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Block structure of Computers :

A present day computer has four main components: Input Unit, Memory, Central Processing Unit CPU and Output Unit. A brief description of each of these components is given below

( Memory InputOutput Unit ALU (arithmetic And Logic Unit) (Registers)Control Unit (CPU))

Input Unit: An input device feeds data and program into the computer. The program contains instruction about what has to be done with data; data can be in the form of text, voice, video, graphic. Some of the popular input devices are:

Keyboard, Mouse, Touch screen, Light Pen, Track Boll Scanner, Digitizer, Joystick, Hard Disk, floppy disk, Punched Card, Microphone, Optical mark Reader.

Memory: Memory is used as storage of programs as well as data. The Memory can both be Internal External depending upon its location in the Computer System. The Internal memory, also known as main Memory of Primary storage or Random Access Memory (RAM), is directly assessable by the CPU and volatile by nature, whereas the external Memory of secondary storage includes hard disk, magnetic drum etc, are not directly assessable to the CPU and are non-volatile. These devices can be accessed by the CPU through Input/output controllers.

Central Processing Unit (CPU): Central processing Unit serves as the brain of the computer system. The main computer of CPU is CU (Control Unit), ALU (Arithmetic and Logic Unit) and Register.

Control Unit Manages and co-ordinates the entire computer system. CU, controls the operation of the Input Unit, Output and ALU.

Arithmetic Logic Unit ALU performs all the arithmetic and logical orations of the data as a result of execution of an instruction i.e. it performs addition, subtraction, division, multiplication comparison etc.

Registers: Registers are a set of temporary memory location of CPU. Number of Registers is available with each register having specific function. E.g. address register holds address of active memory location, Instruction register: holds and instruction while it is begin executed.

Output Unit: output device provides results to the user of the computer system. Some of the popular output devices are VDU (Visual Display Unit, Printer, Plotter, LCD (Liquid Crystal Display), speaker, Hard Disk, Floppy Disk)

Earliest device that qualified as a digital computer was Abacus also known as Sorbian. It was invented around 600 B.C. and was used for simple addition, subtraction (by appropriate positioning of beads of a lack). All devices that were used for computation before 1940s were mechanical devices. A popular mechanical calculator capable of performing addition and subtraction was developed in 1642 by French scientist Belasis Pascal. Later. In 1671, his machine was extended to perform also multiplication and division by German scientist Gottfried Leibniz: IN 1823. Charles Babbage. And English mathematician designed a mechanical computing machine for automatic computation of mathematical tables (by the method of finite differences with additions only) and called it Difference Engine. In 1834 he built a much more powerful mechanical computer known as Analytical Engine. In which was first programmable machine. Its components were similar to those of a modern computer and had a mechanism for conditional branches of instruction. Charles Babbage is known as the Father of modern computer.

//In 1823 Charles Babbage, an English mathematician, designed a mechanical computing machine for automatic compotation of mathematical tables (by the method of finite differences with additions only) and called it Difference Engine. In 1834. He built a much more powerful mechanical computer known as Analytical Engine which was first programmable machine. Its components were similar to those of a modern computer and had a mechanism for conditional branches of instruction. Charles Babbage is known as the Father of modern Computer.//

In late 1890s Harman Hollerith came up with the concepts of punches cards (input media). In1896, he formed the Tabulating machine company to manufacture his machines which was renamed as IBM (international Business Machines) in 1924. In 1938, konard Zuse of Germany built mechanical computer Z1 having binary number system. Whereas earlier computer used decimal number system. Later, in 1941, he developed Z3 which was first general- purpose operational program- controlled computer.

Then the first electronic digital computer came into existence name ENIAC (Electronic Numerical Integrator and Calculator). 1943/46, It was developed at the University of Pennsylvania under the guidance of John Archly and prosper Eckert. It used Vacuum Tubes and decimal number system instead of binary. Since its programs were wired on boards and could store and manjoulate only very limited amount of information. So it was difficult to detect errors and to change programs. These problems were later overcome by the development of EDVAC (Electronic Discrete variable Automatic Computer), 1946-52. It used Stored program concept, developed by Dr. John Bon Neumann of U.S.A, and also use binary instead of decimal. Almost simultaneously the Briteshers developed EDSAC (Electronic Delay Storage Automatic Calculators), 1947-49 other such the computer that use mean lubes were Manchester mark 1 in 1948 UNIAC (Universal Automatic Computer) in 1951

In 1955-65 industries using transistors in place of vacuum tubes to construct computer. Magnetic cores where used for primary storage in 1960 a non- volatile memory was introduced (i.e. even if the power its turned off, stored contents are not destroyed). In1965-75 computer used ICs (Integrated), designed and fabricated by jack S. Kitby at text Instruments. SSI (Small Scale Integrated), MIS (Medium Scale Integrated), LSI (Large Scale integrated), and VLSI (Very large Scale Integrated) were made classified of ICs and computer started using ICs as CPU components. After 1975 they microprocessor (IC chip which contains the entire cup of a computer). Now-a-day microprocessors use VLSI chips, and ULSI (Ultra Large Scale Integrated) chips are under construction.

Generation of computer

Over the past five decades, computer has gone through five generation of development. Good features of the earlier generations have been passed to the later generations. A description of various computer generations is given below

First Generation(1946-1954):

Vacuum tubes as CPU components.

Mercury of delay line, Electrostatic (CRT) for memory.

Machine or Assembly langrage programming.

Fixed-point arithmetic was used.

Magnetic tape/magnetic drum as secondary memory.

No concepts of operating system.

Punched card and paper tape to feed program and data to get results.

Examples: ENIAC (first electronic, computer completed in 1946), used vacuum tubes as CPU components and memory. Stored program concept in 1946 mend programs and date to be stored in stored in separate memories, Examples EDVAC(1951), UNIVAC(1951), IBMs701(1953), IBM704(1955), IBM 709.

Second Generation(1955 to 1964):

Transistor in place of vacuum tubes IN CPU, and other electronic components.

Magnetic ferrite-core memory as main memory, random-access non-volatile memory.

Machine independent high level languages like FORTAN, COBOL, and ALGOLE were developed and used as programming languages.

Floating point arithmetic was used.

I/O processors were introduced to supervise control input./output operations. They relieved CPU from many time- consuming routine works.

System Software like compilers subroutines etc was introduced.

Bach processing was introduced.

Punched cards continued during this period also.

Computer was made effect in industry and commercial organization: payroll, inventory control, marketing, production planning, general ledger system and research.

Example: IBM 1620 (1960) IBM 7090(1960), IBM 7094 (1962), Digital data Corporations (DECs) Programmed Data Processor. PDP 1(1957), PDP 5(1963), PDP 8(1965), Control Data Corporations (CDCs) CDC 1604.

Third Generation (1965-1974):

ICs (SSI and MSI) started replacing transistors in CPU, I/O processors.

Semiconductor memories (RAM and ROMs) using LSI technology replaced magnetic core type main memory.

Certain new techniques such as parallels processing (Multiprocessing) Multiprogramming, Multiuse (time sharing); to increase the effective speed of program execution.

Cache memory was also introduced.

Operating systems for multiprogramming and time shared systems (multi-user systems) were developed.

Computers were widely applicable to multi-user applications, on line systems reservations, interactive query system, and automatic industrial controls.

Examples: CDC 7600(1969), IBM 370 serious (1970), PDP 11(1970, 16-BIT COMPUTER), CDCs CYBER-175 and STAR-100, Intels 4004, 8080.

Fourth Generation (1975-pressent):

Use microprocessor as CPU. Earlier, Cache, MMU (Memory Management Unit), FPU.Floating point Unit i.e. math processor) were on separate ICs Now-a-days all such components and CPU are packed into a single IC of the microprocessor.

Computer network is being-widely used: LAN (local Area Network), MAN (Metropolitan Area network), and WAN (Wide Area Network).

Multifunction peripheral chips providing functions of interrupt controller, DMA controller, Bus controller memory refresh controller etc on a single IC have developed.

Disk drives of gigabytes capacity, as secondary memory.

CE-ROMs, DVD-ROMs have been developed as read only memory