Computer System

Computer System is a group of integrated parts that have common purpose of performing operations itself and the complete set of all  involved parts is called as computer system.

Components of Computer System

A computer system is a combination of six elements.

Hardware : - All the physical  components or parts which can be seen and touch known as hardware. It is tangible. For example,   Input devices, out devices, processing devices and storage devices etc.

Software : Software is the program instructions that tell the hardware, what and how perform a task. Without software, hardware is useless.

Data and Information : Data consists of the raw facts and figures that are processed  information. The processed data  or summarized data known as information.

People : People is user, operator, programmer who operates the computer, creates the software and can flow the procedures when using hardware and software.

Procedure : It is a description of how things are done, steps for accomplishing a result. Documentation manual, reference manual which contains the instructions, rules and guidelines to flow when using hardware and software.

Communication : It becomes a sixth system element because one computer is set up to share data and information electronically with another computer system.

Basic Hardware Components of a Computer System

Input Devices

A device which provides man to machine communications with computer. Input may any form textual, audible, visual/graphics etc. Input devices are given below:

1. Keyboard

One of the most popular input device used in almost all modern computers is keyboard. In this system it works as a type writer. The only difference between them is that, the typewriter directly gives output on the paper where as the keyboard gives output on screen. No back space or delete key is used in type writer but this process is possible in keyboard.

 Keyboard can be divide into five categories.

i.         Standard Type writer : - A -Z , a-z, 0-9, Enter key, Space bar, Back space, Shift key, Capslock, Ctrl etc,

ii.        Function key :- F1 to F12

iii.      Arrow or cursor keys :- Right Arrow, Left Arrow, Up Arrow and Down Arrow

iv.       Numeric or Calculator Key  :- 0 to 9, +, -, *, / and .

v.        Other Special Key :-  Home, End, Insert, Delete, Page Up, Page Down, Print Screen, Scroll Lock, Pause Break .

2. Mouse

 It is a small hand held device that works according to the movement done in the flat surface. It can move the cursor in any direction.

Basically there are 2 types of mouse:

                a. Mechanical mouse:

In this type of mouse, a small ball is kept inside the mouse and this ball is touching the surface or the pad through the hole at the bottom of the mouse.  When the mouse is moved the ball rolls. The movement of the ball is converted into the electrical signals and sent to the computer. Mouse is used for OS that have graphical user interface.

                b. Optical mouse:

 It has no moving part or the ball. It is used with the special pad covered with a grid of vertical and horizontal lines.

3. Track ball:

Track ball is a pointing device that uses a hard sphere to control cursor movement. It is similar to mechanical mouse. Instead of rolling the ball along the surface in case of mechanical mouse, the user rolls the ball in its place as if the mouse were turned upside down. It is used in Laptops and notebooks. The advantage of the track ball over the mouse is that it is stationary so it doesn't require much space to use it. 

4. Touch Screen:

The monitors or screens are designed such that the touch of the particular point on the screen with our finger is recognized by the system. There are very useful when the menu driven programs are run. The menu is displayed and the user can just touch the item to be selected with fingers.

5. Punched card

This method uses a rectangular thin card of size 18.8cm * 8.3cm, which is divided into 8 columns and 12 rows. Information is punched in the card using a card punching machine. The card punching machine has a keyboard similar to the typewriter. The card is inserted into the machine and set in the correct position. When the key of a particular character is pressed, the machine makes punches in one column and also prints that character at the top of the column.

6. Joy stick

A joy stick is a device consisting of a hand held stick which is mounted on cross bars with electrical contact at four points. When the stick is moved in any of the direction it makes contact with a given point and the cursor moves accordingly.

7. Touch Pad

 Many laptop computers use a touchpad in front of the keyboard. You move your finger on the pad to move the cursor on the screen. When you want to click, you can tap pad or use the buttons in front of the pad, which works like mouse buttons.

8. Scanners

It is used to scan pictures and scripts and store them in computers. If pictures have to be stored in the computer, the pictures can be scanned using a scanner. While scanning the pictures is held below the scanner. Immediately the scanner digitizes the pictures and stores it in the computer. We could immediately see the picture in the computers screen.

9. OCR

It stands for Optical Character Recognition/Reader. It reads printed characters directly and converts them into appropriate codes and stores them in a computer. When light source is passed over a document the written or typed characters reflect less light than the background area.

10. OMR

OMR is an input device that read marks such as pencil marks on a page which can be 80 column card or a full page. It is used to read forms and multiple-choice question papers. The user is required to mark the available options to indicate choice. The marks are then exposed to a beam of light for sensing and then optical mark reader scores the test by identifying the position of the mark. It is capable of operating at a speed of 10,000 A4 documents per hour.

11. MICR

It stands for Magnetic Ink character Recognition. The human readable characters are printed on documents using a magnetic ink.  The main limitation of MICR is only the 10 digits and 4 special characters needed for bank processing are used. No alphabetic characters are available. Special type of magnetic ink is required for encoding characters to be read by the machine.

12. Bar Code Reader

An input device used to read bar codes. The device may be a light pen or stylus that scans the code, or a non- contact optical or laser unit such as those used in supermarket checkouts. The bar code is read and converted into a number that the computer can process for inventory control and other statistical information.

13 Digitizer

 A computer peripheral that converts linear pictorial information such as  maps into digital data by tracing the image with a puck. Also known as digitizing tablet.

14 Touch Pad

When you want to click, you can tap pad or use the buttons in front of pad, which works like mouse buttons. You move your figure on the pad to move the cursor on the screen. Many laptop computers use a touch pad of keyboard.

15. Light Pen

Light pen is a pointing device which is very sensitive of light. Light pen is electrically charge pen which is written on the electrically charged writing pad, to store small documents or data. It can be connected to the computer through a cable.

16. Digital and Web Camera

Digital Cameras use CCDs (Charge Coupled Device). Within the digital camera the CCD sits in the same place where the film is in conventional camaras, exposing thousands of light sensitive pixels to the light. Each pixel converts the received information (i.e. color, intensity, etc) into an analogue electrical signal, which is converted into binary by the ADC (Analogue to Digital Converter).

17. Speech input device

 Speech or voice input device convert a person's speech into digital form and the input device when combined with appropriate software from voice recognition systems. Microphone is an input device.

Output device

These are the device through which machines communicates with man. It is the action of getting information from the computer. Once the CPU has executed the program instructions, the program may ask that information be communicated to an output device. The results of the job assigned to the computer are brought out by these devices in man readable form.

Output can exists in two forms. They are:

• Soft copy output
• Hard copy output

Soft copy output:

The outputs displayed on the screen are softcopy output. The output on the screen is lost when the computer is turned off. Monitor is the most commonly used soft copy output.

Monitor

It has a television like screen and the results are displayed in it. This is used to display text, graphics etc.

Monitor is of two types.

1.       Monochromatic soft white screen

2.       Color screen

Now days monochromatic monitors are obsolete and color monitors are widely used by almost all the systems.

For the construction of the screen or visual display unit following technologies are used.

¨       Cathode Ray Tube (CRT)

¨       Liquid crystal display (LCD)

¨       Light emitting diode (LED)

A CRT display unit is a commonly used output devices. it displays data or information received from the computer. A CTT is a vacuum tube, which is filled with an inert gas at very low pressure. The electron beam is directed towards the front of CRT when electrode touch the phosphour compounds, the kinetic energy of electron is converted into a visible light and coating glows to produce the picture or characters. it can be divided into two term.

1. Monochrome : it displays two colors, one for background and other for forground. the colour can be black and white color.

2. Color : Color monitor can displays 16 bit or higher with 1 million different colours, red, green, blue are used to produce color.

The color graphics adapter (CGA) monitors are capable of displaying low quality output.

Video Graphics Adapter (VGA) produces good quality of graphical output and widely used in most of the PCs.

Now days for GUI super VGA monitors are used.

LCD (Liquid Crystal Display)

Most flat panel display use LCD technology and consume less electricity. LCD displays sandwich cells containing tiny crystals between two transparent surfaces. By varying electric current supplied to each crystal, an image forms.

Hard copy output

Hard copy output refers to recording letters, graphics or pictures on a permanent medium such as paper.  Printer is a best example of it.

Printers are the most commonly used output devices that can be found in almost all computers.  There are several kinds of printers that designed for different applications. Depending upon their speed and approach of printing, printers can be classified into different types by their special characters. These special characters are:

·         Quality of type :- The o/p produced by the printer is said to be either letter quality or draft quality. Only daisy wheel , ink jet and laser printer produce letter quality type.

·         Speed  : - The speed of printer is measured in characters per second or page per minute. Daisy wheel printers tends to be slowest , printing about 30 cps. Line printers are fastest up to 3000 lines per minute. Dot matrix can print up to 500 cps and laser printer ranges from about 4 to 20 text pages per minute.

·         Impact or non impact : - Impact printers include all printers that works by striking an ink ribbon. Daisy wheel , dot matrix and line printers are impact printers.

Non impact printers include laser printer and ink jet printers. The difference between impact printer and non impact printers are that impact printers are much noisier.

·         Graphics  : - Some printers like daisy wheel and line printers can print only text whereas other printer can print text as well as graphics.

·         Fonts  :- Some printers like Dot matrix are limited to one or few fonts. In contrast laser and inkjet printers are capable of printing almost unlimited variety of fonts.

Types of printers

There are mainly two types of printers.           1. Impact Printers             &             2. Non-Impact Printer

1. Impact printers

                It refers to the class of printers that work by striking a head or needle against an ink ribbon to mark on the paper. This includes Dot matrix, daisy wheel and Drum printers. It is noisier kind of printer.

Various kinds of impact printers are:

Ø   Dot matrix printer

It is a kind of printer that prints the character in the form of a group of dots. So it is called Dot matrix printer. The writing head of the printer can print only one character at a time. Since the Dot matrix printer prints character by character, it is very slow than other. Its speed varies from 30 to 300 characters per second (cps).

Ø  Daisy wheel printer

It is a kind of printer where the characters are arranged on the ends of the wheel (just like as the petals of the flower daisy).

                To print a character, the printer rotates the disk until the desired result letter is facing the paper. Then the hammer strikes the disk , forcing the character to hit an ink ribbon, leaving an impression of the character on the paper.

Advantages              -      Give better quality of print than Dot matrix printer.

-            Used to produce simple graphics.

Disadvantages         -        Slow in printing

-            Expensive

Ø            Drum printers (Line Printer)

It consists of a drum having letter marked on them and rotates at a uniform speed. The paper passes between the drum and the hammer and strikes the character image through a carbon ribbon, when the required characters pass the print location. It is capable of printing at a very high speed ranging from 125 to 3000 lines/min. It is very expensive.

Non-Impact Printer

The Non-Impact printers uses chemical, head or electrical signals to etch or induce symbols on paper. Many of these require special coated or treated paper. Non-Impact printers are Thermal, Laser, Inkjet, Electrostatic etc.

1.       Thermal :- A type of printer in which the image is produced by localized heating or paper that has very thin thermo-sensitive coating containing two separate and colorless components.

2.       Laser (Page) Printer : - A non-impact printer which uses a deflecting laser beam on to the photosensitive surface of a drum and the latent image attracts the toner to the areas. The toner is then Electro statically transferred to the paper and fixed into a permanent image by heating.

3.       Ink Jet :- These printers are also called bubble jet printers. They print by spraying a controlled stream of tiny ink droplets accurately on the paper forring either dot or solid characters.

Central Processing Unit (CPU)

It is the computing part of any digital computer. It is known as the brain of the computer system. It performs execution, computation and transfers information between all the parts of the computer.

All the components inside CPU are linked together by buses.

·         BUS is basically a collection of wires, chip and slots inside the computer through which data is transmitted to different parts of computer from various devices. There are 3 types of buses:

Data bus: Actual data transfer takes place.

Address bus: Transmit the storage locations for data and instruction.

Control bus: Timing and controlling functions are done here.

Another important device is registers, which is the temporary storage location in the CPU.

·         Registers

It is a electronics device made up of transistors which serve as the temporary storage devices in the CPU. It quickly accepts stores and transmits the data and information that are being used immediately. The various types of Registers are:

Accumulator - A special purpose register which is used to store intermediate results obtained during program execution.

Program Counter - It keeps the address of the next instructions to be executed.

Instruction Register - It keeps the address of the current instruction being executed.

Memory Address Register - It contains the address of the memory location from which the data and instruction are to be taken.

Memory Buffer  Register - It temporary holds all data and instructions and is moved to and from the memory.

Input/Output Register : It is used to communicate with input and output devices. All input information such as instructions and data are transferred to this register by an input device. Similarly , all output information to be transferred to an output device is found in this register.

The main components of the CPU are:

1.       Control Unit (CU)

2.       Arithmetic and Logic Unit (ALU)

3.       Main Memory Unit

Control Unit

                The control unit controls all the parts of the computer. It is known as the nerve center of the computer. It consists of the registers to hold the address of the current instruction.

When the instructions is selected by the register and send to another registers at that time the operation part is sent to the decoder and address part to the address register. Now the control unit issues order to extract the content and send it to the ALU.

Arithmetic and Logic Unit

                                The arithmetic and logic unit (ALU) contain circuitry to perform the arithmetic operations. It also performs logical tests.

Operation done by ALU

                Data to be processed are taken from main memory and passes through the registers where they are initially stored. ALU then performs the required operation to be done as directed by the control unit. It then leaves the result to the data register from where it is again send to the main storage, from where the required result is send to the output device.

Memory Unit

There are two types of memory units (Primary or main memory  and Secondary memory or Auxiliary Memory)

Primary or Main Memory

Main memory is the principal internal memory system of the computer.

The memory consists of several cells and each cell is capable of storing one binary digit. These groups of cells are known as registers.

Basically memory unit is needed to:

-          Store the program and data during execution

-          Store the program of repetitive usage problems.

The basic types of memory used are:

o   Semiconductor Memory                                                                   

o   Cache Memory

o  Semiconductor Memory

The major types of semiconductor memory are:

Random Access Memory (RAM)

RAM is a high speed memory that can be written to and read from at any time. It is random because any of the bits can be accessed non sequentially. RAM is a volatile as well as temporary memory it is due to the reason that if another program is fed in it, the initial program will be erased. After the computer is off the data in the RAM get lost.

The different types of RAM are:

·         SRAM (Static RAM)

SRAM:

• Binary values are stored in the logic gates configurations.
• If the power is maintained then depending on the type of transistors used, data can be stored indefinitely without degradation.
• More expensive and high power consumption.
• Doesn't need refresh cycles
• Access time is 10-100ns.

DRAM:

• Made with cells that stores data as charge on capacitors.
• The existence of the charge on the capacitor is interpreted as a binary 1 or 0. Due to leakage, the capacitors discharges gradually and the memory cells loses the information.
• To preserve the information, the memory has to be refreshed periodically.
• Despite this inconvenience, DRAM is very popular memory technology because it's consequently low price and low power consumption.
• The access time is 40-70ns.

Read Only Memory (ROM)

ROM cannot be written by user, so the user can only read the information stored in it. These are permanently resided in the ROM chips by the manufacturer. When the computer is switched on, the sequence of operation is carried out automatically. It is non volatile because even if the power supply is lost it retains their contents as it is.

ROM is mainly of 3 types:

·         PROM (Programmable ROM)

It is a memory that can be modified once by the user. It means once the operations are written into PROM, they can't be changed.

·         EPROM (Erasable PROM)

It is a memory that can be erased and re-used. Basically used in the projects as well as experimental purposes.

·         EEPROM (Electrically EPROM)

It can be erased and reprogrammed repeatedly with special electrical pulses.

Cache Memory

It is a high speed buffer placed between the main memory and CPU. When the program is executed, cache memory is searched first and the referenced word is accessed in the cache if the word is present otherwise the word is brought into the cache from the main memory. Cache memory is faster than main memory.

Secondary Memory or Auxiliary Memory (Storage devices:)

As human beings used to keep the record in the file or at the drawer, when it is done manually. Similarly, in case of computer system it stores in its memory. Storage capacity of the computer system is faster as well as reliable than human being. In the initial stage the data are stored in the main memory from where it is being lost when the when the power supply is terminated. But another storage device is secondary device i.e. hard disk, floopy disk, CD R/W, magnetic tape etc. It is also known as backing storage or auxiliary storage.

·         Floopy disk

It is one of the popular storage medium also known as diskette. Basically used in general purpose PCs. Various sizes of floopy disks are introduced but largely replaced by 3.25" diskette. It is a square hard plastic coated with magnetically sensitive film and have a capacity of 1.44MB of data. It can also be used as input as well as output device because we can store as well as retrieve data from it. Floopy disk must be handled carefully.

Some points to keep floopy disk safe are:

o   Do not fold the floopy disk.

o   Do not touch the read write area of the floopy disk.

o   Never remove a disk from its drive while the light is on.

·         Hard disk

It is a smooth metal plate coated on both sides with the thin film of magnetic material. It works by the movement of the magnetic read-write head that moves across the surface of a disk coated with magnetically disk coated material.  It is a storage medium that store and provides relatively quick access to large amount of data. It is both input as well as output devices because we can store as well as retrieve data.

Advantages of hard disk over floopy disk

                                Hard disk can store more data than floopy disk.

                                Hard disk can access data 10 times faster than the floopy disk.

• Optical storage disk

It is also an auxiliary storage device works similarly as other storage device but the only difference is that they use light as a media to represent binary information. Optical disk can be found in three forms :

1.        Compact Disk

2.       WORM (Write Once Read Many)

3.       Read and Write Optical Disk

4.       DVD - ROM disk and drives

1. Compact disk

One of the most popular optical storage devices is CD. It is made up of resin and consists of aluminum coated plastic for the reflection of the light in order to read smoothly.

The advantages of the CD-ROM are:

-            The storage capacity is high.

-            Data can be stored up to unlimited time span.

-            It is reliable.

The disadvantages related to the CD-ROM are:

-            That can't be erased once written.

-            It can be damaged by high temperature and strong light sources.

2. WORM (Write Once Read Many)

It allows the user to write the information once and read it as many times as needed. The most popular size is 5.25" which can hold 200 to 800 MB of data.

3. Read and Write Optical Disk

Here the user can read as well as write the information as many times as it is needed. It has the capacity of 650 MB of data.

4. DVD - ROM disk and drives

Digital Video Disk or Digital Versatile Disk (DVD) is the newest format of the optical disk. It is specially designed to work with a video players and television. It can only access with DVD drive along with a computer. The basic advantages associated with the DVD - ROM is that all the data sets are compiled in a single disks. Therefore now the DVD - ROM is replacing the CD-ROM drastically.

Port

A port is a connector which is used to connect the processor to the peripheral devices. Data flow in and out through these ports. Computers  have a few built-in interface called ports. There are two types of ports

1. Parallel ports : A parallel port allows the transfer of all the bits of word simultaneously. A parallel interface is used to transfer data at faster rate for high speed peripheral such as disk. Within the computer data is transferred in parallel so that the computer works at high speed.

2. Serial ports : A serial port allows serial data transfer one bit of data is transferred in serial port. Only one line or pair of lines is used to transmit data. It is used for slow speed peripherals such as terminals. A serial port is used to connect to either a telephone line (through modem) or the mouse.

Interrupt

The interruption of normal program execution of microprocessor caused by external events is called interrupt. When an I/O device wants to transfer data, it informs microprocessor by sending a signal to the microprocessor. Thus, the microprocessor is interrupted by an external device. The microprocessor can also be interrupted by external abnormal conditions. An interrupted caused by an abnormal condition is called exception. Types of interrupt:

a) Software Interrupt : The microprocessor can also be interrupted by inserting a special instruction in a program. This type of interruption is called software interrupt. e.g. power failure, automatically restarting etc.

b) Hardware Interrupt : The interruption which is caused by external device is called hardware interrupt e.g. keyboard error.

Some source of interrupts are:

a) An interrupt  caused by power failure: The Kernel saves vital information using the dying power supply so that it can be restarted when power is returned.

b) Arithmetic or logic errors : When ALU detects that an errors has taken place it generates a signal that causes the interrupt.

c)  Hardware malfunction: I/O control for each device and its associated hardware normally takes care of parity checking. If a parity error is detected, the Kernel is notified by interrupt and so on.

d) External interrupts: Caused by events such as operator passing an interrupt key in console or caused by signals from other process in a multiprocessor or machine.

Modem(Modulation-Demodulation) is a device that allows a computer to transmit information over a telephone line. The modem translates between digital signals that the computer uses, and analog signals suitable for transmission over telephone lines. When transmitting, the modem modulates the digital data onto a carrier signals on telephone line. When receiving, the modem performs the reverse process and demodulate the data from carrier signals. It is mostly used in telecommunication/ internet browsing.

Virtual Memory : The technique which allows a program to use main memory that what a computer really has is known as virtual memory . It  gives an illusion that they have main memory available more than what is physically provided into computer. The entire program and its data are not placed in the main memory. The entire program and its data are not placed in the main memory. Only the instruction and the data which are to be currently executed are brought from secondary memory into main memory. the remaining part of programs resides in secondary6 memory. The address which are used in programs are called logical addresses. The address indicate the position of instructions and data in the program. the logical  addresses are translated into physical addresses. by memory management unit (MMU). the CPU generates logical address while executing programs.

Backup Memory : Backup memory is used to store copy of important programs, operating system, compilers etc. Floppy disks, optical disks and magnetic tapes can be used as backup memory. These programs are generally available in the backup memory. so that they can be reloaded into secondary memory in the case the programs stored in the secondary memory are lost accidentally or due to any other reason.

SIIM :

1. SIMM (Single Inline Memory Module) circuit cards contain anywhere 3 to 9 memory. Memory chips soldered on one side of the card. the complete module is organized as byte-wide memory module.

2. Used in P1, PII computer.

3. It is slower

4. Less pins (30, 70 pin).

DIMM

1. DIMM (Double Inline Memory Module) circuits cards memory chips and contacts are placed on both sides of printed circuit card.

2. Used in PIII, PIV computer.

3. It is faster.

4. More pins (168,184 pins).