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Lecture notes on Advanced Computer Architecture

advanced computer architecture lecture notes and advanced computer architecture and parallel processing | pdf free download
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Published Date:10-07-2017
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? Advanced Computer Architecture Subject: ADVANCED COMPUTER ARCHITECTURE Credits: 4 SYLLABUS Computer System Generation of computers, Classification of computers, Organization, Structure and function, Von Neumann architecture. System bus, Bus Structure, Elements of Bus design (Type, Arbitration, Timing, Width, Data transfer Type), Interrupts, Instruction Cycle state Diagram with interrupts/Without interrupts, Characteristic of Internal memory (ROM, PROM, EPROM, Flash memory), Input / Output: (External / Peripheral Device, Function of I/O module, Programmer I/O, Interrupt Driver I/O DMA) The Central Processing Unit ALU, Binary Arithmetic, Floating point Arithmetic, Basic combinational and sequential Circuit Design, RTL representation, Suggested Reading: John L. Hennessy and David A. Patterson. Computer Architecture: A Quantitative Approach (Third Edition ed.). Morgan Kaufmann Publishers. Laplante, Phillip A. (2001). Dictionary of Computer Science, Engineering, and Technology. Lecture 1 INTRODUCTION TO COMPUTERS Objectives of the lecture: 1. To understand the basics of the computer. Hello Students, In today’s information age, computers are being used in every occupation. They are used by people of all age and profession, in their work as well as their leisure. This new social age have changed the basic concept of ‘Computing’. Computing, in today’s information age, is no more limited to computer programmers and computer engineers. Rather than knowing how to program a computer, most computer users simply need to understand how a computer functions so in this lecture I will be discussing with you about this versatile tool, why is it so powerful and useful, its history and you will also be briefed about the classification of computers its devices in my today’s lecture. What is A Computer? A computer is an electronic machine that accepts information, stores it until the information is needed, processes the information according to the instructions provided by the user, and finally returns the results to the user. The computer can store and manipulate large quantities of data at very high speed, but a computer cannot think. A computer makes decisions based on simple comparisons such as one number being larger than another. Although the computer can help solve a tremendous variety of problems, it is simply a machine. It cannot solve problems on its own. History of Computers Since civilizations began, many of the advances made by science and technology have depended upon the ability to process large amounts of data and perform complex mathematical calculations. For thousands of years, mathematicians, scientists and businessmen have searched for computing machines that could perform calculations and analyze data quickly and efficiently. One such device was the abacus. The abacus was an important counting machine in ancient Babylon, China, and throughout Europe where it was used until the late middle ages. It was followed by a series of improvements in mechanical counting machines that led up to the development of accurate mechanical adding machines in the 1930’s. These machines used a complicated assortment of gears and levers to perform the calculations but they were far to slow to be of much use to scientists. Also, a machine capable of making simple decisions such as which number is larger was needed. A machine capable of making decisions is called a computer. 2 The first computer like machine was the Mark I developed by a team from IBM and Harvard University. It used mechanical telephone relays to store information and it processed data entered on punch cards. This machine was not a true computer since it could not make decisions. In June 1943, work began on the world's first electronic computer. It was built at the University of Pennsylvania as a secret military project during World War II and was to be used to calculate the trajectory of artillery shells. It covered 1500 square feet and weighed 30 tons. The project was not completed until 1946 but the effort was not wasted. In one of its first demonstrations, the computer solved a problem in 20 seconds that took a team of mathematicians three days. This machine was a vast improvement over the mechanical calculating machines of the past because it used vacuum tubes instead of relay switches. It contained over 17,000 of these tubes, which were the same type tubes used in radios at that time. The invention of the transistor made smaller and less expensive computers possible. Although computers shrank in size, they were still huge by today’s standards. Another innovation to computers in the 60’s was storing data on tape instead of punched cards. This gave computers the ability to store and retrieve data quickly and reliably. Classification of Computers ƒ Mainframe Computers ƒ Minicomputers ƒ Microcomputers ƒ Supercomputers Mainframe computers are very large, often filling an entire room. They can store enormous of information, can perform many tasks at the same time, can communicate with many users at the same time, and are very expensive. . The price of a mainframe computer frequently runs into the millions of dollars. Mainframe computers usually have many terminals connected to them. These terminals look like small computers but they are only devices used to send and receive information from the actual computer using wires. Terminals can be located in the same room with the mainframe computer, but they can also be in different rooms, buildings, or cities. Large businesses, government agencies, and universities usually use this type of computer. Minicomputers : are much smaller than mainframe computers and they are also much less expensive. The cost of these computers can vary from a few thousand dollars to several hundred thousand dollars. They possess most of the features found on mainframe computers, but on a more limited scale. They can still have many terminals, but not as many as the mainframes. They can store a tremendous amount of information, but again usually not as much as the mainframe. Medium and small businesses typically use these computers. Microcomputers : These computers are usually divided into desktop models and laptop models. They are terribly limited in what they can do when compared to the larger models discussed 3above because they can only be used by one person at a time, they are much slower than the larger computers, and they cannot store nearly as much information, but they are excellent when used in small businesses, homes, and school classrooms. These computers are inexpensive and easy to use. They have become an indispensable part of modern life. Computer Tasks ƒ Input ƒ Storage ƒ Processing ƒ Output When a computer is asked to do a job, it handles the task in a very special way. 1. It accepts the information from the user. This is called input. 2. It stored the information until it is ready for use. The computer has memory chips, which are designed to hold information until it is needed. 3. It processes the information. The computer has an electronic brain called the Central Processing Unit, which is responsible for processing all data and instructions given to the computer. 4. It then returns the processed information to the user. This is called output. Every computer has special parts to do each of the jobs listed above. Whether it is a multi- million dollar mainframe or a thousand dollar personal computer, it has the following four components, Input, Memory, Central Processing, and Output. The central processing unit is made up of many components, but two of them are worth mentioning at this point. These are the arithmetic and logic unit and the control unit. The control unit controls the electronic flow of information around the computer. The arithmetic and logic unit, ALU, is responsible for mathematical calculations and logical comparisons. Input Devices ƒ Keyboard ƒ Mouse ƒ Scanner ƒ Microphone ƒ CD-ROM ƒ Joystick 4 Memory Read Only Memory (ROM) ROM is a small area of permanent memory that provides startup instructions when the computer is turned on. You can not store any data in ROM. The instructions in ROM are set by the manufacturer and cannot be changed by the user. The last instruction in ROM directs the computer to load the operating system. Every computer needs an operating system. This is a special computer program that must be loaded into memory as soon as the computer is turned on. Its purpose is to translate your instructions in English into Binary so that the computer can understand your instructions. The operating system also translates the results generated by your computer into English when it is finished so that we can understand and use the results. The operating system comes with a computer. Random Access Memory (RAM) This is the area of memory where data and program instructions are stored while the computer is in operation. This is temporary memory. NOTE: The data stored in RAM is lost forever when the power is turned off. For this reason it is very important that you save your work before turning off your computer. This is why we have peripheral storage devices like your computer’s hard disk and floppy diskettes. Permanent Memory (Auxiliary Storage) Your files are stored in permanent memory only when saved to your disk in a: drive or saved to your computer's hard disk, Drive c: To better understand how a computer handles information and to also understand why information is lost if the power goes off, let’s take a closer look at how a computer handles information. Your computer is made of millions of tiny electric circuits. For every circuit in a computer chip, there are two possibilities: 1. an electric circuit flows through the circuit or 2. An electric circuit does not flow through the circuit. When an electric current flows through a circuit, the circuit is on. When no electricity flows, the circuit is off. An “on” circuit is represented by the number one (1) and an off circuit is represented by the number zero (0). The two numbers 1 and 0 are called bits. The word bit comes from “binary digit”. Each time a computer reads an instruction, it translates that instruction into a series of bits, 1’s and 0’s. On most computers every character from the keyboard is translated into eight bits, a combination of eight 1’s and 0’s. Each group of eight bits is called a byte. 5Byte – The amount of space in memory or on a disk needed to store one character. 8 bits = 1 Byte Since computers can handle such large numbers of characters at one time, metric prefixes are combined with the word byte to give some common multiples you will encounter in computer literature. Kilo means 1000 kilobyte (KB) = 1000 Bytes Mega means 1,000,000 megabyte (MB) = 1,000,000 Bytes Giga Means 1,000,000,000 gigabyte (GB) = 1,000,000,000 Bytes At this point it would be good to point out why information stored in RAM is lost if the power goes off. Consider the way the following characters are translated into binary code for use by the computer. A 01000001 B 01000010 C 01000011 X 01011000 Z 01011010 1 00110001 2 00110010 Consider the column at the right, which represents how the computer stores information. Each of the 1’s in the second column represents a circuit that is “on”. If the power goes off, these circuits can NOT be “on” any more because the electricity has been turned off and any data represented by these circuits is lost Central Processing Unit (CPU) The central processing unit is one of the two most important components of your microcomputer. It is the electronic brain of your computer. In addition to processing data, it controls the function of all the other components. The most popular microprocessors in IBM compatible computers are made by Intel. The generations of microprocessors are listed below. 1981 8088 1984 80286 1987 80386 1990 80486 1993 Pentium 1996 P-1 2002 P-4 6 Output Devices Monitor Speakers Printer Impact Daisy Wheel Dot Matrix Non-Impact Ink Jet Laser Storage Devices Floppy disk Tape drive Local drive (c) Network drive (z) CD-ROM Zip disk Telecommunications Telecommunications means that you are communicating over long distances usually using phone lines. This enables you to send data to and receive data from another computer that can be located down the street, in another town, or in another country. Telecommunications requires a communication device called a modem, which connects your computer to a standard phone jack. A modem converts the digital signals that your computer uses into analog signals that can be transmitted over the phone lines. To use a modem, you must also have communication software to handle the transmission process. Computer Software System Software System software will come provided with each computer and is necessary for the computer’s operation. This software acts as an interpreter between the computer and user. It interprets your instructions into binary code and likewise interprets binary code into language the user can understand. In the past you may have used MS-DOS or Microsoft Disk Operating System which was a command line interface. This form of system software required specific commands to be typed. Windows 95 is a more recent version of system software and is known as a graphical interface. This means that it uses graphics or "icons" to represent various operations. You no longer have to memorize commands; you simply point to an icon and click. 7Program Software Program software is software used to write computer programs in specific computer languages. Application Software Application software is any software used for specified applications such as: ƒ Word Processing ƒ Spreadsheet ƒ Database ƒ Presentation Graphics ƒ Communication ƒ Tutorials ƒ Entertainment, Games Emerging Trends The components of a computer are connected by using buses. A bus is a collection of wire that carry electronic signals from one component to another. There are standard buses such as Industry Standard Architecture (ISA), Extended Industry Standard Architecture (EISA), Micro- Channel Architecture (MCA), and so on. The standard bus permits the user to purchase the components from different vendors and connect them easily. The various input and output devices have a standard way of connecting to the CPU and Memory. These are called interface standards. Some popular interface standards are the RS-232C and Small Computer System Interconnect (SCSI). The places where the standard interfaces are provided are called ports. Data Representation Bits and Bytes Data in Computers are represented using only two symbols '0' & '1'. These are called "Binary digiTS" (or) "BITS" for short. A set of 8 bits is called a byte and each byte stores one character. n 2 Unique strings are represented using n bits only. For example, Using 2 bits we can represent 2 4=(2) unique strings as 00, 01, 10, 11. ASCII (American Standards Code for Information Interchange) codes are used to represent each character. The ASCII code includes codes for English Letters (Both Capital & Small), decimal digits, 32 special characters and codes for a number of symbols used to control the operation of a computer which are non-printable. Binary numbers Binary numbers are formed using the positional notation. Powers of 2 are used as weights in the binary number system. A binary number system. A binary number 10111, has a decimal value 84 3 1 0 equal to 12 +02 +12 +12=23. A decimal number is converted into an equivalent binary number by dividing the number by 2 and storing the remainder as the least significant bit of the binary number. For example, consider the decimal number 23. Its equivalent binary number is obtained as show below in figure CONVERSION OF DECIMAL TO BINARY EXAMPLE. 23 = (0111)2 Hexadecimal Numbers High valued binary numbers will be represented by a long sequence of 0's and 1's. A more concise representation is using hexadecimal representation. The base of the hexadecimal system is 16 and the symbols used in this system are 0,1,2,4,5,6,7,8,9,A,B,C,D,E,F. Strings of 4 bits have an equivalent hexadecimal value. For example, 6B is represented by 0110 1011 or 110 1011, 3E1 is represented by 0011 1110 0001 or 11 1110 0001 and 5DBE34 is represented by 101 1101 1011 1110 0011 0100. Decimal fractions can also be converted to binary fractions. Parity Check Bit Errors may occur while recording and reading data and when data is transmitted from one unit to another unit in a computer Detection of a single error in the code for a character is possible by introducing an extra bit in its code. This bit, know as the parity check bit, is appended to the code. The user can set the parity bit either as even or odd. the user chooses this bit so that the total number of ones ('1') in the new code is even or odd depending upon the selection. If a single byte is incorrectly read or written or transmitted, then the error can be identified using the parity check bit. Input Devices Key Board The most common input device is the Keyboard. It is used to input letters, numbers, and commands from the user. Mouse Mouse is a small device held in hand and pushed along a flat surface. It can move the cursor in any direction. In a mouse a small ball is kept inside and the ball touches the pad through a hole at the bottom of the mouse. When the mouse is moved, the ball rolls. This movement of the ball is converted into electronic signals and sent to the computer. Mouse is very popular in the modern computers that use Windows and other Graphical User Interface (GUI) applications. Magnetic Ink Character Recognition (MICR) In this method, human readable characters are printed on documents such In this method, human readable characters are printed on documents such as cheque using special magnetic ink. The cheque can be read using a special input unit, which can recognize magnetic ink characters. This 9method eliminates the need to manually enter data from cheques into a floppy. Besides saving time, this method ensures accuracy of data entry and improves security. Optical Mark Reading and Recognition (OMR) In this method, special pre-printed forms are designed with boxes which can be marked with a dark pencil or ink. Such a document is read by a document reader, which transcribes the marks into electrical pulses which are transmitted to the computer. These documents are applicable in the areas where responses are one out of a small number of alternatives and the volume of data to be processed is large. For example: • Objective type answer papers in examinations in which large number of candidates appear. • Market surveys, population survey etc., • Order forms containing a small choice of items. • Time sheets of factory employees in which start and stop times may be marked. The advantage of this method is that information is entered at its source and no further transcription is required. Optical Character Recognition (OCR) An optical scanner is a device used to read an image, convert it into a set of 0's and 1's and store it in the computer's memory. The image may be hand-written document, a typed or a printed document or a picture. Bar Coding In this method, small bars of varying thickness and spacing are printed on packages, books, badges, tags etc., which are read by optical readers and converted to electrical pulses. The patterns of bars are unique an standardized. For example, each grocery product has been given unique 10-digit code and this is represented in bar code form on every container of this product. Speech Input Unit A unit, which takes spoken words as its input, and converts them to a form that can be understood by a computer is called a speech input unit. By understanding we mean that the unit can uniquely code (as a sequence of bits) each spoken word, interpret the word and initiate action based on the word. 10Output Devices Monitor or Video Display Unit (VDU) Monitors provide a visual display of data. It looks like a television. Monitors are of different types and have different display capabilities. These capabilities are determined by a special circuit called the Adapter card. Some popular adapter cards are, • Color Graphics Adapter (CGA) • Enhanced Graphics Adapter (EGA) • Video Graphics Array (VGA) • Super Video Graphics Array (SVGA) THE LECTURES IN A GO • Defination Of computer • History Of Computers • Classification Of Computers • Explanation about i/p and o/p devices • Explanation about storage devices • Types of computer software 11Questions: 1. When u switch on your computer which software you see first and what is the utility of that software. 2. Suppose on fine day you are working on ur computer and power goes off, again u switch on our computer, what type of booting is done by that computer. 3. Write the essential parts of ur computer system without which u cant work and also list that parts which are optional. 4. How many types of storage are normally there in storage unit of a computer system? Justify the need for each storage type. Explain them. 5. What are the basic components of the CPU of a computer systems ? Describe the roles of each of the components in the functioning of a computer systems. 6. Suppose an entrance exam is held and thousands of students appeared in that exam, Which device u will use to evaluate the answer sheets and why? 7. Hardware and software are like two sides of a coin. Do you agree or disagree, Give reasons. END OF TODAYS LECTURE… References: 1. COMPUTER FUNDAMENTALS Pradeep .K.Sinha and Priti Sinha , BPB PUBLICATIONS 2. COMPUTER ORGANISATION AND ARCHITECTURE William Stallings Prentice PUBLICATIONS 12Lecture 2 GENERATIONS OF COMPUTERS Objectives of the lecture: 1.To learn the generation of the computers. Hellofriends , I am sure now you must be well versed with the History of computers from the previous lecture .Today I will be completing the remaining part of the previous lecture and then starting with the generations which tells how it has evolved from its early days, to become a powerful and useful tool for all types of users in today’s society. So Lets start on.. Contd. Lets start with the defination of Pixels, the smallest dot that can be displayed is called a pixel. The number of pixels that can be displayed vertically and horizontally gives the maximum resolution of the monitor. The resolution of the monitor determines the quality of the display. The higher the resolution the better is the quality of the display. Some popular resolution are 800640 pixels, 1024768 pixels, 12801024 pixels. Printer Line printer • It prints a complete line at a time. • Printing speed varies from 150 lines to 2500 lines per minute with 96 to 160 character on a 15-inch line. • Six to eight lines per vertical inch are printed. • Usually 64 and 96 character sets are used with English letters. • Two types of Line Printers are available. • Drum Printers: It consists of a cylindrical drum. The characters to be printed are embossed on its surface • Chain Printers: I have a steel band on which the character sets are embossed. Serial Printers • It prints one character at a time, with the print head moving across a line. • They are similar to typewriters. 13• They are normally slow (30 to 300 character per second) • The most popular serial printer is "Dot Matrix Printer". • Her character to be printed is made up of a finite number of dots and so, the print head consists of an array of pins. • Characters to be printed are sent one character at a time from the memory to the printer. The character code is decoded by the printer electronics and activates the appropriate pins in the print head. • Many dot matrix printers are bi-directional. i.e. they print form left to right as well as from right to left on return. This enhances the speed of printing. • The printing speed is around 300 characters per second. Letter Quality Printers • Here the characters are represented by sharp continuous lines and so the output is good looking • An example of such a printer is "Inkjet Printer". • It consists of a print head, which has a number of small hole or nozzles. • Individual holes can be heated very rapidly by an integrated circuit resistor. When the resistor heats up, the ink near it vaporizes and is ejected through the nozzle and makes a dot on paper placed near the head. • A high-resolution inkjet printer has around 50 nozzles within a height of 7mm and can print with a resolution of 300 dots per inch. • Latest inkjet printers have multiple heads, on per color, which allows color printing. • The printing speed is around 120 characters per second. Laser Printers • Here an electronically controlled laser beam traces out the desired character to be printed on a photoconductive drum. The drum attracts an ink toner on o the exposed areas. This image is transferred to the paper, which comes in contact with the drum. • Low speed laser printers, which can print 4 to 16 pages per minute, are now very popular and the unit cost is around Rs.0.5 lakh. • Very fast printers print 10,000 lines per minute and cost per unit is around R.5 lakhs. These printers give excellent outputs and can print a variety of fonts. • As these printers do not have type head striking on a ribbon, they are known as non- impact printers. 14Apart from printers, the other output devices are given below: 1. Drum Plotter 2. Flat Bed Plotter 3. Microfilm and Microfiche 4. Graphic Display device (Digitizing Tablet) 5. Speech Output Unit Computer Memory Main Memory A flip-flop made of electronic semiconductor devices is used to fabricated a memory cell. These memory cells organized as a Random Access Memory (RAM). Each cell has a capability to store one bit of information. A main memory or store of a computer is organized using a large number of cells. Each cell stores a binary digit. A memory cell, which does not loose the bit stored in it when no power is supplied to the cell, is know as a non-volatile cell. A word is a group of bits, which are stored and retrieved as a unit. A memory system is organized to store a number of words. A Byte consists of 8 bits. A word may store one or more bytes. The storage capacity of a memory is the number of bytes it can store. The address of the location from where a word is to be retrieved or to be stored is entered in a Memory Address Register (MAR). The data retrieved from memory or to be stored in memory are placed in a Memory Data Register (MDR). The time taken to write a word is known as the Write time. The time to retrieve information is called the Access time of the memory. The time taken to access a word in a memory is independent of the address of the word and hence it is know as a Random Access Memory (RAM). The main memory used to store programs and data in a computer is a RAM. A RAM may be fabricated with permanently stored information, which cannot be erased. Such a memory is called a Read Only Memory (ROM). For more specialized uses, a user can store his won special functions or programs in a ROM. Such ROM's are called Programmable ROM (PROM). A serial access memory is organized by arranging memory cells in a linear sequence. Information is retrieved or stored in such a memory by using a read/write head. Data is presented serially for writing and is retrieved serially during read. Secondary or Auxiliary storage devices Magnetic surface recording devices commonly used in computers are Hard disks, Floppy disks, CD-ROMs and Magnetic tapes. These devices are known as secondary or auxiliary storage devices. We will see some of these devices below. 15Floppy Disk Drive (FDD) In this device, the medium used to record the data is called as floppy disk. It is a flexible circular disk of diameter 3.5 inches made of plastic coated with a magnetic material. This is housed in a square plastic jacket. Each floppy disk can store approximately on million characters. Data recorded on a floppy disk is read and stored in a computer's memory by a device called a floppy disk is read and stored in a computer's memory by a device called a floppy disk drive (FDD). A floppy disk is inserted in a slot of the FDD. The disk is rotated normally at 300 revolutions per minute. A reading head is positioned touching a track. A voltage is induced in a coil wound on the head when a magnetized spot moves below the head. The polarity of the induced voltage when a 0 is read. The voltage sensed by the head coil is amplified, converted to an appropriate signal and stored in computer's memory. • Floppy Disks com with various capacities as mentioned below. 1/4 10 • 5 drive- 360KB, 1.2MB (1 KB= 2 = 1024 bytes) 1/2 20 • 3 drive- 1.44 Mb, 2.88 MB (1MB= 2 bytes) Compact Disk Drive (CDD) CD-ROM (Compact Disk Read Only Memory) used a laser beam to record and read data along 1/4 spiral tracks on a 5 disk. A disk can store around 650 MB of information. CD-ROMs are normally used to store massive text data. (such as encyclopedias) which is permanently recorded and read many times. Recently CD writers have come in the market. Using a CD writer, lot of information can be written on CD-ROM and stored for future reference. Hard Disk Drive (HDD) Unlike a floppy disk that is flexible and removable, the hard disk used in the PC is permanently fixed. The hard disk used in a higher end Pc can have a maximum storage capacity of 17 GB 30 (Giga Byte; 1 GB= 1024 MB = 2 bytes). Now a days, hard disks capacities of 540 MB, 1 GB, 2 GB, 4 GB and 8 GB are quite common. The data transfer rate between the CPU and hard disk is much higher as compared to the between the CPU and the floppy disk drive. The CPU can use the hard disk to load programs and data as well as to store data. The hard disk is a very important Input/Output (I/O) device. The hard disk drive doesn't require any special care other than the requirement that one should operate the PC within a dust-free and cool room (preferably air- conditioned). In summary, a computer system is organized with a balanced configuration of different types of memories. The main memory (RAM) is used to store program being currently executed by the computer. Disks are used to store large data files and program files. Tapes are serial access memories and used to backup the files form the disk. CD-ROMs are used to store user manuals, large text, audio and video data. 16Application and System Software Software & Hardware A set of programs associated with the operation of a computer is called software. The electronic circuits used in building the computer that executes the software is known as the hardware of the computer. For example, a TV bought from a shop is hardware; the various entertainment programs transmitted from the TV station are software. An important point to note is, hardware is a one-time expense and is necessary whereas software is a continuing expense and is vital. Computer software may be classified into two broad categories: Application Software It is the set of programs necessary to carry out operations for a specified application. Example Programs • To solve a set of equations • To process examination results • To prepare a Pay-Bill for an organization • To prepare Electricity-Bill for each month. System Software These are general program written for the system, which provide the environment to facilitate writing of Application software. Some of the system programs are given below: Compiler: It is a translator system program used to translate a High-level language program into a Machine language program. Assembler: It is another translator program used to translate an Assembly language program into a Machine language program. Interpreter: It is also a translator system program used to translate a High level language program into a Machine language program, but it translates and executes line by line. Loader: It is a system program used to store the machine language program into the memory of the computer. 17Computer Languages Machine language The computers can execute a program written using binary digits only. This type of programs is called machine language programs. Since these programs use only '0's and '1's it will be very difficult for developing programs for complex problem solving. Also it will be very difficult for a person to understand a machine language program written by another person. At present, computer users do not write programs using machine language. Also these programs written for execution in one computer cannot be used on another type of computer. i.e., the programs were machine dependent. Assembly Language In assembly language mnemonic codes are used to develop program for problem solving. The program given below shows assembly language program to add two numbers A & B. Program code Description READ A It reads the value of A. ADD B The value of B is added with A. STORE C The result is store in C. PRINT C The result in 'C' is printed. HALT Stop execution. Assembly language is designed mainly to replace each machine code with and understandable mnemonic code. To execute an assembly language program it should first be translates into an equivalent machine language program. Writing and understanding programs in assembly language is easier than that of machine language. The programs written in assembly language are also machine dependent. High Level Languages High level language are developed to allow application programs, which are machine independent. High level language permits the user to use understandable codes using the language structure. In order to execute a high-level language program, it should be translated into a machine language either using a compiler or interpreter. The high level languages commonly used are FORTRAN (FORmula TRANslation), BASIC (Beginner's All-purpose Symbolic Instruction Code), COBOL (COmmon Business Oriented Language). Recently developed programming language such as Visual Foxpro, Visual Basic (VB), Visual C++ (VC++) are more popular among the software developers. The following program written in BASIC language is to add two given numbers. 18Program Code Description 10 INPUT A,B To read the value of A&B 20 LET C=A+B A&B are added and result is stored in C 30 PRINT C Print the value of C 40 END Stop execution Computers and Communications Local Area Network (LAN) & Wide Area Network (WAN) Computers available in remote locations can communicate with each other using a telecommunication line. One way of connecting the computers is by using devices called modems. A modem is used to transfer data from one computer to another using the telephone lines. A modem converts the strings of 0s and 1s into electrical signals which can be transferred over the telephone lines. Both the receiving and the transmitting computer have a telephone connection and a modem. An external modem is connected to the computer like a typical input or an output device. An internal modem is fitted into the circuitry related to the CPU and Memory. Interconnection of computers which are within the same building or nearby locations forms a network of computers and this network is called a Local Area Network (LAN). A LAN permits sharing of data files, computing resources and peripherals. Interconnection of computers located in far away locations using telecommunication system is known as Wide Area Network (WAN). 19