Chapter - 5 Computer System

5.1 CONCEPT OF COMPUTER ARCHITECTURE
The theoretical concept behind the design of computer is called computer architecture.
5.2 CONCEPT OF COMPUTER ORGANIZATION
The subject which deals with about the work, which can be performed by computer system, is called computer organization.
5.3 COMPONENTS OF COMPUTER SYSTEM
Definition of Computer System:
The integration of different computer parts such as keyboard, mouse, monitor, CPU etc to form an entire system to work effectively and efficiency is called Computer System.
Figure: Computer System
# BLOCK DIAGRAM OF COMPUTER SYSTEM
Input Devices
CPU
Output Devices
Memory

a) INPUT DEVICE
An electromechanical device that allows the user to feed data and instruction into the computer for analysis, storage, and to give commands to the computer is called input device.
An input device performs the following functions.
i) It accepts or reads the data and instructions from the user.
ii) It converts these inputs in the computer acceptable form i.e. binary code.
iii) It supplies the converted inputs to the computer system for further processing which are stored temporarily in RAM.
The keyboard is the most commonly used input devices. Various input devices which do not require typing of information have also been developed i.e. mouse, joystick, touch pad etc. which are called pointing device.
Examples of input devices are keyboard, mouse, joystick, Trackball, Touch screen, Light pen, Touch pad etc.
b) CPU(CENTRAL PROCESSING UNIT)
CPU is the main part of computer that controls the overall operations of a computer and interprets program and activates necessary components of ALU for execution of a program. The CPU executes programs (instructions) stored in main memory. It takes input from the input device or memory and sends output to the output device or stores to the memory. CPU contains of ALU, CU, and Registers.
i) ARITHMETIC LOGIC UNIT (ALU)
It is responsible for doing all types of arithmetic operations like addition, subtraction, multiplication and division on integer and real operands. It also can be perform logical operation or comparison like equal to, greater than, less than etc. between two operands. It receives its operands from the register section of the CPU and stores its results back in the register section.
ii) CONTROL UNIT (CU)
 The control unit of the microprocessor co-ordinates and controls the various operations and indicates the appropriate sequence of micro-operation for each task. It controls and guides the interpretation, flow and manipulation of all data. It is responsible for executing the instruction of the program and fetching the data required for the next instruction from memory. It also controls and coordinates the functions of all other units of the computer and ensures smooth operation.
 It is mainly responsible for the following:
·        Data exchange of CPU with the memory or input/output components.
·        Internal operations in the CPU such as moving data between registers.
·        Making ALU perform a particular operation on the given data.
·        Regulating other internal operations.
·        Generate control mechanism to hardware and software.
iii) REGISTER
Electronic memory chips made from transistors and flip-flop which serve as temporary storage location in the CPU are called registers.
·        A semiconductor device used to amplify and switch electronic signals and electrical power is called transistor.
·        The memory element most frequency used in sequential circuit is called Flip-Flop.
The instructions execution is performed in the CPU by taking data from registers. Registers quickly accept, hold and transfer the data and instruction which are being executed currently. The size and number of these registers determine the overall speed of the computer.
The length of a register equals the number of bits it can store. Hence a register that can store 8 bit is called an 8-bit register. Most CPUs sold today have 32-bit or 64-bit registers the length of the register of a computer is sometimes called its word size. The bigger the word size, the faster a computer can process a set of data with all other parameters being same, a CPU with 32-bit registers can process data twice as large as one with 16-bit registers.
Registers in CPU are of two types:
1.     Specific Purpose Register (SPR)
2.     General Purpose Register (GPR)
Some examples of CPU specific purpose registers are:
Ø Memory Buffer Register (MBR) to hold data.
Ø Memory address Register (MAR) to hold address.
Ø Program Counter Register (PCR) to hold next instruction to be fetched and executed.
Ø Stack pointer Register (SPR) to hold address of top of stack memory. Stack is temporary storage area where data is stored temporarily during execution in Last In First Out (LIFO) order.
Other general purpose registers are:
·        Accumulator (Acc) to be used in arithmetic calculation.
·        Counter Register (CX) to count number of repetition of execution.
·        Base Register (BX) to store base address of data.
·        Scratch pad registers to hold operands and result.
The functions of the CPU (Processor) are:
Ø To carry out processing.
Ø To give commands to all parts of the computer system.
Ø To control the sequence of operation.
Ø To control the storage of data or instructions.
Ø To perform arithmetic calculations such as addition, subtraction etc as well as logical operations such as AND, OR, NOT operation.
c) MEMORY/STORAGE
The memory is the computers storing area. This is the place where our data and programs get stored before processing and the result also stored before to its output. Data and instructions are stored in memory in the binary form. Bit is the smallest unit of computer data. The combination of 8 such bits is called a byte. Each byte is also referred to as a single character.
The memory which can retain its memory contents only while the computer is running is called volatile memory. The volatile memory loss its contents after the computer are shut down. It serves for temporary purpose. RAM   and cache memory are the examples of volatile memory devices in computer system.

The memory which can retain its memory contents even after the computer is shut down is called Non-Volatile memory. The non-volatile memory does not lose its contents after the computer is shut down. It serves for permanent purpose. ROM, Hard disk, CD, DVD etc are the example of non-volatile storage devices of computer.     
Only the data and instruction which are being processed and the currently running program are stored in RAM which is also called the memory or main memory of computer system. The main memory in computer is always limited very fast and very expensive in comparison to secondary storage.
Since RAM is volatile, computer system has a separate memory place to permanently save the data, information, programs etc for future use and it is called secondary storage. The examples of secondary storage are Hard disk, Floppy disk, CD, DVD etc.
d) Output Device
The device which shows the results of processing and other information from the computer to the human understandable language form is called output device.
The functions of output device are as follows:-
1)    It accepts the result produced by the computer which is in binary form.
2)    It converts binary form of results to human understandable form.
3)    It shows the converted results to the user.
Computer output can be classified into two forms:-
Soft Copy Output
Hard Copy Output
1. An output which is in unprinted digital document file is called soft copy output.
1.  An output which is in printed form is called hard copy output.
2. It is a temporary form.
2.  It is a permanent form.
3. It can be viewed through an appropriate editing program such as Microsoft Word, PowerPoint etc.
3.  It cannot be viewed through any program.
4. It can be edited.
4.   It cannot be edited.
5. It can be transported from one computer to another through email or through USB drives and other disk drive.
5.  It cannot be transported from one computer to another very easily.
6. Example:-Any file is saves in computer.
6.Examples:- Computer file printouts, fax pages, photo prints etc.

5.4 MICROPROCESSOR:
A multipurpose, programmable logic device that reads instruction from a memory, accepts binary data as input and process data according to those instructions and provides a result as output is called microprocessor. Nowadays microprocessor is also called processor. It is a complete CPU built on a single chip by using VLSI or ULSI technology.

Fig: Microprocessor
Intel Corporation of USA developed the first microprocessor named ‘Intel 4004’ in 1971 AD. It contained about 1600 transistors. It was a 4-bit microprocessor and process only 4-bits of data at a time. The modern processors are available in 32-bit, 64-bit or higher bits world length.
The important characteristics of a microprocessor are the width of address bus, data bus, clock speed and its instruction set architecture. Processors are also often classified as being either RISC (Reduced Instruction Set Computer) or CISC (Complex Instruction Set Computer).
The microprocessor is responsible for the following functions:
a)     To control and co-ordinate all the operations and other components of the computer system.
b)    To carry out processing.
c)     To give commands to all the other components of the computer system.
d)    To control the sequence of operations in the computer system.
e)     To interpret the data, instruction and information in the computer system.
The microprocessor has three components.
Ø Control unit(CU)
Ø Arithmetic and Logic Unit (ALU)
Ø Register
5.5 CONCEPT OF SYSTEM BUSES
As we need a highway to travel from one place to another place. Similarly the input and output of computer system also need a path to transmit data, information and control from one device to another device. The data and instruction have to electrically flow from one device to another device to fulfill the requirements of the user. The data transmit from input device to CPU, CPU to memory, output device to CPU, CPU to output device etc. are common example of system Bus.

A communication pathway used for data transfer among the components of a computer system is called bus.
It is a collection of wires, chips and slots inside the computer through which data and instruction are transmitted from one part of computer to another part. In general electrical pulses are transmitted from source to destination through the system buses.
FUNCTIONS OF BUS
Ø It travels information from one place to another.
Ø It may carry data, address or instruction.
Ø One component of computer interacts with other via bus.
There are three types of system bus.
a)     Data Bus                  b) Address bus         c) Control Bus
a) DATA BUS
The pathway through which transmit of data from one memory location to other occurs is called Data Bus. Data bus is the path through which data transfer takes place from one device to another. When the CPU fetches data from memory, it first outputs the memory address on its address bus. Then memory transfers the data onto the data bus; the CPU can then outputs the data onto the data bus. When writing data to memory, the CPU first outputs the address bus, and then outputs the data onto the data bus. Memory then reads and stores the data to the proper location. The data flows in bidirectional way.
b) ADDRESS BUS
The pathway through which transmit of address of memory location occurs is called Address Bus. All types of memory devices have microscopic memory cells which are identities with unique numbers known as memory address or memory locations. When the CPU reads data or instruction from memory or writes data to memory, it must specify the address of the memory location it is going to access. Unlike the other buses, the address bus always receives memory location from the CPU. There is one way flow in address bus.
c) CONTROL BUS
The pathway through which transmit of control signal to operate and control devices and software takes place is called control bus. It is different from other two buses. The control bus is the collection of individual control signals for timing and controlling function sent by the control unit to other units of the system. These signals indicate whether data is to be read or written, whether the CPU is accessing memory or an input/output device, and whether the I/O device or memory is ready to transfer data.
5.6 MEMORY
The memory is the working area of computer. The place where our data and instruction get stored before processing and the result also gets stored before to its output is called memory. Data and instructions are stored in memory in the binary form.
a)    PRIMARY MEMORY
The primary memory of computer is also called main memory of computer. It is fast and less capacity memory of computer. The primary memories of computer are RAM, ROM, Cache memory, registers etc. They are mainly used to store data during a time of processing. The fast and integrated memory of less capacity is termed as primary or main memory. These memories have great effect on processing speed of computer. The memory also has an input-output port through which the data transfer takes place to and from the addressed location. This port is called Memory Buffer Register (MBR) or Memory Data Register (MDR). The main function of main memory is READ, WRITE and CLEAR.
1.     RAM (Random Access Memory)
It is called RAM because its contents can be accessed randomly. We can read from RAM as well as write into it. Hence, it is also called ‘read – write’ memory. The main drawback of RAM is that it is volatile memory so the contents of RAM are lost when the computer is switched off.
RAM is mainly used to store the following:
Ø Kernel of running operating system.
Ø Data and instruction currently being processed.
Ø Data and instruction waiting to be processed.
Ø Information waiting to its output.
Ø Currently arrived data and instruction from input system.

There are 2 types of RAM.
i) DRAM (Dynamic Random Access Memory)
It is the most common and cheapest type of memory chip. It is made of capacitors which is capable of storing the electric charge. Due to leakage of charges, the capacitors discharge gradually and the memory cells lose their contents. DRAM has to be refreshed periodically by recharging the capacitors to retain its memory contents. It is slower than SRAM but it is dense, consume less electricity, smaller in size and less expensive. Example: - DDR (Dual Data Rate), DDR2 and DDR3 etc.
ii) SRAM (Static Random Access Memory)
It is made of transistors. It is called static because it can remember or retain its memory contents without being refreshed or recharged as long as there is power. SRAM does not need to be refreshed or recharged periodically like DRAM. SRAM is faster than DRAM but more expensive, less in density and bigger in size and consumes more electricity.


Difference between SRAM and DRAM: -
SRAM
DRAM
1. It requires 6 transistors per bit.
1. It requires 1 transistor and 1 capacitor per bit.
2. It is expensive.
2. It is less expensive.
3. It has less density.
3. It has high density.
4. It is fast.
4. It is slow.
5. It does not need periodic refreshing.
5. It needs periodic refreshing.
6. It occupies large space.
6. It occupies small space.
7. It consumes less electrical power.
7. It consumes high electric power.

2. ROM (Read only Memory)
It is called ROM because only read operation can be performed on it. The user can not write anything into ROM. The binary information stored in ROM is written permanently by the manufacture and it can not be altered. The program such as start up or booting instructions which does not need modification are stores in ROM. ROM is necessary to store such software which enables the computer  to boot up. ROM is non-volatile memory because it can retain its contents even after the computer is turned off. It is also made by semiconductor chips.
The program stored permanently in ROM is called Firmware. Hence, firmware is immediately available when a device is powered on to start up the PC or other electric equipment like mobile, PDA and others.
The types of ROM (READ ONLY MEMORY):
i) PROM (Programmable Read Only Memory)
PROM can be written to or programmed via a special device called a PROM programmer. Typically, this device use high voltages to permanently destroy or create internal links within the chip. Consequently, a PROM can only be programmed once.
ii) EPROM (Erasable Programmable ROM)
EPROM is an erasable PROM. The data stored in EPROM can be erased by exposing it to ultraviolet rays. When an EPROM is exposed to ultraviolet light, the entire data are erased. The user can not erase the content of certain selected memory location. It is used to store programs which are permanent but needs frequent updating. It is usually reprogrammed without removing it from the circuit board. It is far more economical than PROM as it can be reused.
iii)EEPROM (Electrically Erasable Programmable ROM)
It is an electrically erasable PROM. It can be erased and reprogrammed on the byte by byte basis. Either a single byte or the entire chip can be erased in one operation. It requires much shorter time, a few milliseconds for erasing as compared to EPROM. Varying with different manufactures, it has a limit on the number of times for which it can be reprogrammed. It need not be removed from the circuit board for erasing as EPROM.
Difference between RAM and ROM
RAM
ROM
1. It allows both read and write operations.
1. It allows only read operation.
2. It is volatile in nature i.e. data are lost when power supply is switched off.
2. It is non-volatile in nature and used for permanent storage.
3. It is used when IO operation is performed known as buffering.
3. It is not used for buffering purpose.
4. It is usually has higher memory space than ROM.
4. It usually has less memory space than RAM.
5. The second operation after booting the computer system is performed in RAM.
5. The first operation in computer system is performed in ROM (during booting process).
6. It is usually cheaper in cost.
6. It is usually expensive.
7. Types of RAM are SRAM and DRAM.
7. Types of ROM are PROM, EPROM AND EEPROM.

3. Cache Memory
Cache memory pronounced as cash, it a special, high speed and expensive semiconductor memory placed between RAM and CPU. It is used to in CPU, so that CPU doesn’t have to wait for data to be delivered. The most frequently used instructions are kept in the cache memory so that the CPU to run faster, because it doesn’t have to take time to swap instructions in and out of RAM. It is made by the components of static RAM. It is usually 5 to 10 times faster  than main memory. Hence, cache memory is placed between RAM and CPU so that the speed of operation of main memory and cache memory together can meet the speed requirements of the high speed CPU.
There are two processes in Cache:
Ø Cache Hit: At first processor, send address of desired data to cache, if data is available than event is called Cache Hit.
Ø Cache Miss: When processor sent address is not present in cache, event is called Cache Miss. After cache Miss, data of main memory is copied to cache for next cache Hit.
Features of Cache Memory:
·        It is temporary memory.
·        It is small size memory.
·        It is fast memory.
·        It increases the speed of computer.
·        Its internal cache is inside microprocessor and external caches are outside microprocessor.
L1 and L2 cache
L1 and L2 are levels of cache memory in a computer. If the computer processor can find the data it needs for its next operation in cache memory, it will save time compared to having to get it from RAM. L1 is “Level-1” cache memory usually built onto the microprocessor chip itself. For example, the Intel MMX microprocessor comes with 32,000 bytes of L1.
L2 is “Level-2” cache memory is on a separate chip (possibly on an expansion card) that can be accessed more quickly than the larger “main” memory. A popular L2 cache memory size is 1,024 KB (1MB).
Buffer
It is a memory that is used in portable computer. It is used to store more than one piece of data. It lies between I/O devices and CPU. It is used to hold data prior being used by CPU.
5.7 Storage Device
Secondary memory, also known as secondary storage, is the slower and cheaper form of memory. CPU does not access the secondary memory directly. The content in it must first be copied into the primary storage RAM for CPU to process. Secondary memory devices include hard drives, floppy disks, CDs and CD-ROMs etc. Secondary memory is the place where programs and data are kept on a long-term basis.






Differentiate between primary and secondary memory
Primary memory
Secondary memory
1.     It connects directly to the processor.
1. Not connect directly to the processor.
2.     They are expensive.
2. They are cheaper.
3.     They are having low capacity.
3. They are having high capacity.
4.     They are fast.
4.  They are slower.
5.     They are semiconductor memory.
5. They are optical or magnetic memory.
6.     Example:- RAM, ROM etc.
 6. Example:- Hard disk, floppy disk, CD etc.

Magnetic Disk
Magnetic disk is the most widely used storage medium for computer. A magnetic disk offers high storage capacity, reliability and fast to access data. It access data randomly. Magnetic disk is flat circular plates of metal or plastic, coated on both side with iron oxide. The types of magnetic disk are hard disk, floppy disk, pen drive etc.
Hard Disk
The hard disk drive is the most popularly used secondary memory device. The main reason for this is the access speed and the reliability of the data it can offer. In the case of the tape drives the access speed is much low and the data transfer is comparatively low than the hard disk drive.
The information that is required to boot a computer is stored in the hard disk boot sector. Also the importance of the hard disk drive is to store the backup of the data or any information that is created by the user. The hard disk specification should also match the expectations of the computers; that is the storage capacity and the access speed.
The internal organization of the any hard disk drive consists of four parts primarily. They are PLATTERS, HEAD ARM, CHASIS, and HEAD ACTUATOR. The hard disk drives are also available in two different types that are internal and also the external hard disk drives. The internal hard disk drives are used for the storage of the data in the computer case. They are not portable and usually are inside the case. The external hard disk drives are portable can be connected to other computer systems as well. There is a hard casing over the hard disk.
The platters are rotated at very high speed varying from 3,000 rpm (revolution per minute) to 15,000 rpm. Information is written to, and read from a platter stack, called read –write head. The read –write head is used to detect and modify the magnetization of the material immediately under it.
The hard disk is a non-volatile and random memory of a computer system. The hard disk has very high storage capacity. A hard disk has a storage capacity in terms of Giga bytes such as 80 GB, 320 GB, 500 GB etc.
ADVANTAGES OF HARD DISK
·        It can store huge amount of data in terms of MB to GB.
·        It is protected from dust, fires etc.
·        It stores data safely in comparison to other devices.
·        External hard disks are portable.
·        Data access time is less in comparison to floppy disk , magnetic tape etc

FLOPPY DISK
The floppy disk is also called a diskette. Floppy disk is used to transfer data from one computer to another, increase hard disk space by saving information to floppy, backup storage media. The disk is cheap, easy to handle and less storage capacity. Floppy disks have largely been replaced with CD, DVD.
Advantages of Floppy disk
·        Direct information encoding onto disk.
·        No handling of bulky media.
·        Error correction is easier than in cards and paper tapes.
·        It can be re-used many times.
Magnetic Tape
Magnetic Tape is a sequential storage device of computer. It is not common storage device now a day. It is mainly used in mainframe computers and super computers. It is slow and sequential flow of data. It is very useful for storing huge volume of data economically. A magnetic tape is a continuous trip of plastic. It is coated with magnetic oxide which can be magnetized to record information as a series of magnetized / non-magnetized spots to represents binary 0 and 1. Extracting information from a file is known as reading.
The magnetic tape used for computer is made from the same material used for audio tape and video tape. Today, it is used mainly to provide backup or duplicate storage. At present, it is available in cassette forms, which is called cartridge tape. The storing capacity can be 500 MB, 2 GB, 4 GB or higher. Data density is 120 Kbits per inch. Speed is 200 inch per second and data transfer rate is 240 KB per seconds.
MEMORY CARD (FLASH MEMORY)
A memory card is a small storage medium used to store data such as text, pictures, audio and video, for use on small, portable or remote computing devices. It is non-volatile memory which means that data is stable on the memory card by a loss of power source, and does not need to be periodically refreshed. Because memory cards are solid state media, they have no moving parts, and therefore, are unlikely to suffer mechanical difficulties.
Memory cards offer a number of advantages over the hard disk drives. They are much smaller and lighter, extremely portable, completely silent, allow more immediate access, and are less prove to mechanical damage. The storage capacity is from MB to GB. They are commonly used in many electronic devices, including digital camera, mobile phones, Laptops, MP3 Players, and Video Game Consoles.

OPTICAL DISK
An optical disk is an electronic data storage medium that can be written to and read using a low powered laser beam. It is mainly used for music, movies and software programs. It is compact, light weight and durable. Optical disks are CD, DVD etc.
CD-ROM (Compact Disk-Read Only Memory)
CD-ROMs are popularly used to distribute computer software including games and multimedia applications through any data can be stored. The diameter of CD-ROM is 12 cm, thickness is 1 mm and weight is 18 gm. The storage capacity of CD-ROM is 650 MB. It is only used for read only purposed in computer system.
CD-RW (Compact Disk Re-Writable)
A CD-RW is a re-writable optical disk. CD-RW Disk required more sensitive laser optics. CD-RWs are composed of a poly carbonate plastic, a thin reflecting metal coating, and a protective outer coating. In a CD-RW the dye is replaced with an alloy that can change back and forth from a crystalline form when exposed to a particular light, through a technology called optical phase change. The pattern created is less distinct than those of others CD formats, requiring more sensitive device for playback.

 VCD (Video Compact Disk)
VCD is a compact disk format based on CD-ROM that is specifically designed to fold MPEG-1 videos data and to include interactive capabilities. Each VCD disk holds 72 – 74 minutes of video and has a data transfer rate of 1.44 Mbps. VCD’s can be played on a VCD player connected to a Television set or computers, on a CD player, on same CD-ROM drives, and same DVD player.
DVD (Digital Video Disk)
DVD is an optical disk storage media format. DVD disks offer higher storage capacity. DVD disks are used in DVD video consumer digital video formats and in DVD audio consumer digital audio formats. DVD disks containing other types of information may be referred to as DVD data disks. It can store any types of data such as text, graphics, audio, video etc.
External storage devices
External Hard Disk
It is a hard disk that is connected outside from a computer. External hard disks usually are used in addition to internal hard drives in order to store more data. They are portable i.e. carry from one computer to another computer. The external hard drive is connected to the computer via an interface cable, which allows the external hard drive to communicate with the computer so that data can be passed back and forth.
Zip Disk and Jaz Disk
A zip disk and Jaz disk are a small, portable disk drive used primarily for backing up and archiving personal computer files. They are used to move data from one computer to another computer.
5.8 Input Devices
The devices through which data are input to computer are called input devices of computer. The main input devices are Keyboard, Mouse, Scanner, Joystick etc.
Keyboard
Keyboard is an input device. It enables users to enter data into a computer. Computer keyboards are similar to typewriter keyboards but contain additional keys. When the user types a character, electronic circuits in the keyboard translate into bit charges. They travel to a buffer, the when they appear in the buffer, and processor sends it to the RAM. At the same time, the processor instructs the monitor to put the character on the screen.
 Keyboard consist the following keys.
·        Alphabetic keys.
·        Numeric keys.
·        Function keys.
·        Control keys etc.
According to keys keyboard are again classified by
·        XT ( Extended keyboard ) : 85 keys
·        AT (Advanced keyboard) : 101 keys
·        Enhanced keyboard : 103 or more keys
Mouse 
A mouse is a pointing device it is held in one hand and move across a flat surface. A mouse may have two or three buttons. Mouse is one of the most widely used input devices of the computer. We can select the command draw pictures, edit text etc by pressing the mouse button.
Type of mouse
·        Mechanical Mouse
·        Optical Mouse
One type of mouse has a round ball under the bottom of the mouse that rolls and turns by wheels which control the direction of the pointer on the screen. This is called mechanical Mouse. Another type of mouse uses an optical system to the track the movement of the mouse. This is called optical mouse. Laptop computers use touch pads, buttons and other devices to control the pointer.
Scanner
Scanner is a device that optically scans images, printed text, handwriting, or an object, and converts it to a digital image. The resulting image can be stored in a file as Bit Map or JPEG (Junction photographic exports Group), displayed on a screen, and manipulated by programs. User can not directly edit text that has been scanned.
Light pen
It is a pointing device which is very sensitive to light. It uses a light sensitive cell to communicate the screen position to computer. When the light pen tip is placed a gains the screen, it is capable of sensing its position or CRT. As the tip is moved over surf face of screen,   its light sensitive element detects the light emitted from screen. We can select the desired option of menu by touching light pen against desired option.
OMR ( Optical Mark Reader)
OMR is an input device which can read marks such as pencil marks on a page. It is used to read forms and multiple- choice question pagers. The marks are then exposed to beam of light for sensing and optical mark reader scores the test by identifying the position of mark.


OCR (Optical Character Reader)
An OCR detects alphanumeric characters printed or typewritten on paper. It may be handled scanner or a page scanner to detect light reflected from a line or from a page of the text. The change in the reelected light is converted to binary data which is sent to the processor. The illuminated by low-frequency light source. The light is absorbed by areas while the light is selected from light areas.
BCR (Bar code Reader)
This method uses a number of bars (lines) of varying thickness and spacing between them to indicate the desired information. A bar code reader can read such bars and convert them into electrical pusses to process by a computer. The bar-code detects as ten digits. The first five digits identify supplier or manufacturer of item and second five digits identify individual product.
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 finger on the pad o move the cursor on the screen. Many laptop computers use a touch pad of keyboard.



Microphone
It is an input device of computer it is used for recording voice and sound. It is mainly used for voice mail, video chat, conferencing etc.

Digital camera
A digital camera is a camera hat takes video or still photographs, or both, digitally by recording images via an electronic image sensor. Most 21st century cameras are digital it stores thousands of images on a single small memory device and deleting images to free storage space. The majority including most compact cameras, can record moving video with soured as well as still photographs.
Joystick
A joystick is also pointing device. It is used to move the cursor on a CRT screen A Joystick is a stick which has spherical ball at its lower end as well as upper end. The lower spherical ball moves in a socket. The Joystick can be moved right or left, forward or backyard. The electronic circuitry inside the joystick detects and measures the displacement of the joystick from its central position, the information is sent to the processor jousting is used in games.
MICR (Magnetic Ink character Reader)
It is an input device which uses a magnetic ink reader to recognize character printed with a special magnetized ink (i.e. iron oxide particles) used to write characters on the cheques and deposition forms which are to be processed by an MICR. To identify the characters these patterns are compared with special pattern stored in the memory. Before cheques are issued to customers the identification number of the bank and depositor’s account number are printed on the lower left-side of the cheques with magnetized ink. When the cheque is entered into an MICR, it passes throng a magnetic field.
The read heads reads the characters written on the cheque with magnetic ink. It interprets the characters and sends the corresponding data directly to computer.
Touch Screen
Touch screen enable the user to select an option by pressing a specific part of the screen with finger. A matrix of beams over the screen in the horizontal and vertical dimensions.  If the beam is interrupted by a finger, the position is determined by the interrupted beams. In an alternative version of touch screen, the display is covered with touch sensitive surface. The user must contact with the screen in order to register a selection. A touch screen comes in two forms, photonic and electrical.

5.9 Output Devices
The device which shows the results of processing and other information from the computer to the human understanding language form is called output device. Example Monitor, printer, plotter & speaker.
Monitor
The display screen that is used to display the text and graphics, allowing users to view the result of the processing is called monitor. It is the most common and popularly used output device for producing soft copy output. It displays the generated output on a television like screen. There are different types of monitors such as LCD, CRT etc.
CRT Monitor
CRT stands for cathode ray tube, describing the technology inside an analog monitor creates a picture out of many rows or lines of tiny colored dots. The more lines of dots per inch, the higher and clearer the resolution. Therefore 1024 X 768 resolutions will be sharper than 800 X 600 resolutions because the former uses more lines creating a denser, more detailed picture. Higher resolutions are important for displaying the suitable details of graphics.
 Advantages
·        High Refresh rates
·        Color clarity and depth.
Disadvantages
·        Very Heavy and Large
·        Use Large Amounts of Energy
·        Generate excess Heat.
Liquid crystal Display (LCD) Monitor
LCD is a thin, flat electronic visual display that uses the light modulating properties of liquid crystals. Liquid crystals do not emit light directly.
They are used in wide ranges of applications, including computer monitors,    television, aircraft cockpit displays etc, they are common in consumer devices such as video players, gaming devices, clocks, watches, calculators art telephones LCDs have displaced CRT in most applications. They are usually more compact, lightweight, portable, less expensive, more reliable, and easier on the eyes. They are available in a wider range of screen sizes than CRT and plasma displays. 

Advantages
·        Smaller and Lighter
·        Less power Consumption
·        Causes Less Eye Fatigue
Disadvantages 
·        Motion Blur on fast Moving Images
·        Some Models have reduced color clarity.
Light Emitting Diode (LED)
A LED is a semiconductor device that emits visible light when an electric current passes throng it. The chip has two reigns imparted by junction. The P region is dominated by positive charges, and N region is dominated by negative charges. The junction acts as barrier to flow of electrons between the P and N region. Only when sufficient Voltage is applied to the semiconductor chip, current flow, and electrons cross the junction into the P region. The light is not particularly bright, but in most LEDs it is monochromatic, occurring a single wave length. The output from a LED can ranges from red to blue violet.
Plasma Display
  A plasma display panel (PDP)  is a type of flat panel display common to large computer and TV displays 30 inches  (76cm) or larger. They are called “plasma” displays because the technology utilizes small cells containing electrically charged ionized gases, or what are in essence chambers more commonly known as Fluorescent lamps.
 Printer
An output device that produces text and graphics on paper is called printer. It is used to produce hard copy output. There are 2 types of printer.
·                    Impact printer
·                    Non-impact printer
Impact printers
An impact printer has mechanisms resembling those of a type writer. It forms characters or images by striking by a hammer or wheel against an inked ribbon, leaving an image on paper. Examples: Dot-matrix Printer, Daisy wheel printer etc.

Non-Impact printer
Non- Impact printer is faster and quieter than impact printer because they have fewer moving parts. It forms characters and images without making direct physical contact between printing mechanism and paper.
Example: - Laser printer, Ink -Jet printer
Difference between impact and Non- Impact printers
Impact printers
Non- impact  printers
1. The head strikes ink coated ribbon to print information.
1. The thermal, chemical
 or optical techniques are applied to print
information.
2. They are noisy.
2. They are less noisy.
3. They are slow.
3. They are fast.
4. They are poor quality printers.
4. They are high quality printers
5. Example:  Dot matrix printer ,Daisy wheel printer etc
5. Example : Laser printer, Ink-Jet printer etc

Dot- Matrix printer

A more common type of microcomputer impact printer is the dot-matrix printer, which contains a print head of small pin that strikes an inked ribbon forming characters or images print heads are available with 9, 18 or 24 pins, with the 24 pin head offering the best print quality.
It is noisy and not good for continuous printing but it can print multi-pages i.e. it is possible to use carbon papers to get extra copies of the same document. It is flexible it does not have fixed characters font. As the font is not fixed, a dot matrix printer can print any shape of character.
Ink-Jet Printer
It is a non-impact character printer. Like dot-matrix printer, ink-jet printer also form images with little dots, but the dots are much smaller and more numerous. It sprays small, electrically charged droplets of ink from four nozzles through holes in a matrix at high speed onto paper. Most color printing is done on ink-jet because the nozzles can hold for different colors, CYMK (Cyan (blue-green), Yellow, Magenta (Purple-red), and Black, other colors are derived by the combination of these four colors. It is even quieter than laser printer and procedures and equally high-quality images.
Laser Printer
Laser printer is a non-impact printer, page printer. An entire page is processed at a time. It produces high quality print. Print resolution ranges from 300 to more than 2400 dpi (dot per inch). It is expensive than other printer.
Similar to a photocopying machine, a laser printer uses the principle of dot-matrix printers in creating dot-like images. However, these images are created on a drum, treated with a magnetically charged ink powder known as toner, and then transferred from drum to paper.
There are good reasons why laser printers are the most common type of non-impact printer. They produce sharp, crisp images of both text and graphics. They are quieter and faster than dot-matrix printer. They can print 4-20 pages per minute for individual micro-computers.
Plotters
A plotter is a specialized output device designed to produce high quality graphics in a variety of colors. Plotter is especially useful for creating maps and architectural drawings, although they may also produces less complicated charts and graphs.
It uses ink pen or ink –jet mechanism to draw graphics or drawings. Either single or multi colors pens can be employed. The pens are driven by motor. Drawing can be prepared on paper or plastic sheets.
Speakers
It produces and amplifies sound output. A good pair of speakers connected to sound card will give you loads of listening pleasure. The audio output of the computer system relayed through the speakers. More advanced speaker systems exit, allowing certain programs (such as music creation, software and games) to use surrounds sound capabilities to beneficial effect.
5.10 Interface and ports
Peripherals are devices that are attached to a computer system to enhance its capabilities. Peripherals include input devices, output devices, storage devices, communication devices. All peripherals must have some way to access the data bus of computer. To do this, peripherals are connected by ports.
A port is a socket on the outside of the system unit that is connected to an expansion boards on the inside of the system unit.
Parallel Ports
A parallel port allows to lines to be connected that will enable 8 bits to be transmitted simultaneously. Parallel lines moves information faster than serial lines do, but they can transmit information efficiently only up to 15 feet. They, parallel ports are used principally, for faster sped devices such as connecting printers. It has 25 pins.

Serial Ports
A serial port, or RS-232 port, enables a line to be connected that will send bits one after the other on a single line. Serial lines are used to link slow speed devices and equipment that is not closed by serial ports are used principally for communication lines, modems and mice. It has 9 pins.

USB (Universal Serial Bus)
USB is a newer type of serial connection that is much faster than the old serial ports. USB is also much smarter and more versatile since it allows the “daisy chaining” of up to 127 USB peripherals connected to one port. USB ports can supply the connection of many kinds of devices such as Keyboard, mouse, printer, memory card etc. New PCs and Macs include several USB ports.
IEEE-1394 Interface (Fire-Wire Ports)
The IEEE -1394 interface is a serial bus standard for high speed communications and isochronous real –time data transfer , frequently used by personal computers as well as in digital audio , digital video and aeronautics applications. It replaced parallel SCSI in many applications, because of lower implementation cost and a simplified, more adaptable cabling system.

SCSI (Small computer System Interface) Port
Pronounced “scuzzy” a SCSI port provides an interface for transferring data at higher speeds up to eight SCSI-compatible devices. These devices include external hard disk drives, magnetic-tape backup units, and CD-ROMS devices.


Peripheral componets Interconnect (PCI) slot 
Conventional PCI is a computer bus for attaching hardware devices. These devices can take either the form of an integrated circuit fitted onto the motherboard itself, called a planner device in the PCI specification or an expansion card that fits into a socket. PCI slot is in PC to connect device like network cards, sound cards, internal modems etc.
5.11 Software
Software is the collection of computers programs, procedures and documentation that performs different tasks on a computer system. The types software is system software and application software.
System Software
System software is computer software designed to operate the computer hardware and to provide a platform for running application software. It is a collection of operating systems, device drives, utility, servers etc.

Some common types of system software are given below:
·        Operating System
·        Utility software
·        Language processor
Operating System
An operating system is a collection of programs that controls the overall operation of the computer system. It acts as an interface between user and hardware. Examples: Windows XP, Windows 7, Unix, Linux etc.
Utility Software
Utility software help to manage maintain and control computer resources. These programs are available to help you with the day to day chores associated with personal computing and to keep your system running at peak performer.
Some Examples of Utility Software:
·        Virus Scanning Software
·        Backup Software
·        Scan Disk
·        Disk Defragmenter Software etc

Language Translator
It is a special type of computer system software that can use to translate the program written in one language into another language. It is most compulsory for low level language and high level language. The types of language translator are given below:
·        Compiler
·        Interpreter
·        Assembler
Application Software
The program which is designed to do only specific task is called application software. The application software made for one purpose can’t do other task. For example: - accounting of your college can not prepare the result sheet of your college. Application system works on system software.
There are two types of application software.
a)     Customized or Tailored software: Tailored software is specific purpose software. These types of software are developed for specific tasks. They are written in high level language such as JAVA, C, and C++ etc. Examples Banking software, Hospital Management Software etc

b)    Packaged Software: Packaged software is mainly designed by software companies to generalize the tasks. They are general purpose software. Some common types of packaged software are as follows.

·        Word processing Software Eg: MS-Word
·        Database software Eg: MS-Access, Oracle etc
·        Spreadsheet Software Eg: MS-Excel
Computer Virus
Computer virus is a kind of program that destroys the functioning power of computer smoothly. Viruses have many purposes – some are designed to play annoying tricks where as others have destructive intent.
Anti- Virus
Anti-virus is protective software designed to defend your computer against malicious software. In order to be an effective defense virus, your antivirus software needs to run in the background at all times, and should be kept updated so it recognizes new version of malicious software. Some antivirus software are Norton, MacAfee, Avira, Kaspersky etc
MODEM
The word modem stands for modulator and demodulator. It is a device, which translates data from binary (digital) code to analog data that can be transmitted over the telephone network. Modulation is the conversion from binary (digital) to analog and demodulation is analog to binary (digital). When you send a file from your PC to analog signal, it is converted to analog signal by your modem and converted back to digital when it arrive at the destination by the modem connected that computer. A similar procedure occurs when you send a fax from your PC.

HOMEWORK [UNIT-5]
1.     Draw a block diagram and explain the main components of computer system.        
2.     Differentiate between Impact and Non- Impact printers.
3.     What is printer? Differentiate between soft copy and hard copy output.
4.     Define the term “BUS”. Explain different type of BUS.  
5.     What is cache memory? How it increases the performance of computer?  
6.     Define software. Explain the different types of software in details.
7.     Define memory. Explain the types of memory in details.10
8.     What is output? Differentiate between CRT and LCD monitor.          
9.     What is CPU? Write down the functions of CPU? 
10. What is software? Explain the different types of software.
11. Write short notes on:                                  
a)     Light Pen
b)    Joy stick
c)     MICR
d)    Modem
e)     Track Ball
f)      Cache memory
g)    USB
h)    Speaker
i)       Scanner
j)       OCR
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