

Compiler

          It translates all lines of high level language program at a time in to its equivalent machine code. The language like C++ and VB are used compiler.

Interpreter

          It translates high level language program in line by line in to its equivalent machine code. The language like BASIC used interpreter.

Compiler v/s Interpreter

Compiler	Interpreter
It translates whole code at a time.	Only one statement is translated at a time.
Errors listed only after the compilation.	Errors listed in each line.
Compiler generates an object program.	It does not generate an object program.
Execution of program requires source program or compiler.	Execution of program requires both source program and interpreter.

Application Software

          Application software is a set of one or more programs, designed to solve a specified problem, or do a specific task for an individual or an organization. They are classified in to three. They are 1) Pre- written packages (General Purpose), 2) Customised software, 3) Tools and Utilities.

Pre- written packages (General Purpose packages)

          It needs for too many users. They are.

·        Word processors

(MS Word, Open Office Writer, Soft word, LEAP, ISM)

·        Electronic Spread sheets

(MS Excel, Open Office Calc, VisiCalc, Lotus and Quattro pro)

·        Graphics and Multimedia

(Adobe Photoshop, Gimp, Paint shop pro, Maya, Ulead Photo impact)

·        Desk Top Publishing (DTP)

(Adobe PageMaker, CorelDraw, Quark Express, MS Publisher)

·        Database management systems

(DBMS) (MS Access, My SQL, Oracle)

Customised Software

          They are developed to solve an individual or organization’s need.

Example: Pay roll, Air line reservation, sales and inventory control, Hospital management, Library management, Supermarket management, banking, insurance, accounting etc.

Utilities

          They are used to assist the computer for back upping files, scanning viruses etc. they are Text editor, Backup Utility, Compression Utility(Zipping), Disk Defragmenter, Antivirus software etc.