Computer Studies Notes 6271i

www.studyguide.pk The International School Seychelles

IGCSE Computer Studies CIE 0420

Theory Notes Contents 1

Applications Of Computers, Their Social & Economic Implications................................ 1 1.1 The Range And Scope Of Computer Applications ............................................................................. 1 1.2 The Social And Economic Implications Of The Use Of Computers....................................................... 3

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System Analysis ......................................................... 6 2.1 Systems Analysis ................................................................... 6

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Problem Solution, Algorithm Design & Programming Concepts............................. 8 3.1 Making An Overall Plan .......................................................... 8 3.2 Programming ........................................................................ 10

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Generic Software And The Organisation Of Data ................................................ 15 4.1 Generic Software .................................................................. 15 4.2 Data ........................................................................................ 18

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Hardware, Systems And Communications....................................................... 22 5.1 Hardware ............................................................................... 22 5.2 Systems And Communications ........................................... 26 5.3 Types Of System................................................................... 28

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IGCSE Computer Studies

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1 Applications Of Computers, Their Social & Economic Implications 1.1 The Range And Scope Of Computer Applications General Application Areas The range of computer applications is vast. You should have an awareness of as many applications as possible, and for each, try to know the following: 1 The purpose of the application. 2 The required outcome. 3 The overall system design, including both the computerised and the noncomputerised parts of the application. 4 The necessary inputs to the system and the means by which any data is captured. 5 The overall organisation and processing of the data within the system. 6 The use and organisation of the major software and hardware components of the system. 7 The need for recovery in the event of a system failure. 8 The interface between the system and its s. 9 The effectiveness of the system in practice. 10 The effects of the application on individuals and organisations.

Communication And Information Systems Data is exchanged between computers. The data can be in the form of text, images, sound or video. Electronic mail (e-mail) Electronic conferencing

simple message exchange. Can attach other files can either be a text, voice or video conference

On-Line Services & Remote Databases Computers are often placed in locations where the public can use them to lookup information. Information retrieval Library systems Multimedia systems

online databases, encyclopaedias, dictionaries, bus timetables, etc. documents are stored digitally where they can be searched stored information contains text, sound, video, etc.

Commercial And General Data Processing These are often batch-processing systems involving data being collected over a period of time, and then processed later. The collected data is stored in a transaction file, and this is used to update the master file to give a new master file. Banking systems Personnel records Stock control Order processing

bank s are updated based on daily transactions employee's pay is calculated based on hours worked when items are sent / received, the stock is updated received orders are dealt with

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Industrial, Technical And Scientific Uses Data is entered into the computer and the computer can then manipulate the data. In the case of CAD systems, products can be designed and tested completely within the computer's memory. In forecasting systems, predictions are made about the future based on previous data, and a model of how the system works. Weather Forecasting Computer-aided design

try to predict weather, taking data from sensors, using computer models of weather systems (CAD) designs for objects can be designed, altered and tested within the computer, prior to manufacture

Monitoring And Control Systems A computer is used to monitor a system through the use of sensors (such as light, heat, etc.) The computer can then operate devices to control the system (such as pumps, valves, etc.) Monitoring hospital patients checks heart rate, etc. setting off an alarm if low Nuclear power station control monitors temperature, etc. and adjusts coolant Traffic survey and control checks the number of cars, alters traffic light timings

Automation And Robotics The use of computers to control other mechanical devices. Usually requires some sort of interface which allows the computer to receive data from input sensors, and to control output devices such as motors. Domestic automation Automatic navigation Industrial robots

washing machines, microwaves aircraft, ships, cars (GSM satellite navigation) used to work on manufacturing lines. Can work non-stop, 24 hours a day, 365 days a year. Can work in hazardous areas

Expert Systems And Artificial Intelligence Artificial Intelligence (AI) is the attempt to simulate the human brain, and its thought processes, using computer hardware and software. Expert systems use AI techniques to replace a human expert. All of the human's knowledge on a subject is entered into the computer as a series of rules. Medical diagnosis

Speech recognition

data is fed into the system, and questions are answered. The expert system rules then come up with the best diagnosis (usually with a % confidence level) checks voice patterns to determine what was spoken

Miscellaneous Areas Some other applications of computers are: Computer-aided learning (CAL) CD-ROMs, etc. where you can learn at your own pace, like having your own personal tutor Computer animation for films and TV. Special effects, etc.


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1.2 The Social And Economic Implications Of The Use Of Computers Social And Economic Effects Effects on people, organisations and on society in general. Redundancies De-skilling Electronic 'scabbing' Tactical striking 'New tech,' agreements

jobs lost when staff replaced by computer-based systems replacement of skilled staff by computers. Staff are then left to do less skilled jobs if staff are striking, work can easily switched to non-striking staff via a network, even in a different country unions maximise impact of strikes by selecting computing staff first. Whole company is then affected benefits to workers (cleaner, safer workplace) and management (more cost-effective) of using computerbased systems

Economic Reasons For The Use Of Computers There are many financial reasons for using computers, and computer controlled systems, even though they are expensive to set-up initially. More efficient tasks completed quicker, and with lest wastage Work longer than people automated systems need no rest, can work for 24 hours a day Save on wages computers can often do the work of several people, so people are made redundant

Changes To Existing Methods, Products And Services Businesses change the way they work, and provide different, and better services to clients. On-line banking E-commerce EPOS

EFT

more convenient for customer, cheaper for banks, less real staff required to deal with cash selling goods online means less overheads, so better prices for customers, and more profit for company (Electronic Point of Sale) tills in shops where the good purchased are automatically taken from the database of stock. Can automatically generate orders for new stock (Electronic Funds Transfer) allows people to pay for goods using a card which is 'swiped' in the store, authorising the transfer of money from the customer's bank

Development Of New Products And Services Computers have lead to the development of new markets and businesses. • • •

Internet service providers Web-design services Internet cafes


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Changes In The Working Environment Using computers within businesses has altered the environment that we work in Cleaner and safer Work injuries can go up

dangerous / messy jobs done by computer systems due to prolonged computer use – RSI, back ache, etc.

Changes In Employment The use of computers in the workplace has an impact on the way people work Retraining of staff Individual training

software packages upgraded, staff need to be trained. This is often a regular thing. However… training can be personalised to staff's needs through the use of CAL systems, CD-ROMs, etc.

Privacy And Integrity Of Data So much personal data is stored within computer systems that companies and governments have to have guidelines and laws to protect the privacy of people and their information.

Data Protection Legislation The Data Protection Act gives the following requirements to anyone who stores data about someone else: • • • • • • •

Person must give permission for data to be stored Data must not be used for purposes other than those it was given for Must not store more data than is necessary for the purpose Data must be kept up-to-date Data should not be kept for longer than is necessary Data must be protected against unlawful access, or accidental loss Data must not be transferred outside of EU unless the country also has data laws

Security And Reliability Data must be protected. This involves a number of procedures. Back-ups of critical data

Batch-Processing

Archiving of old data

should be performed on a regular basis. A full copy of all files is taken (can use CD-R, magnetic tape, etc.) Usual technique is to keep 3 generations of back-ups (today's, yesterday's and the day before's). This is the grandfather, father, son system. Backup of both transaction files and master files required. If today's master file is lost, we can re-build it from yesterday's backed-up master file and transaction file. to reduce the amount of system resources required (disk space, etc.) and keep the system running smoothly, old data that is no longer used (but may be required for future reference) is moved into an archive file.


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Consequences Of System Failure The requirements for security and reliability vary considerably depending on the nature of the application. For example, a failure during a batch update of a sequential master file is irritating and will cause delay, whereas a failure in an air traffic control system could well have catastrophic results. Safety-critical systems (such as air-traffic control, or medical systems) have much more emphasis (and money) placed on reliability.

Hacking And Other Computer Crime Computer crime includes activities such as the cracking of ineffective security systems to gain unauthorised access to commercially sensitive or confidential personal files, and fraud through the improper transfer of funds from one to another. Computer criminals may work within the organisation or may be outsiders. Precautions can be taken. Physical security Complex security codes Firewall soft/hardware Encryption of data Monitoring of all access

only allow authorised s near the computers use of good s (not just 'Fred' or '') prevents access from outside the network can only be read by someone with the trace s who are accessing and misusing system

Computer viruses Sensible precautions should be taken. • • •

Up-to-date virus protection Limited use of disks, etc from outside your network Firewall software / hardware – prevents viruses using your Internet connection


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2 System Analysis 2.1 Systems Analysis Systems Analysis describes the process followed when replacing or upgrading a system with a computer-based one. The steps followed are: • • • • • • • •

Fact-finding Feasibility study Analysis System design Implementation Testing Documentation Evaluation

Identification Of The Problem (Fact-Finding) What exactly is/are the problem(s) that needs to be solved? How do we find out as much as possible about the present system? We need to do some fact-finding: • • • •

Interviewing people who work for the company Questionnaires Observation of people at work within the company Inspecting documents that are used within the present system

Deciding And Stating Specific Desired Outcomes (Feasibility Study) Before continuing with solving the problem identified, it is important that it will be worthwhile. The feasibility Study, and the resulting Feasibility Report, will give an idea of: • • • • • • • • •

A brief description of the business and the sections which will be affected A description of what the system is required to do (objectives) Some early system designs to allow estimations of time and cost Details of why some designs were chosen, and some rejected Approximately how long the project will take Approximately how much it will cost Cost / benefit analysis detailing whether the solution will save money or not Plan of further action Conclusions as to whether to continue, and if so, how

Based on the contents of the study and report, the company will decide if they are going to proceed with the project.

Analysing The Flow Of Information And Data In Existing Solutions and Evaluation Of Existing Solutions If the project is to continue, then a much more detailed look at the business and its systems is required. Data will be collected and analysed thoroughly to aid in the design of a replacement system. In particular this stage will provide: • •

Charts and diagrams of the present system, and the proposed new system (structure diagrams, data-flow diagrams, etc.) Detailed objectives for the new system


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IGCSE Computer Studies • • • •

Syllabus 0420

Details of exactly which parts of the old system will be replaced Constraints of the new system (things it will not be able to do) An updated cost / benefit analysis A new plan for further action (including timing, responsibilities, etc.)

Consideration Of Alternative Solutions Designs will need to be produced including: • • • • •

Required outputs (documents, reports, etc.) Required inputs (data, method of entry, validation, etc.) File / database design (number of tables, links between them, etc.) Hardware configuration (computers, networks, printers, etc.) Software required (operating systems, application packages, custom written, etc.)

A solution will need to be selected, based on the objectives that were set: Which solution fulfils all of the objectives?

Implementation This can be done in one of three ways: Direct implementation Phased implementation

Parallel running

the new system simply replaces the old one. Only suitable for small systems parts of the old system are slowly replaced by the new one, taking time to make sure each bit works before the next is begun the new system runs alongside the old one to check it works as well. When all problems have been solved, the old one stops.

Testing We must ensure that any new system functions as expected, both in normal use, and when potential problems arise. We test the system with: Normal data Data with errors Large amounts of data Rare / uncommon events Extreme data

to see if correct, expected results are achieved to check that our validation and error-checking routines to ensure that the system can cope with high loading like yearly reports, just to see that they work as expected to make sure that validation works, and calculations cope with the large / small values

Documentation There are two main type of document that are required: documentation explaining how to use the new system (a guide) Technical documentation to allow people to alter / upgrade the system in the future

Evaluation Finally, the new system has to be assessed. Did it meet its objectives? Ask the new s! The International School Seychelles

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3 Problem Solution, Algorithm Design & Programming Concepts 3.1 Making An Overall Plan Defining The Scope Of Separate Modules Software problems should always be broken down into small modules of code (subroutines). These modules should be small and as self-sufficient as possible as: The module is easy to produce, not being dependent upon other modules The module is easy to test for the same reason The code is re-usable - the same module can be used in another project

• • •

e.g. Software for teaching children about shapes could be broken down into the following modules: One that deals with drawing shapes (further broken down into shape modules) One that deals with asking questions about shapes, and uses the drawing modules One that saves and loads the student's progress to / from disk

• • •

Each module has a clear purpose and it's dependency on other modules is clear. If the modules are well defined, then the overall problem becomes easier to solve.

Deg Algorithms When writing algorithms to solve software problem, you must always bear in mind that your solution must satisfy the requirements of the system. If the software is required to calculate the average scores of 1000 students, then that is what your algorithms must do.

Explaining Algorithms And How They Relate To The System The design of any algorithm should be driven by: Inputs supplied Outputs required

what data will be ed to the algorithm to work with? what data is expected from the algorithm?

The inputs and outputs will be defined by the algorithm's location within the overall system, and it's connections to other modules. e.g. If an algorithm is required to draw a square on the screen, then we need to define how it will be used. We want to use it several times for different sized squares, in different places on the screen, so we'll need to give it some sizes and a location: • •

Inputs: Outputs:

side length l, and position of top-left corner (x, y) a square! Sides l, located at (x, y)

This algorithm can now be easily used to draw any square, anywhere, as long as we supply it with the correct data. The actual algorithm produced is almost irrelevant as long at it produces the desired results, from the inputs supplied (there are issues of speed, good programming practice, etc. to be considered though)


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Explaining How Hardware Needs Arise From The Output Required Any software system will need some hardware to run on. The configuration of the hardware is dependent upon: Outputs required Processing needs

Environment Space available Costs

items like printers, monitors, loudspeakers, etc. will be required depending on the output of the system a modern computer will have a processor running at about 3GHz. That's a lot of processing power! If less speed is required, money can be saved Certain items of hardware are suited to certain workplaces (e.g. extra-rugged keyboards for factories) some hardware (e.g. flat-screen monitors) is very space efficient hardware costs very greatly. A CRT monitor is much cheaper than a flat-screen one.

Algorithm Tools Top-down design

Structure diagrams Flowcharts Menus

Libraries of subroutines

a software problem is solved by breaking it down into smaller and smaller modules, until each of these is easy to solve, and write software for. The final solution is formed by ing the modules together. Usually represented using a structure diagram. (see Section 3.2 below) (see Section 3.2 below) these are an effective way of breaking up a software task. The has menu options, each of which is handled by a separate bit of the software (like the top-down structure) Many common software problems have been solved already, and you can buy in libraries of software modules that can be 'glued' together to form an application

Interpreting And Testing Algorithms You should be able to work out the purpose of an algorithm and to suggest and apply suitable test data. Valid data

Extreme data

Abnormal data Invalid data

data that tests the normal operation of the algorithm (e.g. if entering test scores, your test data could include 0%, 20%, 56%, 89%, 100%) including values that are at the limits of the expected input (e.g. 0% or 100% for exam result, or a text string that is the data that may be valid, but is not usual (e.g. all test scores 0%, or all 100%, or no test scores at all) this includes data that is not of the correct type or format (e.g. text into a number field)


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3.2 Programming The Concept Of A Program A computer program is a collection of commands that the computer is to follow. The main requirements of a programming language are: • • • •

To allow manipulation of data of various types and structures The control of inputs and outputs To provide for selection and repetition (see below) To allow links between subprograms

Sequence i.e.

Selection i.e.

Following one command by another by another, etc. Instruction 1 Instruction 2 Instruction 3 etc…

The computer is given a question as then performs different tasks based on the answer Has something happened? Yes Instruction 1 Instruction 2 etc…

Repetition

the computer executes each instruction in order

No Instruction 1 Instruction 2 etc…

the instructions that are executed depend answer to the question

Following the same group of commands over and over again (loop) The loop may go on forever (infinite loop) or may stop due to a certain condition (conditional loop)

i.e. Instruction 1 Instruction 2 …

an infinite loop

Instruction 1 Instruction 2 … Stop now? No

a conditional loop

Low-Level Languages Low level languages are close to the actual language that computers use (machine code) • • •

Hard to program in Most common is assembly language An assembler converts assembly language into machine code


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High-Level Languages These are languages that are closer to human language than computer. • • • • •

There are many different types, some dedicated to specific applications (e.g. Fortran is used for mathematics, Cobol is used for business databases) Much easier to use than low level languages Very easy to create the programming structures such as loops Lots of easy-to-use tools to help debug programs Require either an interpreter or a compiler to convert language into machine code

Assemblers Convert low-level languages into machine code. This is called assembly.

Interpreters Convert high-level languages into machine code • • •

Convert one instruction at a time, executing it immediately To run programs that use an interpreter, requires the to have an interpreter themselves The final program runs more slowly than a compiled program

Compilers Convert high-level languages into machine code • • • •

Convert the whole program in one go The program cannot be run until it is fully compiled Once a program is compiled, the does not need a compiler to run it (the program is 'stand-alone' The resulting programs run very quickly compared to interpreted programs

Structure Diagrams These diagrams help us design software, by letting us break the task down. • • •

The programming problem is broken down into a number of sub-tasks These sub-tasks can also be broken down into smaller tasks And so on…

The smaller tasks are then simple to solve, and when they are all ed together, we should end up with a solution to the overall problem. Main Problem

Sub-Task A

A1

A2

Sub-Task B

A3


B1

B2

Sub-Task C

C1

C2

C3

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Variables Within any computer program, data needs to be stored whilst it is being processed. We store it in special locations in memory called variables (their contents can change, so they are 'variable'!) • • • •

Variables have names – pick names based on what you want to store (e.g. 'score') They can store different types of data: numbers, text ('strings'), etc. You can use variables within calculations (e.g. 'total = score1 + score 2') The content of a variable can be set (e.g. 'Set score to 0', or 'score = 0')

Flowcharts These diagrams show the flow of our program (the order that instructions are executed) •

The start and end of the program is shown as a 'sausage'

•

Any tasks that are just simply executed are shown in rectangles

•

Inputs and outputs are shown in parallelograms

•

Places where a decision is made (selection) are shown using diamonds

•

•

•

•

Places where files are saved to or read from disk are shown using cylinders When files are master files or transaction files, they are shown as reels of tape Places where print-outs are generated are shown as a piece of paper Arrows the symbols together

Pseudocode

Start Instruction

Start

Output "Enter score"

In/Out

Question

File

File

Input Score Add Score to Total

Was Score = 0?

Print-out Output Total Start

This is a way of describing an algorithm (or program) using words. It is an alternative to a flowchart, and is often easier to write, and more understandable, since it is close to English. • • • • •

Words are close to English Uses some standard words for program structures such as loops (see below) Indentation is used to highlight the structure of the code Inputs and outputs are shown using the words Input ( data ) and Output ( data ) Mathematical operations can be written in words (e.g. 'Add 6 to score')


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Pseudocode Structures There are several standard pseudocode structures that are commonly used: Input ( variable )

Gets data from the and stores it in the variable

Output ( variable / "text" )

Prints the contents of the variable, or the text on the screen for the to see

Repeat ... Until

Repeats the instructions between the 'Repeat' and 'Until' lines until a condition is met e.g. Repeat Output ( "Please enter a score:" ) Input ( score ) Until score equals 0

If ... Then ... Else ... Endif

Allows for selection or decision-making. A condition is tested and if it is true then one set of commands is executed, else if it is false another set is executed. e.g. If Score is greater than 60% Then Output "You have ed. Well done!" Else Ouput "You have failed. Sorry!" Endif

Case of ... Otherwise ... Endcase If you want to have different things happening depending on the value of a variable. e.g. Case of score is greater than 70 Output ( "You have an A" ) score is greater than 60 Ouptut ( "You have a B" ) score is greater than 50 Output ( "You have a C" ) Otherwise Output ( "You have failed" ) Endcase While ... Do ... Endwhile

This is another form of loop (like Repeat…Until) except that this one keep going whilst a condition is true, and the condition is checked before the loop starts e.g. Set count to 10 While count is greater than 0 Do Output ( count ) Take 1 away from count Endwhile Output ( "Blast off!!!" )


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Documentation Any piece of software requires some kind of documentation. • • •

Instructions on how to set-up and install the software Instructions on using the software ( Guide) Troubleshooting tips (for when things don't work as expected)

Technical Documentation This is required by those responsible for improving and maintaining a software solution in working order, or for developing the solution to meet new needs. • • • • •

A guide showing the structure of the software (using structure diagrams for example) What inputs are expected What outputs are generated What algorithms are used to process data, etc. (using pseudocode or flowcharts) Any actual code should contain comments. These are notes explaining what a particular set of instructions is supposed to do.


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4 Generic Software And The Organisation Of Data 4.1 Generic Software Word-processing (e.g. Word) • • • •

Letters, etc. can be typed Full control over font, font size, styles, etc. Most WP applications also include some graphics and DTP functions Has resulted in many people doing their own typing, making office staff redundant

Database management (e.g. Access) • • • •

From simple databases similar to card files (flat databases) through to complex databases, with many groups of data linked together (relational databases) A database consists of a collection of records which in turn are made up of fields Database can be interrogated using queries Can easily generate reports containing selected data

Spreadsheets (e.g. Excel) • • • • • • • •

Made up of individual cells which can contain data (number, text, etc.) or formulae Allows complex and powerful calculations on collections of data Can experiment with data values and see the results ('what-if' analysis) A mathematical 'model' of a situation can be produced (e.g. company future profit) Rows are numbered, columns are lettered, cells by letter/number (e.g. A3) Formulae all begin with '=' Many built-in functions such as SUM, AVERAGE, etc. Numbers can be formatted as currency, percentage, etc. (e.g. a cell containing 0.5 could be shown as $0.50, 0.5000, 5e-1, 50%)

Graphics (e.g. Photoshop) • • • •

Images can be produced from scratch, or photos manipulated Effects can be added (e.g. blurring, colour alterations, etc.) Input can be from a graphics tablet for more natural lines, etc. Images can be exported into many file formats (e.g. GIF, JPEG, TIFF, etc.)

Communications (e.g. E-Mail, Internet) • • • •

Very quick delivery of messages Used both across the Internet, and within businesses (internal e-mail) Normal e-mail is text-based, but other files can be 'attached' People now communicate more regularly since it is so easy, but are possibly less sociable since they now use the phone / meet less often (less human )

Multimedia Authoring • •

The production of CD-ROM / websites that include text, images, video, sound, etc. Common applications are education, CAL, reference (encyclopaedia, etc.)


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Data-logging • • •

A computer uses sensors to detect events, etc. Examples could be measuring traffic flow, science experiments, etc. Data is more accurate than that taken by a person, and can be analysed instantly (graphs, etc.)

CAD • • • • •

Products are designed on the screen rather than on paper Much faster, and far more flexible than drafting designs by hand The products sizes (dimensions) can be altered easily (no rubbing out!) The product can be viewed from different angles with lighting effects, etc. Some CAD systems allow products to be tested using modelling techniques, so that weaknesses can be found before anything is actually physically manufactured

Programming • •

Software can be produced using programming languages Many modern programming languages are now so easy to use that applications can be produced in minutes

Desktop Publishing (e.g. Publisher) • • • • • •

Allows the to layout text, images, etc. on the page with total flexibility Items are placed in 'frames' which can be layered, moved, re-shaped, etc. Authors can produce books that are fully laid out without the need for publishers Anyone can easily make flyers, posters, etc. Used to produce printed matter from labels to magazines Has made publishers almost redundant as designers can now do every stage of the work, ing on the final files directly to the printers

Typical Problems Capable Of Solution By Packages Many problems can be solved using generic software packages: Small businesses

Schools / charities

s can be calculated using a spreadsheet, letters written using a word-processor, stock details kept using a database don’t need to employ expensive consultants to produce professional looking reports, posters, flyers, etc.

Use Of Standard Techniques Or Routines Cut / copy / paste Mail-merge

Sorting of data Import / export of data

data can be easily moved between applications a standard letter can be sent to a number of people by merging a database of names and address with the letter template. Special codes in the template are replaced each time by the appropriate name / address a list of items (e.g. names) can easily be sorted into order Applications can save data in a format that is suitable for another application (export), or can open files that they wouldn't normally use (import)


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IGCSE Computer Studies Spell / grammar check Wizards

Macros

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within many applications, the computer will automatically check and correct your spelling and grammar Many complex tasks are made extremely easy for novices through the use of wizards. The is asked a series of questions and their answers result in an appropriate complex set of actions (e.g. producing a poster) When a number of operations need to be repeated over and over again, they can be 'recorded' and stored in a macro. This macro can be 'replayed' any number of times

Integrated Packages Versus Custom-Written Software Integrated packages are completely general-purpose, so they are: • • • •

Cheap to buy (many copies will be sold, so the unit cost is low) Well documented (since many people will use them – lots of training courses) Stable (mostly bug-free) because of the large -base Not always suitable for every business's needs, unlike custom-written software

Some business's needs cannot be satisfied by generic software, and custom-written applications are necessary. These are: • • • •

Very expensive since they are usually one-offs Perfectly suited to the business's needs Easy to learn since they can be exactly matched to the skills of the workforce Sometime quite unstable (buggy) since they have only limited testing


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4.2 Data The Relationship Between Information And Data Information is data that has been processed. e.g. 234567 is simply data (a number) but it could mean many things – a hip number, a mathematical result, etc. Once it is given this meaning, it becomes information.

The Collection Of Data There are many methods for collecting data for computer systems: Questionnaires OMR forms Bar-codes Scanners OCR of documents

Sensors

These can be closely matched to the on-screen computer form so the data from them is easily transferred, or… where marks are made with a pencil (e.g. multiple choice test) and read into a computer by a special reader Bar-code readers can scan the codes in very rapidly, e.g. at a till in a store, or from books in a library Paper documents can be scanned into a computer. They become large, graphic images which can then be… (Optical Character Recognition) after text documents are scanned, the image file can be processed by software that recognises the shapes of the characters. Output is a text file containing the original text. Not always 100% right these can record things such as temperature, light levels, sound, etc. and the readings can be entered into a computer for logging and processing

Methods Of Ensuring Data Correctness Data can be checked in two ways: validation (is it valid and logical?) and verification (is it correct?)

Data Validation To ensure that the data being processed is accurate it must be checked to see that it makes sense. It is asking too much of a computer to check if it's true - somebody may just be lying! Certain validation checks can be carried out: Range-check Type-check Format-check Presence-check Checksum Check-digit

e.g. a driver's age should be between 17 and 99 e.g. a name should only contain alphabetic characters e.g. a data should be entered as dd/mm/yyyy e.g. certain data must be entered (e-mail address) e.g. the numeric values in a file should always add up to a known amount e.g. the last digit of an number is calculated from the previous digits, and then recalculated on input to see if it matches


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Data Verification Once we know our data is valid (i.e. it is logical, and in the right format, etc.) then we also need to check if it is correct (it maybe a valid date-of-birth, but is it your date-of-birth?!) Proof-reading Double-entry

the person entering the data simply reads through what they have entered to check it is correct the data is entered twice, either by the same, or another operator. The computer compares the data, and any that doesn’t match is questioned.

The Coding Of Data For Input Before data is entered into a computer system, it needs to be put into a suitable format: • •

•

Dates should all be of the same format (e.g. dd/mm/yy or dd/mm/yyyy) Text that is to be entered into a database field may be limited to a certain number of characters (if your paper form is designed well with little squares for each letter so the knows how much space they have, this will be easy) Data from sensors needs to be converted into digital data for the computer

Storing Data In Files Data, particularly data held in files, requires access in different ways depending on the particular application. Some applications require the data to be ordered in a particular way for processing, whilst some need the data to be very easily searchable for rapid access to specific items The medium on which the data is stored, and particularly the way in which it is organised, depend on the requirements for access. You would not store a file on magnetic tape, if you needed quick access to any part of the data – it would take too long to find items. Sequential access

Sequential indexed

Random access

is the storage of data in a way that you can only read one item at a time (like recording a film on video tape – you have to work your way through the film from the start) A magnetic tape is a sequential access device. You read it from the start until you find the data you want, re-winding and starting again if you need to do another search the file is still accessed sequentially, but each item of data has an index (a number or code to identify it). This index can be used to search for data more rapidly any item of data can be accessed immediately. A hard drive is a random access device – you do not need to read all of the data from the start to get to a particular file, you simply jump straight to it.

Some common file processing: Sorting Merging

the data on the file is arranged into a logical order (perhaps by numeric index, or alphabetically) two files are combined, creating a new file. The data is often sorted at the same time so the two sets are mixed in the correct order


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IGCSE Computer Studies Updating Insertion

Deletion

Syllabus 0420

when data records in a file are altered because the data has changed, the file is said to be updated if new data is to be added in the correct place in a sequential file, the file is first searched to find the right location and the data added. The remaining data must be moved to make room when an item of data needs to be removed, the sequential file is searched until the item is found, it is removed and the remaining data is moved to fill in the gap left behind

The Presentation Of Useful Information From Processed Data There are many ways in which processed data can be presented so that it may be better understood: Graphs and charts Images

numeric data is presented visually as bar charts, line charts, etc. data can be converted into an image, for example weather information from sensors can be shown as lines / colours on a map to help aid weather prediction

Analogue-To-Digital And Digital-To-Analogue Conversions There are two categories of data: Analogue data Digital data

this is constantly varying data, such as temperature, time, etc. this is discrete data that has exact values, such as exam grades (A, or B, or C… There is no level in-between

Analogue data can be made digital if it is sampled. This involves measuring the level at a particular moment in time Since computers are digital devices, whilst many other devices are analogue, we need to be able to convert analogue data into digital for input into a computer, and back again. Analogue to Digital Converter (ADC) The analogue signal is sampled at regular time intervals to give a sequence of digital values. An example: •

Temperature sensor supplies an analogue voltage dependent upon the temperature. This voltage es through an ADC to become binary (digital) values

Digital to Analogue Converter (DAC) The computer's digital data (binary numbers) is converted into an analogue signal. An example: •

Sounds for a computer game are generated as digital data. These are ed to a DAC which produces an analogue signal which can drive a loudspeaker (all sound cards in PCs are essentially DAC devices)


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Data Types Data can take many types including: Numbers

Characters Strings Arrays

these can be integers (whole numbers) or real numbers (with fractional parts). Numbers can be formatted to be displayed as currency, dates, etc. are the symbols on the keyboard, including letters, numbers, punctuation, etc. Characters can be ed together to make… a load of characters ed together (e.g. "Steve", "HHGF45sju" ) are a collection of a particular data type. So an array of integers would be a whole set of numbers stored together. Arrays have a name, and the individual parts are numbered (e.g. Scores(3) is the third item in the 'Score' array)

The Need For Different Data Types And Structures As part of our system design, we need to specify what types of data are to be used to store information. e.g. A person's name A person's age A person's height in metres A collection of temperature values


stored in a string (as it's a collection of characters) stored in an integer (since always whole number of years) stored in a real number (since will have a decimal part) stored in an array of real numbers (since can be fractions of a degree)

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5 Hardware, Systems And Communications 5.1 Hardware Computer Comprised of a number of components (parts): Central Processing Unit

Peripheral devices

Backing storage

(U) the 'brains' of the computer, comprises • Arithmetic and Logic Unit (ALU) which processes instructions and performs calculations • Control Unit (CU) which co-ordinates the operation of the different parts of the computer • Primary memory (RAM) which holds data that is currently being used. all of the devices that the computer interacts with, and that make it useful • Input devices which get data into the computer (e.g. a keyboard) • Output devices which convert data from the computer into other forms (e.g. a printer) devices that allow us to store data that is not currently being processed, or which needs to be saved when the power is turned off (non-volatile storage) • Magnetic storage such as hard drives, floppy disks, magnetic tape • Optical storage such as CD-ROMs, DVD-ROMs, etc. • Memory cards such as the ones found in digital cameras. Use a form of ROM that can be changed electronically. These are replacing floppy disks

Microcomputer The original digital computers from the 1940s were huge beasts that took up whole rooms, or even buildings. Since then, computers have got a lot smaller, a lot more powerful and a lot more -friendly. Mainframes Minicomputers Microcomputer Laptop Palmtop PDA

the original, large computers were called mainframes the next generation of computers, and the first ones that were sold in large number to businesses the small computer that you will be used to using, one that is self-contained and sits on your desk a portable microcomputer a portable computer that is small enough to be hand-held (Personal Digital Assistant) tiny computer that usually has no keyboard, using a touch screen instead


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Microprocessor (Embedded Computer) Not all computers need to have floppy drives, monitors, etc. Some are designed to do just one specific job and don’t need to be able to load new software, etc. These computers are very small and are often on a single chip. Examples of devices where microprocessors are found are: Digital watches Digital cameras

Engine Management

only task of the software is to tell the time. Inputs come from buttons, and the outputs are the display and a buzzer the tiny microprocessor takes inputs from sensors about the light level, etc. and then outputs control the shutter speed when the picture is taken. in cars a microcomputer controls all of the engine's systems, taking a range of inputs from sensors around the car (speed, etc.) Outputs devices control the flow of petrol, etc.

Standard Input Devices You need to be aware of a range of input devices, and the data they are provide, including: • • • • • • •

Keyboard Mouse and other pointing devices (graphics tablet, touchpad, touch screen, etc.) Microphone Joystick Barcode reader Scanner Sensors which detect or measure things (e.g. light sensors, heat sensors, etc.)

Standard Output Devices You need to be aware of a range of output devices, including: • • • • •

Monitor Printer, including dot-matrix, inkjet, laser printer Plotter Loudspeakers Robot arms or other actuators used to perform tasks or control things

Processor Power A processor deals with instructions one at a time. The quicker it can read and process instructions, the quicker tasks will be completed. Because of this, people are always keen to get quicker and quicker processors. We measure processor speed in Hertz (Hz), and this measures how many instructions a processor can handle per second. Most processors will handle millions or even billions of instructions per second, so you will see their speed quoted in megahertz (MHz, millions) or gigahertz (GHz, billions)


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The Functions And Characteristics Of Storage Media There are many ways we can store date. It depends on what we are doing, and how we need to be able to access the data. The size of storage is measured in bytes, kilobytes (kB, thousands of bytes) or megabytes (MB, millions of bytes). RAM

this memory is volatile (loses it's contents when the power goes off) so only suitable for temporary storage whilst the computer is switched on. RAM is used to hold:   

Some of the operating system (e.g. Windows) The applications currently running (e.g. Word) data (e.g. the letter you're typing)

The computer can access RAM extremely quickly. Commonly computers have about 256MB of RAM ROM

this memory is non-volatile (keeps it's contents when the power goes off) so it is used to hold data that is permanently required. The main use is to hold the small boot-up program that a computer runs when first switched on.

Hard disk

a form of magnetic storage that can hold huge amounts of data (up to 500GB, or 500,000,000,000 bytes of data!) Data can be accessed very rapidly (not as fast as RAM though) so it is good for most storage. s need larger and larger hard drives since they are starting to save digital images, music and even video, all of which take up a lot of space. For example:   

Digital photo, A4 size 4 minute MP3 music track 10 minutes of a movie, full screen (VCD)

500kB (0.5MB) 4MB 80MB

Floppy disk

a form of magnetic storage that is removable, so it can be used to transfer data between computers. Very old, crude technology so not very reliable. Can hold 1.44MB of data (1,440,000 bytes)

Zip disk

similar to a floppy, but holds much more data (20MB or more)

Magnetic tape a form of magnetic storage that is a little like a small video cassette. It can hold huge amounts of data (24GB), but the data can only be accessed sequentially (one bit after another) so tape is usually only used for backing up data. CD-ROM

optical storage (read by a laser) that is commonly used to hold data such as software, reference data, etc. Normally holds abut 700MB of data. CD-ROMs are read-only, so data cannot be saved onto them.

CD-R and RW CDs that can be written to, so data can be saved onto them. CD-Rs can be used saved to once, CD-RWs can be saved to many times. DVD-ROM

similar to a CD-ROM, but can hold up to 17GB of data (25 x bigger)

Memory card

a type of ROM that can be written to electronically. These little devices are found in digital cameras, MP3 players, etc. and allow data to be easily moved about. They are replacing floppy disks. Still quite expensive. Come in capacities from 16MB up to 2GB


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The Characteristics And Performance Of A Range Of Peripherals You should be able to discuss the suitability of different peripherals for various applications. e.g. Laser Printer Bar-code reader Flat-screen monitor Touch-screen

fast, quite, cheap to run suitable for an office environment allows very rapid entry of data codes suitable for POS terminals in stores although expensive, saves space, more energy efficient suitable for office desks and shops where space is limited very easy for novices to use, natural input, tough suitable for public information points (no mouse required)

Etc…


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5.2 Systems And Communications Operating Systems An operating system (OS) performs the following tasks: •

Acts are the interface between the computer's hardware, and the application software: Applications can run on a range of different hardware without needing any alterations (the OS 'hides' the differences

•

Manages system resources, such as memory, U processing time, etc.

•

Manages the transfer of data to / from peripheral devices

•

Manages system security (, file access, etc.)

Examples of common OSs are Windows 98, Windows XP, Mac OS, Unix, Linux, etc.

The Interface Between The Operating System And The Most computer OSs require interaction at some point. This human / computer interface (HCI) usually takes one of three forms: Command-line interface

Menu interfaces

Graphical interface

The types in commands to tell the computer what to do. Very quick for experts, but hard to learn for novices. Examples are MS-DOS, and basic UNIX the is presented with a number of options. They pick one, and another set of options appear, and so on until they find the operation / data that they want. Very -friendly, even with a large number of options, and easy to add / remove options. Examples of a menu-driven interface are cash-point machines at banks, and mobile phones (GUI) where computer operations and data is represented by pictures (icons); these are clicked by a pointer controlled by a pointing device (e.g. a mouse); information is shown in overlapping areas called windows and there is usually menus too. Often called a WIMP (Windows, Icons, Menus, Pointer). Very easy to learn for novices since it mimics the real world (desktop, waste bin, etc.) An example of a GUI is Windows

Management Of Files and File Directories Files stored on the backing store of a computer include operating system files, programs (e.g. Word) and data (e.g. a letter that you typed). There are often 10,000s of files on a computer's hard drive, so they have to be kept organised: • • •

Usually they are grouped in folders (or 'directories') Folders can contain sub-folders allowing for a hierarchy of folders (a 'tree' structure). So a file could be stored in /My Letters/2003/January/ Files should be named to make it clear what they are (e.g. "Letter to Mum.doc")


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Files often need to be moved, copied or deleted. This can be accomplished through a GUI (e.g. in Windows you can delete a file by dragging it to the waste bin) or by typing commands (e.g. In Unix you type "rm filename", in MS-DOS you type "del filename"). Sometime you need to see the contents of a file, or to print it out. In a GUI you might drag the file to a suitable application (e.g. In windows you would use Notepad to view text files) or you may type a command (e.g. In Unix you would type "cat filename" to view it)

Peripheral Device Control There must be some central control of peripherals so that data is ed between them without any problems (like data being lost)

Use Of Buffers A buffer is a small arrear of memory that is used to temporarily hold data that is either being sent to a peripheral (e.g. a printer) or coming from a peripheral (e.g. a keyboard) The buffer allows the computer (which is very fast) and the peripheral (which is often very slow) to operate independently without having to wait for each other. For example, when printing, the OS places the print data in a buffer and the printer begins to print it. The OS does not have to wait about for the printer to finish. It can get on with more useful tasks.

Interrupts And Interrupt Priorities This is one way that peripherals can interact with the U. If anything occurs that the computer needs to know about, the peripheral sends a control signal which 'interrupts' the U (hence the name 'interrupt'). The U then deals with the peripheral before continuing normal operation. Some interrupts have more importance than other. This is called the interrupt's priority. High priority interrupts get dealt with more rapidly by the U.

Polling Another way of peripherals interacting with the U is for the U to regularly 'ask' the peripheral if anything has happened that it needs to deal with. It goes around each of its peripherals in turn. This is called 'polling'. Polling is much more inefficient than an interrupt system, as often nothing will have happened, so the U wastes time asking.

Handshaking When establishing a communication link, two computers will 'greet' each other, and 'introduce' themselves. Once both computers are happy, and ready, communication can continue. This initial process is know as 'handshaking'

Check Sums Adding together all of the elements (e.g. the bytes) of a block of data produces a single element known as the check sum. This can be stored with the data block and provides a check when the block is transferred. The receiver re-calculates the data's checksum and compares it with the one sent. If they differ, a communication error probably occurred. The International School Seychelles

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5.3 Types Of System You must be able to distinguish between the different types of system: • • •

What is needed to them? Which is the most suitable for any given application Implications for the

Batch Processing Systems In broad , a batch processing system is one in which a job is processed without any direct interaction between the job and the .

MF

The operation is controlled by a Job Control Language (JCL)

TF

Merge

Typical applications are payroll and billing systems. A master file of data is updated using a transaction file that contains all of the changes that have occurred to the data since the last update. The result is a new master file.

New MF

Single- Online Systems Online systems provide for interaction between the job and the , which may influence the future course of processing. e.g. word processing Such systems may be single- (for example, a personal computer)

Multi- Online Systems Multi- online systems allow several s to interact with the computer at one time, often from completely different locations. Typical applications are online information retrieval (e.g. flight booking in a travel agents)

Multi-Tasking Systems If a can run several tasks at once (concurrently) on the same computer, it is aid to be multi-tasking. An example is when you have a word-processor, a spreadsheet and a CD player all running at once. The computer is multi-tasking.

Real-Time Systems A real-time transaction system is an online system in which individual, discrete transactions are processed as they occur. An airline booking system and an online stock control system are typical examples. A real-time control system is a system in which physical quantities are continuously monitored and processed sufficiently rapidly to be capable of influencing the sources of data. A chemical plant, or an autopilot system in an aircraft are examples. The International School Seychelles

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Control Systems In a control system, one or more computers is used to control the operation of some non-computer equipment, usually involving some monitoring and logging of physical quantities, providing some analysis of performance and allowing some interaction. is an essential element in most control systems. Timing considerations are often critical and the term real-time control system is sometimes used to indicate this. Control systems are used in applications such as oil refining, chemical processing and integrated traffic-control systems.

Automated Systems Automated systems are broadly similar to control systems but are dedicated to a particular task, and lack the ability to collect and analyse data and the flexibility to allow for and act on interaction beyond a very simple level. Examples are the systems found in equipment such as washing machines and cameras.

Multimedia Systems Candidates should be able to specify minimum hardware and software requirements for multimedia applications, and describe typical features and uses of multimedia systems.

Network Systems A network system is one in which several computers are ed together by communication links, and can then share resources such as file storage, printers and information resources. Local resources Remote resources

hardware and software that reside on your computer those items that you access via the network (e.g. a printer)

Star Networks The computers are arranged around a central hub. This hub may be a computer or a dedicated piece of hardware. Star networks are very fast as data between computers can always take the most direct route. They are also quite secure (safe from people seeing your data) as data is only read by the computers sending / receiving. Linear (Bus) Networks The computers connect to a long, common cable. All data is sent onto this cable, and each computer listens out for messages for itself. Bus networks are cheap to install, and if a computer breaks, the rest of the network can still operate. Ring Networks Computers are connected to a circular cable. Data can go either way, so this network can cope better with cable breakages than other network types.


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