1. GENERATIONS OF COMPUTERS:The tem generation indicates the type of technology used in the computer construction. As new technology was emerging, it was being used in the making of computer. The new technology improved the speed, accuracy and storage capacity of the computers. Different technologies have been used for computers in different times.

Therefore, computers can be divided into five generations depending upon the technologies used. These are:

First Generation (1942 1955)Second Generation (1955 1964) Third Generation (1964 1975)Fourth Generation (Since 1975)Fifth Generation (Since 1980)

1. First Generation Computers (1942 1955):

The vacuum tube technology was used in first-generation computers. Mark-1m, ENIAC, EDSAC, EDVAC, UNIVAC-1 etc. machines belong to the first generation of computers. The machine language only was used in first-generation computers.Principle:-The vacuum tubes consisted of filaments inside it which on heating emit electrons. These electrons were responsible for the amplification and deamplification of electrical signals.Input:-Punched cardsOutput:-Print-outs

Advantages:*These computers were the fastest of their time.*They were programmed using machine language.*The electronic digital computers were introduced due to the vacuum tube technology.

Disadvantages:*Very large in size*Not reliable*Consumed large amount of energy*Constant maintenance required*More heat generated and air-conditioning was required*More costly*Very slow in speed (data processing)*It was difficult to programmed, because they used only machine language*Non-portable*Limited commercial use

2. Second Generation Computers (1955 1964):

The transistor technology was used in second-generation computers. The electronic component transistor was invented in 1948 at Bell Laboratories. The transistor is smaller in size and more reliable than vacuum tube. Therefore, the transistor technology was used in computer in place of vacuum tube technology. The programming assembly language was also introduced in second-generation of computers.The transistors consisted of BASE, COLLECTOR and EMITTER. Their functions include:-BASE: input gate for the transistor.COLLECTOR: collect the amplified signalsEMITTER: output gate for emitting the amplified signals to the external environment.

Advantages:*Low in cost*Smaller in size*Fast in speed*Less heat generated more reliable and accurate in calculations*Consume low power etc.*Used for commercial purposes*Portable*Assembly language was introduced. This language is easy to write program than machine language

Disadvantages:*Air-conditioning required*Commercial production was difficult and these were very costly*Constant (or frequent) maintenance required*Only used for special purposes

3. Third Generation Computers (1964 1975):

The IC (Integrated Circuits) technology was used in third-generation computers. In a small IC chip (5 mm square size) a circuit is designed having large number of electronic components like transistors, capacitors, diodes, resistors etc. Initially, an IC contained only about ten to twenty components. Thus the IC technology was named as Small Scale Integration (SSI). The third-generation was based on IC technology and the computers were designed using this technology.

Advantages:*Smaller in size*Production cost was low*Very fast in computational power*More reliable*Low power consumption*Maintenance cost was low because failure rate of hardware was very low*Magnetic disk, used for external storage*More storage capacity*Easily portable*Easy to operate*Upgraded easily*Widely used for various commercial applications all over the world*Lower heat generated*High-level languages were commonly used*Many input/output devices were introduced such as mouse and keyboard etc.

Disadvantages:*Air-conditioning required*Highly sophisticated technology required for the manufacturer chips

4. Fourth Generation Computers (1975 onwards):

The microchip technology was introduced in this generation of computers. With the advancement in IC technology, LSI (Large Scale Integration) chips were developed. It was possible to integrate over 30,000 or more components on to single LSI chip. After LSI, the VLSI (Very Large Scale Integration) was developed and the development of microprocessor possible. It is expected that more than one million components will be integrated on a single chip of VLSI. Using VLSI technology, the entire CPU is designed on a single silicon chip. The use of microprocessor as CPU introduced another class of computers called the microcomputers. Thus fourth-generation may be called Microcomputer generation. IBM introduced its personal computer for use in 1981.

Advantages:*Smaller in size*Production cost is very low*Very reliable*Hardware failure is negligible*Easily portable because of their small size*Totally general purpose*Air conditioning is not compulsory*Very high processing speed*Very large internal and external storage capacity*Used advanced input & output devices such as optical readers, laser printers, CD-ROM/DVD-ROM drives etc.

Disadvantages:*Highly sophisticated technology required for the manufacturer of microprocessor chips

5. Fifth Generation Computers (In process):

The main drawback of first to fourth generation computers is that the computers have not their own thinking power. These are totally depending upon the instructions given by the users.

Fifth generation computers are supposed to be the ideal computers, but do not exist. The scientists are working to design such computers that will have the following features.*Having their own thinking power*Making decisions themselves*Having capabilities of learning*Having capabilities of reasoning*Having large capacity of internal storage*Having extra high processing speed*Having capabilities of parallel processing

Technologies used in fifth generation computers:

*ULSIC (Ultra Large Scale Integrated Circuits) technology*Artificial Intelligence (AI) technology also called the knowledge Processor. The AI means automatic programs that let the machines to think and decide themselves. The programming languages LISP (List Processor) and PROLOG (Programming with Logic) are used for artificial intelligence. The scientists at ICOT in Japan use the PROLOG to develop the Artificial Intelligence software.

Advantages:*Laptops, Pocket computers and PDA were developed.*Development of parallel processors*Development of centralized servers*Development of optical disc technology*Invention of internet and its advantages.

2. CLASSIFICATION OF COMPUTERS:

Computers can be classified based on the following criteria:

According to Technology: $Analog Computers $Digital Computers $Hybrid ComputersAccording to Purpose: $General purpose Computers $Special ComputersAccording to size: $Supercomputers $Mainframe Computers $Minicomputers $Microcomputers, or Personal Computers

Based on operating principles and technology:

Analog computers:$These computers represent data in the form of continuous electrical signals.$These are fast and multi-tasked.$Results displayed by these computers are less accurate.$Powerful in solving differential equations.$These computers use OP-AMP (Operational Amplifier).$The features of OP-AMP include: *High voltage gain. The voltage gain is defined as the ratio of output voltage to input voltage. *Infinite input resistance. The input resistance is defined as the ratio of change in input voltage to the change in input current. *Zero output resistance. The output resistance is defined as the nominal resistance measured with nil loads.$The basic OP-AMP circuit is represented as:

Wherein, Rin is the input resistance, RF is the feedback resistance A is the amplifier which is used to invert the incoming signals of voltage Vin to Vout.

Digital computers:$These are called the Digital information processing systems.$These systems store and process the data in digital form (strings of 0's and 1's)$They are capable of processing analog signals but the analog signals have to be converted into digital signals using an ADC before feeding into the digital computers.Hardware components:$Arithmetic Logical unit (ALU)$Control unit$Memory unit$Input unit$I/O units

Hybrid computers:$These are the combination of both Analog and Digital computers encompassing the best features of both the computers.$Fast, efficient and reliable computer systems.$The Data is measured and processed in the form of electrical signals and stored with digital components.$The input is accepted in the form of varying electrical signals and is converted into discrete values for performing operations.$They are used in hospitals to measure heartbeat and have engineering and scientific applications.

Based on Applications:

General purpose computers:$Work in all environments$Versatile computers$Store number of programs to perform distinct operations$More expensive$Not efficient and consume large time to generate results

Special purpose computers:$Work on specific tasks$Non-versatile$Speed and memory of these computers depend on the task performed$More efficient and consumes less time to process results$Less expensive

Based on size and capability:

Micro-computers:$These are small cheap digital computers for individualsHardware components:Microprocessor, storage unit, I/O channels, power supply, connecting cables.Software components:Operating System (OS), Utility software, Device driversAvailable in the forms of:PC's, Work stations, Notebook computers.

The various components are:Microprocessors:$This incorporates all functions of the CPU into a single unit. The various units of microprocessor are:$ALU: performs arithmetic and logical operations.$Registers: Store data and instructions temporarily needed by the ALU.It includes several types like: Accumulator (ACC), Program Counter (PC), etc.$CU: Control unit-Used to manage and control the functions of microprocessors, I/O devices$Memory:Used to store data and instructions. It is of two types: *Primary memory: It stores temporarily the data and instructions needed by the microprocessor *Secondary memory: It stores data permanently. Examples include magnetic tapes, floppy, CD, USB, etc,Peripheral devices: $Input devices: They are used to transfer data into the computer. Examples: keyboard, mouse, etc, $Output devices: Used to display the results processed by the computer. Examples: Monitor, printer, etc,System bus:$It is also called the FRONTSIDE BUS, MEMORY BUS, LOCAL or HOST BUS.$It is used to connect microprocessor, memory and peripheral devices into a unit. SYSTEM BUS= ADDRESS BUS+DATA BUS+CONTROL BUS

$Address bus: Unidirectional bus to identify the peripheral devices and memory.$Data bus: Bi-directional bus used to transfer data among the microprocessor, peripheral devices and memory.$Control bus: bus used by the microprocessor to send control signals to various devices.

Depending on size, Microcomputers are of three types:*Desktop computers:They are used in single location, cheap, and have good storage. Examples include: Apple, IBM.*Laptop computers:Portable computers also called as the notebook computers or mobile computers. They are smaller in size, more expensive and are rechargable.Examples include Apple, Acer, Hewlett Packard (HP)*Hand-held computers:Also called as Personal Digital Assistants (PDA's), Palmtop or Mobile device. They are smaller in size, have smaller display and input device is generally an electronic stylus. Their storage capacity is small. Examples include Apple Newton, Franklin eBook man.

Mini computers:$These were introduced by Digital Equipment Corporation (DEC) in 1960.$They can process more data and can support more I/O devicesThey are less powerful than mainframe computers but more powerful than micro computers. Hence they are called MID-RANGE COMPUTERS.They cater to the needs of 4-200 users at a time$They are used in business as a centralized computer or as an internet server$They are less expensive than Mainframe computers. Examples include PDP 11, IBM 8000 series, etc,

Mainframe computers:$These are capable of handling millions of records a day.$These are bigger and more expensive than mini-computers$They require a large space and closely monitored humidity and temperature.Characteristics: *A typical mainframe computer consists of 16 microprocessors and even more than that. *RAM capacity is 128MB to 8GB *They can run multiple operating systems. Hence they are called-VIRTUAL MACHINES *They handle a large amount of I/O devices which are arranged in separate CABINETS or FREMES and hence the name.Applications: *they are used in large financial transactions *Enterprise Resource Planning (ERP) *Industry and consumer statistics *Census

Supercomputers:$These are the fastest and complex computers with very high speed$First Supercomputer was presented by SEYMOUR CRAY in 1960 in Control Data Corporation (CDC)$Used exclusively in applications where large complex calculations have to be performed to get the output$These are very expensive and designed to perform only small number of programs at a time$The manufacturers of Supercomputers are: IBM, SILICON GRAPHICS, FUJSTU, INTEL, etc.These are the fastest as they employ 1000's of processors, 100s of GB of RAM, 1000s of GB of secondary storage$the principles used in these computers are: *Pipelining: This enables the processor to execute second instruction even before the first is completed provided, it has the required resources *Parallelism: Enables the processor to execute several instructions at a time$Examples include: CRAY 3, Cyber-205, PARAM, etc.

3. Characteristics of computers:Computers have distinguishing characters which make them ideal machines. But they do not certain characters which human possess. Some of the important characteristics of computers are:

(I)Automatic:#Computers are automatic machines which once started a job, carry it on until it is complete provided they are given the required instructions by the users.

(II)Speed:#The computers are capable of taking logical decissions, performing arithmetic and non-arithmetic operations on alphabets and copying at an unbelievable speed#The units of speed for a computer are in microseconds (10 -6) or even in nano and Pico second (10 -9 or 10 -12)#A powerful computer can perform 3 to 4 million arithmetic operations per second#The speed of the computers is attributed to the fact that THE COMPUTERS ARE ELECTRONIC DEVICES WORKING ON ELECTRICAL PULSES WHICH TRAVEL AT HIGH SPEED.

(III)Accuracy:#The computer produces highly accurate and reliable results#The errors in the calculations may be due to the error in the logic of the human but not due to the computer.#The computers perform accurate calculations 'n' number of times

(IV)Versatility:#A computer is capable of performing a wide variety of functions #It can accept data and produce results.#It can perform the basic arithmetic and logic operations#It can transfer data internally#Several applications can be run at a time. For example MS paint Adobe Photoshop and VLC media player can be run at a time.

(V)Diligence:#A computer is capable of performing the same task over and over again with the same degree of accuracy and reliability as the first one#This is because, unlike human beings, computers are free from monotony, tiredness, lack of concentration, etc, and can work hours together without creating bugs.

(VI)Large and perfect memory:#A computer can store and recall any amount of information because of its secondary storage capability with perfect accuracy unlike human beings.#The storage capacity of the computer is enormous and is perfect#A computer recalls a data with greater accuracy even after several times and does not lose any information unless it is prompted to do so.

(VII)No I.Q and feelings:#A computer is not intelligent on its own and cannot think on its own#It can only perform the tasks specified by the human but the difference is that it does this with greater accuracy and speed.#It cannot decide on its own and only the user can determine what the computer must do#A computer has no feelings but a human does.

4. Basic computer organization:

The organization of computers involves the interfacing of various components of the computer and the co-ordination of the operations performed by them. The various functional units of a computer include:(I)Input unit(II)Memory unit(III)Central processing unit(IV)Output unit

BASIC COMPUTER ORGANISATION

Input devices:@These devices are used to feed the data inside the computer.@The most commonly used input devices are: *keyboard *mouse *light pen *digitizer *trackball *joystick *OCR (Optical Character Recognizer) *MICR (Magnetic Ink Character Recognizer) *OMR (Optical Mark recognizer)

Keyboard:@It is used to enter alphanumeric data into the computer and to perform special functions:@Alphanumeric keys: Used to enter alphabets and Numbers@Function keys: Used to perform special functions. These include F1 to F12.For example F5 is used for refreshing a page or desktop@Modifier keys: SHIFT and CONTROL keys are called modifier keys and they perform special functions. For example CTRL+X for cut@Spacebar key: Used to move by one space in a document or worksheet or DBMS.@Enter key: Used to open something like a file or a web-page or to move to the succeeding line in a document.

Mouse:@It is an electronic device used for selecting and pointing purposes. Hence it is called the POINTING DEVICE.@Left mouse button: Used for selection purpose@Right mouse button: Used to perform special functions like OPEN, EXPLORE, COMPRESS, etc.@Ball at the bottom: The ball at the bottom of the mouse moves and the cursor moves on the screen in whichever direction the ball rotates@Wheel at the top: To scroll a web-page or a document.

Scanner:@Used to scan images and documents@The scanned images are converted into the DIGITISED IMAGES understandable by the computers.@Color images can also be scanned using the scanner depending upon the RED GREEN BLUE (RGB) PROPORTIONS@The principle used in the BARCODE READER is similar to that of a scanner.

Memory unit:

MEMORY PYRAMID OF A COMPUTER

The memory unit is used to store data on a temporary or permanent basis. The various types of memory can be depicted as:

MEMORYTYPESPRIMARY MEMORYINTERNAL PROCESSOR MEMORYSECONDARY MEMORY

MEMORY:PRIMARY MEMORY @Read only memory *MASK PROM *PROM *EPROM *EEPROM @Read/Write memory *Static RAM *Dynamic RAM *Cache memorySECONDARY MEMORY @Disk Drives *Floppy *Hard disk *CD-ROM (Read only and Rewritable) @Tape drives: *Cartridge Drive *Cassette driveINTERNAL PROCESSOR MEMORY @Registers *ACC *PC *IR *MAR *MBR *MDR

Primary memory:@This memory is also called as MAIN MEMORY.@The information stored in this memory that are needed by the microprocessor during the time of processing are temporary and the information in the ROM are permanent

Read Only Memory (ROM):@It is a non-volatile memory@The contents of this memory are permanent@These are cost-effective@They are available in high storage capacity@Processing speed is very low@Generally, the OS supporting programmes and the Basic Input Output System (BIOS) programs are stored in this@In this trigonometric and logarithmic functions are also stored@In PROM (Programmable Read Only Memory), The flexibility of data alteration is also provided.

Random Access Memory (RAM):@This is the part of computer's temporary storage where the ALL THE DATA, INSTRUCTIONS NEEDED BY THE MICROPROCESSOR AND THE RESULTS EXECUTED BY IT are stored.@It is a volatile memory@The contents are temporary@Cost is very high@They are available in small storage capacity@processing speed is high@User defined programmes can be stored at any time

Principle:-Each bit in a RAM stores information by means of electric charge, where the presence of electrical signals indicate'1' and the absence of electrical signals indicate'0'.Types of RAM:DYNAMIC RAM:@The electric charges tend to be leak out in a few milliseconds, so the information present in RAM is lost@Hence, the set of data stored in Dynamic ram has to be refreshed periodically before all the charges have leaked.STATIC RAM:@Special refresh circuitry is provided@They are not leaky and hence do not require any refreshing

Cache memory:A small memory between the CPU and the main memory is called the cache memory.@It is faster than main memory and the access time of this memory is close to the processing speed of the CPU@It acts as a HIGH-SPEED BUFFER between the CPU and the main memoryIt is used to store temporary and active (most frequently used) data during the time of processing

The other components of primary memory include:@PROM (Programmable Read Only Memory)@EPROM (Erasable PROM)@EEPROM (Electrically Erasable PROM)

Secondary memory:@The secondary memory is also called as the AUXILLARY MEMORY@It may be in-built or may be introduced into the computer and is used to store the data on a PERMANENT basis@The various types of secondary memory include:

Magnetic storage device:@Use magnetism property to store data@The data stored in these devices can be stored, erased, rewritten many number of times@Examples include: Magnetic tapes, magnetic disks, floppy disk, etc

Optical storage device:@Uses the LASER BEAMS to store data @The data stored in these devices can be erased and rewritten many number of times@Examples include: CD, CD-ROM-CD-RW, DVD, etc.

Magneto-Optical storage device:@This uses both MAGNETISM and LASER BEAMS to store data@These devices are generally used in the cases of BACKUP, DATA RECOVERY, etc,@The data stored in the can be erased and rewritten many times

Universal Serial Bus (USB):@These are commonly called as pen drives@These are compact and store large data than the other storage devices@These are connected to the USB PORT in the CPU

Central processing unit:The CPU is the main functional unit of the computer. The basic functions of CPU include:@fetch the data from the user@Decode the data into a computer-understandable form@Process the data or execute the operation@Store the resultThis sequence of functions is called the INSTRUCTION CYCLE and is represented as: DECODE CU (CONTROL UNIT)ALU (ARITHMETIC LOGICAL UNIT)

EXECUTEFETCH

MAIN MEMORY

STORE

The various units of the CPU include:

ARITHMETIC LOGICAL UNIT (ALU):@It is used to perform the various arithmetic and logical operations.@Arithmetic operations like +,-,*, / are performed by ALU@Logical operations like >,