Thursday, January 24, 2008

Computer Notes for Class IX

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History of Computers
This chapter covers the history of computers.
Computer and Its Types
What are the different types of computers? They are classified according to Purpose, Size and Type
Data Processing
This chapter has details about Electronic Data Processing and its Types.
Input and Output Devices
Input devices as Keyboard, Mouse, Light Pens and Output devices such as Monitor, Printer are discussed in this chapter.
Programming and BASIC
Flowcharts, Programming, Variables and everything related to BASIC

Computer Notes for Class IX
Topic No.Topic
1 History of Computers
2 Dark Age (3000 BC to 1890 AD)
3 Middle Age (1890 AD TO 1944 AD)
4 Modern Ages (Since 1944 AD)
History of Computers


At the early age people used pebbles, stones, sticks, scratches, symbols and finger tips to count, which were later replaced by numbers.
The history of computing is divided into three ages during which man invented and improved different types of calculating machines. These ages are,
  • Dark age 300 BC to 1890 AD
  • Middle age 1890 AD to 1944 AD
  • Modern age since 1944 AD
Dark Age (3000 BC to 1890 AD)


About 3000 years BC, Chinese developed the first calculating machine named Abacus or Soroban.
Abacus consists of a rectangular wooden frame having rods which carry round beads. Counting is done by shifting the beads from one side to another.


In 1632 AD William Oughtred, an English mathematician developed a slide rule. This device consists of two movable rules placed side by side on which number were marked.


Blasé Pascal (1623-1662), a French developed the first mechanical calculating machine in 1642. This machine consists of gears, wheels and dials. It was capable of adding and subtracting operations.


In 1671, a German, Gottfried Von Leibnitz (1646-1716) improved Pascal’s calculator to make it capable of performing all maths operations.


In 1801, a French, Joseph Marie Jacquard developed the first punch card machine.


Charles Babbage (1792-1871) an English mathematician also called Father of modern computer. As he gave the true concept of computer at Cambridge University, he developed Babbage Difference Engine in 1823 and Babbage Analytical Engine in 1833.
Lady Ada Augusta an assistant of Babbage is called the first programmer.
Modern Ages (Since 1944 AD)


In 1945, Dr. John Von Neuman suggested the concept of Automatic Data Processing (ADP) according to the stored program and data.


Electronic Numerical Integrator And Calculator (ENIAC) was the first electronic computer made in 1946 by John Presper Eckert and John Williams Mauchly, at the University of Pennsylvania, USA. This was based on decimal number system and it has no memory.
It could perform 5000 additions or 350 multiplications in one second. It contained 18000 vacuum tubes, 70,000 resistors, 10,000 capacitors and 60,000 switches and occupied a two room car garage. It consumed 150 kW of power. It weighed 27 tons.


Electronic Delay Storage Automatic Computer (EDSAC) was first computer based on stored program concept. It was completed by Mourice Wilkes at Cambridge University in 1949.


Electronic Discrete Variable Automatic Computer (EDVAC) was built by John Williams Mauchly, John Presper Eckert at Moore School, Pennsylvania in 1951.


UNIVersal Automatic Computer (UNIVAC) was the first commercially used computer made by John Presper Eckert and John Williams Mauchly in June 14, 1951.

Middle Age (1890 AD TO 1944 AD)


In 1880s Herman Hollerith an American developed a machine which used punch card system. The machine could sense and punch holes, recognize the number and make required calculations. This machine was first used in 1890s by American Census Bureau.


In 1937, Professor Howard Aiken build the first electro-mechanical computer Mark-1, by trying to combine Babbage’s theory and Hollerith’s punching technologies. He completed his project in 1944 with the help of IBM Engineers.
Mark 1 could multiply two, twenty digit numbers in 5 seconds and made a lot of noise. It had a shape like a monster about 50 feet long, 8 feet high, having wiring of length equal to distance from Lahore to Gilgit or Karachi to Bahawalpur (800km) and had thousand ends of electro-magnetic relays.


ABC a special purpose computer was developed in 1938 by Dr. John Vincent Atanasoff and Clifford Berry at Iowa State College, USA.
Computer and Its Types
Topic No.Topic
1 Computer
2 Classification According to Purpose
3 Classification According to Type
4 Classification According to Size
Computer is an electronic data processing machine which takes the input, stores the data in its memory then process it with the help of Arithmetic Logic Unit (A.L.U) under the supervision of Control Unit (C.U) and then give us the output according to the instructions (Program).
The word computer is derived from a Latin word “Computare” which means to count or to calculate. The computer is an extension of human mind which can perform several tasks with a great speed. Computers are just like a dumb servant who cannot take any decision by itself they only do what they are told to do so.
Classification According to Purpose


General purpose computers are designed to solve a large variety of problems. The different programs can be used to solve many problems. Most digital computers are general purpose computers and used in business and commercial data processing.


A computer designed for machine control or process control would be different than a general purpose computer. The special purpose computers are designed to solve specific problems. The computer program for solving a specific problem is built right into the computer. Most analog computers are special purpose computers. These special purpose computers are widely used in industrial robotics.

Classification According to Type


Analog computers are used to process continuous data. Analog computers represent variables by physical quantities. Thus any computer which solve problem by translating physical conditions such as flow, temperature, pressure, angular position or voltage into related mechanical or electrical related circuits as an analog for the physical phenomenon being investigated in general it is a computer which uses an analog quantity and produces analog values as output. Thus an analog computer measures continuously. Analog computers are very much speedy. They produce their results very fast. But their results are approximately correct. All the analog computers are special purpose computers.


A computer which process information represented by combination of *** or discontinuous data. It is a device for performing sequence of arithmetic and logical operations, not only on data but on its ***. Digital computer that is capable of performing sequences of internally stored instructions for calculation. Digital computers are not very much fast but their results are very much accurate. All the digital computers are general purpose computers.


Various specifically designed computers are with both digital and analog characteristics combining the advantages of analog and digital computers when working as a system. Hybrid computers are being used extensively in process control system where it is necessary to have a close representation with the physical world.
The hybrid system provides the good precision that can be attained with analog computers and the greater control that is possible with digital computers, plus the ability to accept the input data in either form.
Classification According to Size


Large scientific and research laboratories as well as the government organizations have extra ordinary demand for processing data which required tremendous processing speed, memory and other services which may not be provided with any other category to meet their needs. Therefore very large computers used are called Super Computers. These computers are extremely expensive and the speed is measured in billions of instructions per seconds (B.I.P.S).


The most expensive, largest and the most quickest or speedy computer are called mainframe computers. These computers are used in large companies, factories, organizations etc. the mainframe computers are the most expensive computers, they cost more than 20 million rupees. In this computers 150 users are able to work on one C.P.U. The mainframes are able to process 1 to 8 bits at a time. They have several hundreds of megabytes of primary storage and operate at a speed measured in nano second.


Mini computers are smaller than mainframes, both in size and other facilities such as speed, storage capacity and other services. They are versatile that they can be fitted where ever they are needed. Their speeds are rated between one and fifty million instructions per second (MIPS). They have primary storage in hundred to three hundred megabytes range with direct access storage device.


These are the smallest range of computers. They were introduced in the early 70’s having less storing space and processing speed. Micro computers of todays are equivalent to the mini computers of yesterday in terms of performing and processing. They are also called “computer of a chip” because its entire circuitry is contained in one tiny chip. The micro computers have a wide range of applications including uses as portable computer that can be plugged into any wall.
3 Data Processing
This chapter has details about Electronic Data Processing and its Types.
Topic No.Topic
1 Data Processing and EDP
2 Elements of EDP
Data Processing and EDP
Data processing often referred as D.P is a process of collecting the data together and converting the data into information. The method used for collecting the data may be manual, mechanical or electronic.
Data processing is a term mostly associated with business and commercial work. Since computers are being used in the processing of data the term “electronic data processing” may also be used.


Data processing means transformation of data into more meaningful results for carrying out scientific, business activities. The result of data processing is called “information”. The transformation of data consist of a sequence of operations. The sequence is called “procedure”.
Input ® Processing ® Output
Data processing is a system which takes data as an input, carries out the required processing on the data and produces the information. The system is called “MANUAL” when processing is performed by human beings and “AUTOMATIC” when machines are used. When computers are used for data processing the system is called “Electronic Data Processing”.

Elements of EDP


There are five basic elements in a processing system which uses a computer for processing data. These are hardware, software, user program, procedure and personnels.


All the physical parts which makes up a computer system called hardware i.e. all the devices or peripherals which performs the data processing operations.


Software consists of programs and routines whose purpose is to make the computer useable for the user. These software normally supplied by computer manufacturers or software manufacturers.


A program consist of a related instructions to perform operations. A data processing job may require a number of programs.


The operations of data processing system requires procedure for use, in preparing data, for operating the computer and distributing the output after processing.


E.D.P basically needs three kinds of skilled personnels.
a) System Analyst
b) Progammer
c) Operator


System Analyst studies information needs and data processing requirements, design a data processing system and prepare specification.


A Programmer writes a programmer on specification by System Analyst.


An Operator is a person who operates the computer system.
4 Input and Output Devices
Input devices as Keyboard, Mouse, Light Pens and Output devices such as Monitor, Printer are discussed in this chapter.
Topic No.Topic
1 Printer and its Types
2 Secondary Storage Devices
3 Input Devices
4 Output Devices
5 Hard Disk
Printer and its Types


A computer peripheral that puts text or a computer generated image on paper or on another medium, such as a transparency. Printer can be categorized in several ways the most common distinction is IMPACT and NON-IMPACT.


Is the method used by the conventional type writers. In some type of impact printing a metal “hammer” embossed with a character strikes a print ribbon, which presses the characters image into paper. In other types the hammer strikes the paper and presses it into the ribbon characters created through impact printing can be formed by either a solid font or dot matrix printing mechanism.


Does not depend on the impact of metal on paper. In fact no physical contact at all occurs between the printing mechanism and the paper. The most popular non-impact methods today utilize thermal transfer, ink-jet.


Any printer that produces character made up of dots using a wire pin printed head. The quality of output from a dot matrix printer depends largely on the number of dots in the matrix, which might be low enough to show individual dots or might be high enough to approach the look of fully formed characters. Dot matrix printers are often categorized by the number of pins in the printer head typically, 9 or 24.


Any printer that prints one line at one time, as opposed to one character at a time or one page at a time. Line printer typically produce the 11 by 17 inch “computer” printout. They are high speed devices and are often used with mainframes, minicomputers, or networked machines rather than with single user system. Types of line printers include chain printers and band printer.


An electrophotographic printer that is based on the technology used by photocopiers. A focussed laser beam and a rotating mirror are used to draw an image of the desired page on a photosensitive drum. This image is converted on the drum into an electrostatic charge, which attracts and holds toner. A piece of electrostatically charged paper is rolled against the drum, which pulls the toner away from the drum and onto the paper. Heat is then applied to fuse the toner to the paper. Finally, the electrified charge is removed from the drum and the excess toner is collected. By omitting the final step and repeating only the toner application and paper handling steps, the printer can make multiple copies.


Daisy wheel printer are some times called letter quality printer because they are often used to produce attractive correspondence. The D.W.P is a flat circular device made of metal with character embossed on it. As this wheel spins at a very high speed the hammer hits the specific character against the ribbon which presses against the paper.


It is a kind of non-impact printer. In electrothermal printing, characters are burned on to a special paper by heated rods on a print heat. They transfer ink from a wax-based ribbon onto plain paper. These printer can support high quality graphic.


It is a kind of non-impact spray small dots of electrically charged ink onto a paper to form images. Ink jet printer are flexible enough to be used as plotters.
Secondary Storage Devices


Secondary storage devices are also called backup storage because it is used to store data. Volume of data on permanent basis which can be partially transferred to the primary storage, when required for data processing. Afterwards these devices are comparatively cheap and provide greater space to store the data /instructions are stored on secondary storage devices in the same binary codes as in primary storage.


Random Access Devices are those devices on which we can directly access the data. These devices are comparatively provide the fast communication.
For example, hard disk, floppy disk, optical disk.


Sequential Access Devices are those in which we can access the data one by one in a sequence. These devices provide slow communication as compared to Random Access Device.


The storage capacity of the primary storage of today’s computer is not sufficient. To store a large volume of data as a result additional memory called secondary storage is needed with most of the computer system.
These devices also provides the fast communication than I/O devices. The internal memory of a computer is a volatile memory. Therefore, we cannot save the data permanently. In that case we require secondary storage device which provide the facility to store the data for future use.


A floppy disk, also called simply a diskette or disk, is a small flexible Mylar disk coated with iron oxide on which data are stored. The floppy disk has been around since early 1970s, today it is available in three 3½ inch, 5¼ inch and 8 inch sizes. The 5¼ and 8 inch diskettes are covered by stiff protective jacket with different holes. The central big hole called hub ring which is used to hold by disk drive during rotation. The elongated read write window is used to read and write data through read/write head. The small hole next to the hub ring is called index hole which is used to locating data through computer. The cut out on the side of the floppy disk is called write protect notch. If we cover this opening with a piece of paper then we can’t write data on to disk.
In small diskette a hard plastic cover and protective metal is used to protect disk. Before using a disk we have to format a disk in which disk is divided into tracks and sectors for storing the data. Diskettes may be double sided and single sided while the storage capacity become less or more.
Floppy diskettes are more convenient to use with microcomputers. A floppy disk which is a random access device can access data fast than magnetic tape.


Magnetic tape is a sequential access device about one half or one fourth inch in size and made of Mylar (a plastic material) coated with a thin layer of iron oxide. Data can be read and write through a device which is called tape drive. The read/write head of tape drive which is an electromagnetic component read, write and erase data from magnetic tape. Magnetic tape is divided into nine separate strips or tracks in which eight tracks are used to store data and ninth track is used for error checking bit.
Magnetic tape can store large quantities of data therefore they are erasable, usable and durable secondary storage device. But it can use with large computers.
Input Devices


Input devices are used to provide data or information to the computer. The computer follows the instructions given to it by and input device. A variety of input devices are used with the computer depending on the type and purpose of input information. For example, a keyboard is commonly used to transfer data or information from human readable form to machine readable form. Other examples of input devices are: mouse, joystick, trackball, light pens, digitizers, scanners, optical character reader (OCR), touch window, etc.


The mouse is an input device that usually contains one or two buttons. As a user moves the mouse on a flat surface, the mouse controls the cursor movement on the screen. When the user presses one of the buttons, the mouse either marks a place on the screen or makes selection from data or menu on the screen. A mouse has a sphere on its underside. This rotates as the mouse is moved along a flat surface.
The mouse translate the direction and speed of rotation into a digital signals that identifies the position or control, the cursor on the computer.
A mouse can be used for many application, ranging from games to drawing and designing products with computer graphics. It provides an alternative for people who are uncomfortable with a keyboard but it also can be used in combination with a keyboard to input data.


A keyboard is the most commonly used input device which helps us in simply keying in required information in a computer. This information is subsequently stored in the computer’s memory. A keyboard can be used effectively to communicate with the computer but considered to be relatively slow as compared to other input devices. The keyboard is divided into following divisions:


These keys are similar to a standard typewriter and is used to type general information.


These keys are used to input numeric data only. These are very useful in case of large numeric data input because all numeric keys can be accessed by one hand only. These keys can also be used as an alternative to the screen navigation and editing keys.


These are keys marked as F1 - F12, located normally at the top of the keyboard. These are special keys provided to a programmer which allow him to attach special functions to each key. Each of these function keys are also given some special function in different packages.


These keys are provided to move around in the screen. May programs use these keys to let the user move around the screen display. In some keyboards these keys are also provided inside the numeric keypad as alternate keys.

Output Devices


An output device is used to display the data or information that we receive from the computer. An output device can be used to display or print the intermediate or final results performed by computer. A variety of output devices are used with computer. The use of these devices depends on the type and purpose of output. Some examples of output devices are: Monitors, Printer, Plotters, Visual display unit (VDU), liquid crystal display (LCD), etc.


To display result or output from computer, a T.V like device is used which is called monitor. The monitors are also referred as C.R.T (Cathode Ray Tube), V.D.U (Visual Display Unit). The monitor assist during input from the keyboard, this display is called a soft copy. The monitor can be of various kinds, depending on the type of application. Monitors are categorized into two groups:
· Monochrome monitors
· Colour monitors


Monochrome monitors are used specially for text editing purpose. These monitors can display only one colour. Normally amber, green or paper white.


Colour monitors serves a wide range of selection according to the application. Such as red, green, blue, Enhance Graphic Array (EGA), Colour Graphic Array (CGA), Video Graphics Array (VGA), Super Video Graphics Array (SVGA).
Hard Disk


Hard disk is a thin circular metal plate coated both side with a magnetic material. A hard disk pack consist of a number of disk mounted on central shaft which rotate at a speed of 2400 rpm or more. In a hard disk information is stored on both the surfaces of each disk plate except the upper and the lower surfaces of the bottom plate which are not used. Information is recorded on the track of the disk surfaces in the form of invisible tiny magnetic spot. The presence of a magnetic spot represent 1 bit and its absence represents 0 bit.
Hard disk are potentially very high capacity storage devices typically in the range of 20 megabyte to 1 gigabyte. Data are recorded on the tracks of a spinning disk surface and read from the surface by one or more read/write heads. There are two basic types of disk system.
1. Moving head
2. Fixed head


The moving head consist of one read/write head for each disk surface mounted on an axis and which can be moved in and out. In this system each read/write head moves horizontally across the surface of the disk. So that it is able to access each track individually. Information stored on the tracks which constitute a cylindrical shape through the disk pack are therefore accessed simultaneously.


In the fixed head system the axis are non-movable. A large number of read/write heads are distributed over the disk surfaces. One head for each track as a result no head movement is required and therefore information is accessed more quickly.
5 Programming and BASIC
Flowcharts, Programming, Variables and everything related to BASIC

Programming and BASIC
Topic No.
Flow Chart
Bits and Bytes
Library Functions
Computer Language
Program Development Process
Modes of Operation
Steps of Programming
Variables and Constants
Introduction to BASIC


While writing a program it is necessary to repeat a part of a program for a number of times. The best solution for this is to use a loop it is a method which executes a part of the program repeatedly as specified by the programmer.


a) Pre-Test Loop
b) Post-Test Loop


As its name indicates, checks the looping condition at the start of the loop if the condition is true the loop execution starts otherwise stops. This loop is used in a situation when part of program is to be repeated for a fixed number of time.
e.g., FOR …… NEXT.


This loop checks the looping condition at the end of loop and decides whether or not to continue the execution. This loop is used when a part of the program needs to be repeated until a specified condition is met.
e.g., WHILE …… WEND.
Flow Chart


Flow chart is a symbolic representation of flow of programme. It can graphically represent data processing procedure Flow chart serves two purposes, i.e., the planning of the program structure as an aid in writing the computer program. Secondly the documentation of the program logic and work flow for the purpose of the communication with other person and to recall a program at a later time. If the program needs to be changed. The second purpose is very important because it is very difficult to check the logic of the program.



The terminal symbol is used to indicate the beginning, ending of the program logic flow. It is the first and last symbol of the flow chart.


The input/output box is used to denote any function of an input and output in the program. If there is a program instruction to input or output the data from any I/O device that step will be indicated on this symbol.


A processing symbol is used in a flow chart to represent arithmetical and data movement instructions.


Flow lines are used to indicate the flow of operations. The exact sequence in which the instructions are to be executed. The flow of flow chart is normally from top to bottom and left to right.


The decision box is used in a flow chart to indicate a point at which a decision has to be made and to branch to one or more alternative points that is possible during execution. The appropriate path followed, depends on the result of the decision.


A flow chart becomes complex, when the number and direction of flow lines is confusing or it spread over more than one page. It is useful to utilize the connector symbol as a substitute for flow lines.


System Flow Chart is used to describe complete data processing cycle including the hardware devices and media used. A System Flow Chart shows flow of data in a system where and in what form it is received by the system, how it is input to the computer and what storage media are used to hold it. The symbol used in system flow chart indicates the peripherals to be used in data processing procedure, where output will be produced. System flow chart do not explain the logic of the programs.


Errors are the mistakes made by the programmer during program writing. A program relays run perfectly for the first time and each program needs to be tested to ensure that it is correct and contains no errors. There are three types of errors generally encountered.
a) Syntax Error
b) Logical Error
c) Run Time Error


The syntax of a programming language is the set of rules to be followed when writing a program in that language. These rules are similar to the grammar rules of Urdu or English language. When a program violates these rules, computer generates a syntax error. Syntax must be correct before program execution.


These errors are the most difficult ones to find, if a programmer enters a sign of multiply instead of addition at the time of making the program, the answer will not be correct because interpreter will not be able the detect these types of errors. A logical error will not stop the execution of the program but the result will not be accurate.


These types of errors stop the execution of the program. It may be due to entering the invalid data at the time of executing a program. For example, if a program is expecting the user to enter the numeric data and the user enters a string data, computer will display an error message.
Bits and Bytes


The smallest piece of data that can be recognized and used by a computer, is the bit, a binary digit. A bit is a single binary value, either a 1 or 0. A group of eight bits is called a byte. The byte is the basic unit for measure of the size of the memory, with today’s computer memory sizes. It is more common to hear the term kilobyte (KB) or megabyte (MB).


A computer word is the number of adjacent bits that can be stored and manipulated as a unit. Just an English vocabulary words are of varying lengths so are computer words. Many microcomputers have the ability to manipulate a 32 bit word, while some models have words length of 8 and 16 bits.
Library Functions


GW-BASIC provides many built in functions which are called binary functions. Each function is composed of a format or a short program, that requires at least one parameter to be passed. A parameter is a value which is in that formula or program and this formula or program returns a result. This result can be stored in any appropriate variable or directly displayed on monitor. A library function which has “$” sign is called string library function while others are called numeric library function. A string function returns string value while a numeric function returns numeric value. Following are some library functions.


An expression is a combination of constants and variables linked by arithmetic operators like (+, -, *, /, \). Expressions are used to perform different operations. The expressions are evaluated from left to right but some operators have priority over the others. Parenthesis are evaluated first then multiplication and division have the same priority from left to right. Similarly the addition and subtraction are evaluated with equal priorities. If all the operators are used in an expression have the same priority then the expression be executed from left to right.
The expressions can be of three types.
· Arithmetic expressions
· Logical expressions
· Relational expressions.


In an arithmetic expression the following operators are used in conjunction with the operands.
+ Addition
- Subtraction
* Multiplication
/ Division
^ Exponentiation
( Left parenthesis
) Right parenthesis


A Relational expression is composed of operands linked by the relational operators. The relational operators used in the relational expression are given.
= equal to
> greater than
<> not equal to
>= greater than or equal to
<= less than or equal to
A > B
B <> C


When a selections is based upon one or more condition being true. It is possible to combine the condition together using logical operators and the resulting condition would either be true or false the most commonly used logical operators are AND, OR and NOT.
Computer Language


Language is a system for representation and communication of information or data. Like human beings, a language or signal is required to communicate between two persons. Similarly, we cannot obtain any result by computer without langtage. Computer does not understand directly what we are communicating with computer as English or Arabic, it understands only machine language (binary codes 0-1). Computer translates English language into machine codes through interpreter then process instructions and give us the results.
The computer languages can be divided into two main levels.
· Machine language (0-1)
· Symbolic language (A-Z)
Symbolic languages are further divide into two main levels
· High-level language
· Low-level language


Although computers can be programmed to understand many different computer language. There is only one language understood by the computer without using a translation program. This language is called the machine language or the machine codes. Machine codes are the fundamental language of the computer and is normally written as strings of binary 0-1.


Programs written in machine language can be executed very fast by the computer. This is mainly because machine instructions are directly understood by the CPU and no translation of program is required.
However, writing a program in machine language has several disadvantage.


Because the internal design of every type of computer is different from every other type of computer and needs different electrical signals to operate. The machine language also is different from computer to computer.


Although easily used by the computer, machine language is difficult to program. It is necessary for the programmer either to memorize the dozens of code number for the commands in the machine’s instruction set or to constantly refer to a reference card.


It is difficult to correct or modify machine language programs. Checking machine instructions to locate errors is difficult as writing them initially.
In short, writing a program in machine language is so difficult and time consuming.


In symbolic languages, alphabets are used (a-z). symbolic languages are further divide into two main levels.
· High level languages
· Low level languages


A language which is one step higher than machine language in human readability is called Assembly Language or a low-level language. In an assembly language binary numbers are replaced by human readable symbols called mnemonics. Thus a low-level language is better in understanding than a machine language for humans and almost has the same efficiency as machine language for computer operation. An assembly language is a combination of mnemonic, operation codes and symbolic codes for addresses. Each computer uses and has a mnemonic code for each instruction, which may vary from computer to computer. Some of the commonly used codes are given in the following table.
Subtract SUB
Multiply MUL
Compare Registry CR
Compare COMP
Branch Condition BC
Code Register LR
Move Characters MVE
Store Characters STC
Store Accumulator STA
An assembly language is very efficient but it is difficult to work with and it requires good skills for programming. A program written in an assembly language is translated into a machine language before execution. A computer program which translates any assembly language into its equivalent machine code is known as an assembler.


A language is one step higher than low-level languages in human readability is called high-level language. High – level languages are easy to understand. They are also called English oriented languages in which instruction are given using words. Such as add, subtract, input, print, etc. high level language are very easy for programming, programmer prefer them for software designing that’s why these languages are also called user’s friendly languages. Every high level language must be converted into machine language before execution, therefore every high level language has its own separate translating program called compiler or interpreter. That’s why some time these languages are called compiler langauges. COBOL, BASIC, PASCAL, RPG, FORTRAN are some high level languages.


An interpreter is a set of programs which translates the high-level language into machine acceptable form. The interpreters are slow in speed as compared to compilers. The interpreter takes a single line of the source code, translates that line into object code and carries it out immediately. The process is repeated line by line until the whole program has been translated and run. If the program loops back to earlier statements, they will be translated afresh each time round. This means that both the source program and the interpreter must remain in the main memory together which may limit the space available for data. Perhaps the biggest drawback of an interpreter is the time it takes to translate and run a program including all the repetition which can be involved.
Program Development Process


In order to develop a computer program, a programmer has to go through the following stages:


In this step a programmer studies the problem and decides how the problem will be best solved. Studying a problem is necessary because it helps a programmer to decide about:
· The facts and figures to be collected.
· The way in which the program will be designed.
· The language in which the program will be most suitable.
· What is the desired output and in which form it is needed, etc.


An algorithm is a set of instructions or sequence of steps that must be carried out before a programmer starts preparing his program. The programmer designs an algorithm to help visualize possible alternatives in a program.


A flow chart is a graphical representation of a program which helps a programmer to decide on various data processing procedures with the help of labeled geometrical diagrams. A flow chart is mainly used to describe the complete data processing system including the hardware devices and media used. It is very necessary for a programmer to know about the available devices before developing a program.


The next job after analysing the problem is to write the program in a high-level language, usually called coding. This is achieved by translating the flow chart in an appropriate high-level language, of course according to the syntax rules of the language.


The process of execution of any program to find out for errors or bugs (mistakes) is called test execution. This is very important because it helps a programmer to check the logic of the program and to ensure that the program is error-free and workable.


It is a term which is used extensively in programming. Debugging is the process of detecting, locating and correcting the bugs by running the programs again and again.


It is written information about any computer software. Final document guides the user about how to use the program in the most efficient way.
Modes of Operation


There are two modes of operation for BASIC. The mode that you are in determines what BASIC will do with the instruction you give it. When you start BASIC you receive the OK prompt. You then have two modes available to you immediately.


In the direct mode BASIC acts like a calculator. No line numbers are required. Direct mode is not of course the main purpose of BASIC, but it is useful at times particularly when you are debugging program or short problems in which you want to perform quick calculation e.g., PRINT 3+4.


In this mode you first put a line number on each statement. Once you have a program you can run it and get your results. The indirect mode saves your instructions in the computer along with their line number, you can execute the program as many times as you wish simply by typing RUN.
Steps of Programming


There are five steps in preparing a computer programme which are also called ABCDE of Programming.


In this step the system analyst tries to become familiar with the problem. He has to study the problem and prepare some notes upon that problem. He also notes that what is given, what is required and what will computer can do.


In this step the programmer converts the analyst report to a series of steps through which the computer will give the required result. The steps are commonly known as Algorithm. There are different ways to write those detailed sequential steps. The most common method used is flow charting. A flow chart is symbolic representation of flow of a programme.


In this step the programmer writes the programme in any computer language. This step is known as coding. After this programme is fed into the computer and is compiled with the help of a given compiler.


Debugging is a step in which a programmer corrects a syntax error which may come after the compilation.


Testing is a step where the programmer is finally testing the programme for execution (there may be any logical mistake which compiler cannot trace).


In this step we send the programme for execution where company’s data will be fed and process.


Since the computer hardware is capable of understanding only machine level instructions so it is necessary to convert the instruction of a programme written in high level language to machine instruction before the programme can be executed by the computer. This job is carried out by compiler.
A compiler is translating programme that translates the instruction of a high level language into machine language. A high level programme is called a source programme. After the source programme has to be converted into machine language by a compiler. It is referred to as an object programme.
A compiler can translate only those programme which has been written in the language for which the computer is meant e.g., FORTRAN compiler is only capable of translating source programme which have been written in FORTRAN. Each machine requires a separate compiler for each high level language.
The compiler analysis each statement in the source programme and generate a sequence of machine instruction. As the compiler analysis each statement it uncovers certain types of errors. Compiler can diagnose the following kinds of errors in a source programme.
1. Illegal characters.
2. Improper sequence of instruction in a programme.
A source programme containing an error diagnosed by compiler will not be compiled into an object programme. The compiler will print out (display) a suitable message.
Variables and Constants


Variables are names used to represent values used in BASIC programme. The values of a variable may be assigned by the programmer or it may be assigned as the result of calculation in the programme. Before a variable is assigned a value is assumed to be zero (or null for string variable).


Variable names may be of any length up 40 characters are significant. Variable name can contain letters, numbers and decimal point. However, the first character must be a letter. A variable should not be a reserve word.
Variable may represent either a numeric value or a string.


Numeric variable names may be declared as integer, single precision, double precision values. The type of declaration characters for these variable names are as follows.


In programming we often use to handle alphanumeric or string variables. String variables consist of names, addresses or in general any string or series of letters, number or special characters.
Without the use of string variable whenever any differentiation needs to be made e.g., from employee to employee or from application no. to application no., a number has to be assign to each employee or each application.
A string variable name starts with an alphabet and terminate with a dollar sign ($). In a string the value must be in quotation marks. No arithmetic operations except addition and in addition the values do not add they are just written together. The maximum number of character allowed in a string varies from system to system but a typical value is 256.


Constants are those that do not change while executing the programme. BASIC deals with two types of constant.
1. Numeric constant
2. String constant


All numbers belong to this category because their values remain constant during execution.
They may take any of the following forms:
1. Commas are not allowed in a numeric constant.
2. The use of the positive sign is optional but negative sign should be used if the number is negative.


A string constant is any set of valid basic characters enclosed in quotation marks. The quotation marks do not form part of string. Strings are used to represent non-numeric information such:
1. Name of persons and places
2. Addresses
3. Days, Months, Years.
Introduction to BASIC
BASIC is a high level language used for purpose of writing a program on a computer. It stands for Beginners All-purpose Symbolic Instruction Code. BASIC is an easy to use “friendly” language where instruction resembles elementary, algebraic formulas and certain English keywords such Let, Read, Print, Goto etc.


The language was developed at Dartmouth College in 1967, under the direction of John Kemeny and Thomas Kurtz. It was quickly discovered and adopted. All the major computer manufacturers offered their own version of BASIC for their particular computers.
In 1978, the American National Standard Institute standardized an essential subset of BASIC in order to promote uniformity from one version of BASIC into another.
In recent years some new version of BASIC have been developed which included a variety of features that are not included in more traditional versions.


Each instruction in a BASIC program is written as a separate statement. Thus a program will be composed of a sequence of statements.
The following rules apply to all BASIC statements:
1. Every statement must appear on a separate line number.
2. Each statement number must be followed by a BASIC keyword which indicates the type of instruction.
3. On one single line number we cannot write two statements, if we need we must use “colon”.
4. We can write 255 characters on one line.

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