57135443 a Level Computing 9691 Paper 1 Notes

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A Level. Computing (9691/ 1)

www. r evi s i on- not es . c o. c c

Computer Components

System Development

System Software

Data Representation

Data Capture

Data Management

Hardware

Data Transmission

Implication of Computers

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CPT1: Computing Fundamentals

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ADDITIONAL NOTES:

1.1 COMPONENTS OF A COMPUTER SYSTEM

Candidates should be able to:

1. Define the terms hardware, software, input device, storage

device and output device.

2. Describe the purpose of input devices, storage devices andoutput devices.

Hardware

Hardware are the physical components of a computer eg the input devices, output devices,primary storage (memory) and secondary storage (backing store), Central Processing Unit etc.

Note that input and output devices are collectively known as peripherals.

Software

Software are the sets of instructions/programs that are loaded into the memory of the computer inorder to perform a task or to control the operation of the computer.

Peripheral

A peripheral is a device that is external to the computers box and connected to it via one of theInput/Output (I/O) ports.

Common peripherals include:

mouse; keyboard; VDU; printer; scanner.

Input device

An input device is hardware that allows data to be entered into a computer.

Common input devices include:

keyboard; mouse; scanner; digital camera



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ADDITIONAL NOTES:

Storage device

A storage device is hardware that allows data to be saved, long-term, after it has been inputted intothe computer.

Common storage devices include: Hard disk drive; CD-ROM (or CD-R, CD-RW) USB Flash pen

Output device

An output device is hardware that allows a computer to present data to a user.

Common output devices include:

Visual display unit (VDU/monitor); Printer; LCD projector.



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ADDITIONAL NOTES:

Categories of Software


3. Describe the different roles and functions of systems softwareand applications packages.

Systems software

The set of programs needed to control and monitor the operation of the hardware (and applicationssoftware) of a computer.

Systems software consists of the following:

Operating System software Utility programs Programming tools Library programs

Operating System software

The Operating System is the software that controls the operation of the hardware and hides itscomplexities from the user.

The operating system is loaded into main memory during start-up.

Examples of Operating Systems are DOS, Windows, Unix, Linux, Mac OS. Note that the operating

system is not just one large program it consists of a set of many programs all of which arenecessary to get the computer to work.

Utility programs

Utility programs are non-essentials small programs that are designed to perform common tasks thatthousands of computer users benefit from at one time or another.

Some utility programs help maintain the functioning of the system and others make life easier forthe computer users.

Utility programs include:

file backup; file compression; disk formatters; disk defragmentation; file recovery; virus detection and cleaning etc..

Performance monitoringprograms can also be classified as utility software this is software that is

used to monitor disk, memory and processor use.



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ADDITIONAL NOTES:

Programming tools

Programming tools include language translators such as compilers and assemblers that are

necessary to build programs. They also include interpreters which are necessary to run programswritten in languages such as Visual Basic.

Note that the MS DOS versions of DEBUG and QBASIC were clearly systems software although

some people argue that Visual Basic is closer to applications software.

Library programs

Library programs are files that contain program code, which is available to all applications to share.

They allow different applications to communicate and share resources. MS Windows uses manylibrary files known as dynamic link libraries (*.dlls pronounced dee-el-els).

Applications packages

Application packages (software) are the programs that are consciously used by the user to solve

problems or perform work related tasks writing a letter, keeping accounts, printing invoices, etc.

Small-scale applications

Even though they are complex in their programming and can contain millions of lines of code,

some applications are designed to be installed on a single computer for one user to use. These

applications include:

word processing; spreadsheets; desktop publishing (DTP); presentation software; drawing packages.

Large-scale applications

Some of the large scale applications can be used by hundreds of people at the same time and storemillions upon millions of records. Such applications are often central to large organisations such as

banks, supermarkets and other types of large business organisations. These applications include:

stock control; payroll; order processing and tracking; utility billing.



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ADDITIONAL NOTES:

1.2 THE SYSTEMS DEVELOPMENT LIFE CYCLE


4. Describe the stages of the systems life cycle.

The stages in the systems life-cycle

Most IT projects use the Systems Life-cycle approach to developing a new system. This approachconsists of several distinct stages, which follow one after the other.

During the development life-cycle, a team is not permitted to go back to a previous stage thiscould cause the project to over-run in terms of both cost and time.

The stages in the Systems Life-Cycle are as follows:

Problem identification Feasibility Study (Initial investigation)

Analysis (detailed investigation)

Design

Coding (software development)

Testing

Conversion

Review (Evaluation)

Maintenance

Note that each stage of the Systems Life-cycle has a distinct end-point, which can be shown to the

customer and signed off. This helps to ensure that the final product is what the customer actually

wanted!

Problem identification

The problem identification is a statement of the existing problems and description of userrequirements as outlined by the customer.

Feasibility Study

A feasibility study is an initial investigation of a problem in order to ascertain whether the proposed

system is viable, before spending too much time or money on its development.



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ADDITIONAL NOTES:

Analysis

The analysis is a detailed, fact-finding, investigation of the existing system in order to ascertain itsstrengths and weaknesses and to produce the list of requirements for the new system.

Design

Design is the production of diagrams, tables and algorithms, which show how the new system is tolook and work.

The design will show:

how the interfaces and reports should look; the structure of and relationships between the data; the processing to be used to manipulate/transform the data; the methods to be used for ensuring the security and validity of the data.

CodingCoding is the creation and editing of the interfaces, code and reports so they look and work asindicated in the design stage.

Note that user and technical documentation will also be produced during the coding stage.

Testing

Testing is the process to ensure that the system meets the requirements that were stated in theanalysis and also to discover (and eliminate) any errors that might be present.

Conversion

Conversion is the process of installing the new system into the customers organisation and trainingthe employees to use it.

Review

Post-implementation review (also known as evaluation) is a critical examination of a system after ithas been in operation for a period of time.

MaintenanceMaintenance is the process of making improvements to a system that is in use.

The reasons for maintenance could be to fix bugs, to add new features or to make the system runquicker.



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ADDITIONAL NOTES:

Defining the problem


5. Explain the importance of defining a problem accurately.

An accurate problem definition is needed so that the developers know exactly what is expectedfrom the system. This means that the system that is delivered is going to be what the customer

expected. Without an accurately defined problem, it is likely that the software that is developed will

not fully satisfy the needs of the end users.

Note that there has to be a two-way dialogue between the analyst and the users because:

The users do not know a lot about computers and their capabilities; Programmers will not know very much about the way the business works, for which they

are developing the software



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ADDITIONAL NOTES:

Feasibility Study


6. Describe the function and purpose of a feasibility study.

After the problem/task has been defined and before a complete detailed study of exactly what isneeded within a new system, a feasibility study is undertaken to verify that the system that isrequired is, in fact, viable and that it is worth proceeding.

There are five factors that are considered in a feasibility study:

Technical feasibility this investigates whether the hardware and

software exists to create the system that is wanted. Economic feasibility this investigates the cost of developing a

new system (including the purchase of new hardware) and then determines whether thebenefits of a new system would outweigh the costs.

Legal feasibility investigates if there is a conflict between what is wanted and the law.For example, would the new system satisfy the requirements of the Data Protection Act?

Operational feasibility investigates whether the current working practises within theorganisation are adequate to support the new computer system. It is possible that the new

system would require employees to perform duties in a different way this may not beacceptable!

Schedule feasibility this investigates the amount of time that the new system is likely to

take to develop and determines whether it can be developed within the timescale that isavailable.

Estimating cost effectiveness

Costs Benefits

New Hardware Reduced Staffing

The New software Better service to customers

Training (time and money) Improved management information

Conversion (Time)Faster processing that speeds up payments fromcustomers

Maintenance (Money)



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ADDITIONAL NOTES:

Fact finding during analysis


7. Explain the importance of determining the informationrequirements of a system and describe different methods offact finding, including questionnaires, observation, andstructured interviews, highlighting the advantages anddisadvantages of each method.

When attempting to create a new IT system, it is vital that sufficient information is gained about theway the present system operates. The usual methods of obtaining this information include:

Interviewing staff

Observation of current procedures

Examination of paperwork Surveying (with questionnaires)

Interviewing staff

All levels of staff from end-user to senior management need to be involved during the analysisstage. A systems analyst should try to interview as many of them as possible so that all their needs

can be ascertained. Interviews are time-consuming, but very effective.

It should be remembered that many of the staff may not be sure exactly what they require and so the

system analyst may have to tease out some of their requirements with carefully thought out

questions.

Benefits Drawbacks

User can express their opinions in a detailed

way

Time consuming for the analyst

Extension questions can be asked as a resultof the users answers to the original questions

Users may feel intimidated and not tell the truthabout what they feel is lacking in the existingsystem

Users may feel valued and involved with thenew system

Observation of current procedures

Time and motion studies could be undertaken to see how data and documents move around the

existing system and to detect where bottlenecks occur and determine where procedures could be

made more efficient. This could be done by using a test document that has its movement tracked

through all its stages as it moves around the system.



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ADDITIONAL NOTES:

Observing the staff at work can often prove more advantageous than just interviewing them,

because it may uncover processes that they do sub-consciously and do not remember to tell theanalyst about during interview.

Benefits Drawbacks

Allows the analyst to see the current system atfirst hand and what processes that are oftendone sub-consciously

Users may work differently if they know thatthey are being watched.

Analyst can get a feel for user-competence

Analyst can measure the time taken to dotasks

Examination of paperwork

This will help to show the inputs and outputs of the current system and so help determine the inputsand outputs required by the new one. The paperwork will include documents that are received froman organisations customers such as membership application forms or orders. They will also

include documents that are produced by the current system such as the invoices sent to customersand the current reports that are produced for the management team.

Benefits Drawbacks

Analyst can get an idea of the volume of databeing stored and processed

Can be time consuming if there is a largevolume of files to go through

Analyst can see what output is required

Analyst can see how data is currently validated

Surveying (with questionnaires)

If there are many users of the system then surveying staff by asking them to complete a

questionnaire would be a more efficient method of gathering information than conducting personalinterviews. The analyst could then choose to follow up some of the responses with an interview.

Benefits Drawbacks

Efficient in terms of time Responses to questions are less flexible thanin an interview

Answers can be anonymous There is often a low return rate

It can be difficult to design a good

questionnaire



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ADDITIONAL NOTES:

Requirements analysis


8. Describe what is involved when analysing the requirements ofa system, explaining the nature of the requirementsspecification and its content, including current data structures,inputs, outputs and processing represented in diagrammaticform (data flow diagrams, system flowcharts), identifyinefficiencies/problems in the current system.

The deliverable at the end of analysis is documentation that shows an investigation into the current

system and a list of system requirements for the new one. These requirements need stated in a clear,specific and measurable way.

In order to ascertain these requirements, the systems analyst needs to examine the current data

structures and relationships between them. They must also trace the flow of data through the

existing system this will begin by determining the source of the various data, identifying the waysin which the data is processed; and finish by identifying the destinations of the final outputs.

The analysis documentation will contain the following:

Identification of existing and prospective users; Identification of current data and its structure; Identification of inputs, outputs and processes within the current system; Identification of data flows including the sources and destinations of the data; Identification of the strengths and the weakness of the current system; Listing of objectives.

Current data and its structures

This is the description of the current data within the system its data type, validation techniques

used, relationships with other data within the system. The analysis data dictionary is usuallypresented in tabular form, with each different category of data being described in its own table.



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ADDITIONAL NOTES:

Data flow diagrams

A data flow diagram is a drawing that depicts the transformation of data within an existing systemby using three different graphical symbols connected by labelled, directed lines.

A data flow diagram is an analysis tool that represents what a system does, not how it does it. Theyare:

graphical eliminating thousands of words;

logical representations not physical models;

hierarchical showing systems at any level of detail; and

jargonless allowing user understanding and reviewing.

An example of a data flow diagram depicting the sale of turkeys on a farm:



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ADDITIONAL NOTES:

System flowcharts

A system flowchart shows an outline of how a system operates.

The following shows the system flowchart for a schools registration system that stores studentattendance data on a magnetic disk in a database called Register.

KEY:

Input/output

On-line storage

Process



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ADDITIONAL NOTES:

Design


9. Describe a design specification including input design,diagrammatic depiction of the overall system, processing, datastructure design and output design.

The deliverable at the end of the design stage is documentation that could be used, unaided, by athird-party programmer to create the system as the designer intends.

This means that each section of the design must be detailed and clear. Explanations of what must bedone and why this method is chosen need to be included.

The design documentation will contain the following:

user interface designs (input forms and menus); specification of data structures (including the relationships between different types of data); validation procedures; output/report designs; algorithms; security methods.

User interfaces

Humans will need an interface that allows them to:

give instructions Print, Save, Open, Delete, Copy, Paste etc. enter data file names, number of pages to print; make choices Yes, No, Cancel etc.

Computers need ways to:

inform of errors illegal operation, invalid data input, printer out of paper, wrong password: tell on progress copying, deleting, installing, downloading: display the results of processing

ask for options e.g. number of pages to be printed, which file to open. provide help with performing tasks this help could be in the form of Status bar text, yellow

Tip boxes, an Office Assistant or even a full-blown help file accessed via the Help menu.

The user interface designs will need clearly annotated drawings to tell the programmer exactly what

is to be done.



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ADDITIONAL NOTES:

Example of an interface for data entry:

Processing (algorithm) design

These algorithms outline the programming methods that need to be used to process the data.

Algorithms are usually written in pseudo-code, which are instructions that are half-way between

English and a programming language. The advantage of pseudo-code is that it can be used todescribe an algorithm in a way that is not specific to any particular programming language.



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ADDITIONAL NOTES:

Data structures and validation

Data can be of many types and can have many properties. Typically, a table is constructed for eachcategory of data. The properties of the data and the necessary validation techniques are stated as

shown:

Field Name Data Type Validation Comments

MemberID AutoNumber(Integer)

Uniquenesscheck

Automatically generatedPrimary Key

FirstName Text (size 25) Presence check Enter members first name maximum 25 characters)

LastName Text (size 25) Presence check

DateOfBirth Date (Short) Data-type +Presence check

Gender Text (size 1) Existence check(on M or F) Default M; choice of M/Fselect from radio-button.

---

---



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ADDITIONAL NOTES:

Output design

This needs to be detailed in the same way as the input designs. Example of a mail-merge letteroutput:



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ADDITIONAL NOTES:

Testing


12. Explain the importance of system testing and installationplanning.

Testing is undertaken to ensure that a system satisfies the users requirements and to discover any

errors that might be present.

Testing needs to be undertaken by both the programming team and by the end-users. Note that

testing a program can never adequately prove or demonstrate the correctness of the system it canonly reveal the existence of errors.

Testing by the programming team

Testing by the programming team is ongoing as the system is developed, but it is still very

important to undertake tests at the end, after the programming team believe that the system is

finished. This formal testing at the end of development is known as alpha-testing.

Alpha-testing

This is the formal testing at the end of development. It:

is undertaken by the programming team; uses data that the programmers perceive to be realistic;

is designed to ensure that the requirements/objectives have been met.



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ADDITIONAL NOTES:

Testing by the end-users

Testing by the end-users is either beta-testing or acceptance testing.

Beta-testing

Beta testing is when the software is tested under real conditions, using real data, by a selection ofreal end-users.

This phase of testing is necessary because software developers will probably not anticipate all thecombinations of conditions that will occur when the software is in use in its real environment.

This is the stage of testing where problems with different hardware combinations are usually

discovered, as are problems with clashing software.

During beta testing, users generally agree to report problems and bugs to the developers. These willthen be corrected and the software may then undergo a second round of beta testing before the

package is eventually released.

These end-users who test the system during beta-testing:

use the system with realistic volumes of real data; use the system with a variety of different hardware and configurations; report faults/errors back to analyst; check that there is a reasonable response time; ensure that the user interface is clear; ensure that the output as expected.

Acceptance testing

Acceptance testing is where the customer specifies tests to check that the supplied system meets

his/her requirements as specified at the analysis stage and that the system works in their ownenvironment.

Note that the tests and data are specified by the customer, but the testing itself may be carried outby the customer or by the developer under the scrutiny of the customer



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ADDITIONAL NOTES:

Conversion

Conversion is the process of changing from an existing system to a new system.

Note that conversion may take place within a day, or it might be that it is several months before allparts of the new system have replaced the old.

Problems arising when converting/changing over

Converting from an existing system to a new one is not always smooth. The following problemscould arise:

Data may have to be converted because the format in the new system may be differentto the format of the old system.

Users will have to be trained so that they will be able to use the new system the

organisation may even need to employ additional staff. Data may be lost during conversion must make sure there is a full backup made

before changing to the new system;

Hardware may need to be replaced/upgraded if it does not satisfy the demands ofthe new system;

System software may need to be replaced/upgraded i.e. new system may be createdto take advantage of the features within Windows XP and so will not run correctly on an

earlier operating system.

Old data may need to be archived;



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ADDITIONAL NOTES:

Methods of conversion/changeover

The change from the use of an existing system to the use of a new system cannot always be

instantaneous. There are several different methods of converting to a new system, the four mostcommon are:

Parallel; Direct; Pilot; Phased.

Parallel

Parallel conversion is when the old system and the new system operate along side each other for aperiod of time, until all issues with using the new system have been resolved.

Parallel conversion allows an organisation to revert to the old system if the new system fails.

Direct

Direct changeover is when the old system is stopped being used one day and is replaced, in full, bythe new system the next day.

There is no going back when direct changeover is used.

Pilot

Pilot conversion is one department within an organisation changes to the new system before theothers.

This department will discover any problems with the use of the systems and these problems can be

ironed out before the rest of the organisation converts to the new system.

Phased

Phased conversion is when the old system is gradually replaced, in stages, by the new system.

This type of changeover is convenient when the system comprises of several different modules.

This will allow the organisation to convert to one of the new modules first, but maintain the use of

the other existing ones. This type of conversion means that training can be concentrated on one newmodule at a time.

Note that if phased conversion is used, it is vital that the new system and the old system can sharedata.



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ADDITIONAL NOTES:

Evaluation (review)


10. Explain the importance of evaluating the system, and how toidentify the criteria used for evaluation.

Evaluation (also known as post-implementation review) is a critical examination of a system after ithas been in operation for a period of time.

Purpose

The purpose of an evaluation is to assess the success of a system. Specifically, it will assess the

suitability, effectiveness, usability and maintainability of the system.

The evaluation will ask many questions including:

can it carry out the all the requirements that were set?

is it an improvement on the existing system?

is it cost effective?

is it easy to use?

is the new system compatible with the existing systems?

is the system easy to maintain?

The evaluation will also consider:

what limitations there are in the system; what enhancements could be made to the system in the future.

Note that feedback from the end-users should also be included.

Timing

The evaluation will occur after a new system has been in operation for some time usually a periodof between three and six months.

The waiting period allows users and technical staff to learn how to use the system, get used to new

ways of working and understand new procedures required. It allows management a chance to

evaluate the usefulness of the reports and on-line queries that they can make and go through severalmonth-end periods when various routine reports will have been produced. Shortcomings of the

system, if they exist, will be becoming apparent at all levels of the organisation.



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ADDITIONAL NOTES:

System documentation


11. Explain the content and importance of different types ofdocumentation at different stages in the system life cycle,including the technical and user manuals.

There are two types of documentation that are necessary:

Technical documentation aimed at a future system developer

User documentation/Manual aimed at the end user.

Technical documentation:

Note that technical/system documentation is very valuable for the maintenance process. This isbecause it will show how each part of a system has been constructed and the reasons why certain

choices have been made. The technical documentation should include:

Annotated program listing if the system is coded.

Data flow diagram

System flow diagram

Structure charts/pseudo code/algorithm designs

Test plan

Data dictionary i.e. the field definitions (including data-type, field length, validation)

Entity relationship diagrams

User Documentation

As well as a contents page and index, the user documentation should include:

Overview of the system

Instructions on how to install

Instructions on how to backup the data

Instructions on how to operate the program

Details on possible errors and how to deal with them

Glossary of terms used within the documentation



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ADDITIONAL NOTES:

Maintenance


13. Explain the purpose of maintaining the system, and explain theneed for system review and reassessment, understanding thatsoftware has a limited life span.

Maintenance is the process of making improvements to (or modifying) a system that is in use.

The need for maintenance

Maintenance is needed because:

Bugs are discovered in the software code these bugs will have been identified only afterthe system is in full use. They will be fixed and a patch will be issued that changes theappropriate lines of code within the end-users programs.

The user requirements may change this often happens after a system has been inoperation for some time and the users see further uses of the data that the system produces.In some cases, the additional requirements may have been identified during the original

development, but they were not implemented because the systems life-cycle approach to

projects does not allow a change in requirements once they have been agreed (such a change

would extend development time and cause a project to miss its deadline).

Some in-built parameters change e.g. VAT rate; Hardware is changed the system will be updated to take advantage of new hardware

developments. This could be a new input device, output device or even communicationsdevice.

The performance needs tuning often some of the original code, although workingwithout error, uses some quite cumbersome routines that are slow to execute. System

performance can often be improved by finding more efficient algorithms for such routines.

Operating system is upgraded the system will be modified to take advantage of theadditional capabilities of the operating system.



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ADDITIONAL NOTES:

Types of maintenance

There are three types of maintenance:

corrective maintenance; adaptive maintenance;

perfective maintenance.

Corrective maintenance

Corrective maintenance is the removal of some of the known bugs in a program.

Software that has been released to the public will still contain bugs. Some of these bugs will be

previously unknown to the developers while others will be known to exist, but solutions will not yethave been discovered. The software will be released with these known bugs because of the

necessity to meet pre-set target dates and the need to gain some income from the software tocontinue to pay the developers.

Eventually some of the bugs will be fixed and the solutions will need to be incorporated into the

publics version of the programs. This type of maintenance is often done by releasing a patch

which is a very small program that actually changes lines of code within the main program. These

patches are available from Internet sites or from the CD-ROMs that are provided with computermagazines.

Adaptive maintenance

Adaptive maintenance is the addition of new features to a program because of a change in users

requirements.

The new version of the program may contain an added (or modified) feature or it may contain achange in the interface. Adaptive maintenance could be needed because of:

a change in the organisations/users requirements; a change in the law; a change in processes such as the method of tax calculation; to take account of new technologies.

Perfective maintenance

Perfective maintenance is when internal routines are changed to make them more efficient, so that

the application operates faster.

In the initial release of the software some of the processes, although error-free, may have used long

and slow routines. Perfective maintenance will make improvements in the way that the software

performs by tidying up some of the internal routines. Changes to the interface may also be made.



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ADDITIONAL NOTES:

Prototyping


14. Describe prototyping to demonstrate how a solution willappear.

15. Describe the spiral and waterfall models of the systems lifecycle.

A prototype is a simplified working model of a proposed system used as a rough indicator of howthe new system will work.

The prototype will consist of a set of screens and processes that show the user (and developer) what

might be possible. It will help a customer to gain a clearer idea of a proposed system so that they

can give feedback before the development has gone too far.In the prototyping approach, the analysis establishes an outline specification. A model is then built

in order to evaluate it or have it approved before building the production model.

Prototyping can involve the repeated re-development of a system with new features being added asthe initially vague requirements are refined.



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ADDITIONAL NOTES:

1.4 SYSTEMS SOFTWARE

Operating system


20. Describe the purpose of operating systems.

The operating system is an integrated set of programs that is used to control and manage theresources and overall operations of a computer.

Its role is to provide a virtual machine by hiding the complexities of the hardware from the user. It

does this by providing a buffer between the user and the hardware allowing the user to deal with a

simplified system, but without loosing any of its computational powers.

In addition to providing the HCI, the Operating System manages the hardware resources in order to

provide for an orderly and controlled allocation of the memory, storage media, processor time, andI/O devices among the various processes competing for them.

The function of an operating system

The operating system (OS) for a standalone computer will be much simpler than that of asupercomputer which is controlling hundreds of networked terminals and executing many different

kinds of job simultaneously. Nevertheless, all operating systems perform the same basic functions.These include:

memory management; file management (sometimes known as backing-store management); allocation of processor time; input and output management.

An operating system also manages:

interrupts; errors; the human/computer interface.



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ADDITIONAL NOTES:

Memory management

The operating system has to allocate memory to each running application and to itself. Each

application is loaded into its own memory space this means that if one program freezes, the others

can, in theory, keep working normally. It also means that you can terminate a frozen program by

pressing [Ctrl]+[Alt]+[Delete]. Doing this will not effect any other programs.To manage memory effectively, the operating system must:

assign programs their own area of memory; prevent two programs from using the same area of memory; reallocate memory when a program is quit.

File management

The OS controls the transfer of data from disk to memory and back again. It also has to maintain a

directory of the disk so that files and free space can be quickly located. The directory is called the

File Allocation Table (FAT for short). To manage files effectively, when files are written to a diskthe OS ensures that:

existing files are not over-written; when files are deleted from the disk the storage blocks are made available for new files.

Allocation of processor time

When several processes are executing on a computer at the same time (eg downloading a file,

printing and listening to an .mp3), then they will all need to receive time from the processor. Some

processes, such as printing, require very little processor time, while other processes require the

processor continuously. In such situations the Operating System would place a high priority onprinting requests to get them cleared and then allow it to concentrate on the more demandingprocesses.

Input output device management

When two programs want to print to the same printer, the operating system has to ensure that thetwo jobs do not interfere with each other. It would normally do this by putting the jobs into a

queue and then pass them to the printer when the printer is ready.



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ADDITIONAL NOTES:

Interrupt handling

The OS detects and responds to many different kinds of interrupt such as:

a user pressing a key on the keyboard; a printer sending a message that it is out of paper;

a hardware or software malfunction.

In the case of a keyboard interrupt the operating system may simply display the appropriate

character on the VDU, but in the case of the printer sending an out of paper interrupt, theoperating system will display a message to inform the end-user.

Error handling

Application software should normally include routines to deal with their own errors. When this is

not done, it is necessary for the operating system to deal with them or else the whole computercould crash.

The operating system should be able to freeze the program that causes an error and display amessage to the end user. A message such as General Protection Fault might be displayed when a

badly-written application tries to use an area of memory that has been assigned to the operating

system itself.

The human/computer interface

The HCI allows a user to communicate with the computer. In early operating systems, users gave

instructions to the computer by typing command words. Most modern operating systems provide a

Graphical User Interface (GUI), which allows a user to choose commands by moving a pointer andclicking on menus.



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ADDITIONAL NOTES:

Types of operating system


21. Describe the characteristics of different types of operatingsystems and their uses: batch, real-time, single-user, multi-user, multi-tasking and distributed systems.

22. Describe a range of applications requiring batch processing,and applications in which a rapid response is required.

Batch operating system

A batch operating system is one which allows input of data as batches and processes the data onlywhen all input has been collected so that the processing is carried out from beginning to end

without user interaction.

In a batch system the data is collected and input into the computer over a period of time. It is storedas a job to be processed later. Several batch jobs are usually executed at the same time so that the

processor and other resources are kept as busy as possible by switching between the different jobs.

Batch processing is typically used for:

processing OMR forms such as those that contain answers to a multiple-choice exam; payroll;

utility billing

In the case of processing multiple-choice OMR forms, the batch operating system can process theforms incredibly fast, but it needs to reject forms that it cannot read marks may be too faint, two

marks may exist on the same line etc. It does this by rejecting unreadable forms into a hopper andthe data from these forms then need to be handled manually.

In the case of payroll



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ADDITIONAL NOTES:

Interactive operating system

An interactive operating system is one that allows the user and the computer to be in direct two-waycommunication.

The user enters commands and data as the application is executed and the results of processing aredisplayed immediately.

Categories of interactive system are single-user, multi-user, multi-tasking and real-time.

Single-user

A single-user operating system is one that can only be used by one person at a time.

Multi-user

A multi-user operating system is one that allows two or more users to communicate with the

computer at any one time, with each user interacting with the computer via separate terminals.

Multi-tasking

Multitasking is the apparent concurrent execution of two or more programs, on the same computer,

in such a way that communication and data sharing is possible.

Real-time

A real-time operating system is one in which requests are executed immediately and can therefore

produce a response within a specified, short, interval of time.

Some definitions sate that:

A real-time operating system (RTOS) is an operating system that guarantees a certaincapability within a specified time constraint.

Almost any general purpose operating system such asMicrosoft Window orMacOScan be

considered real-time to some extent even if an operating system doesnt fully qualify as real-time,

it may have characteristics that enable it to be considered as a solution to a particular real-timeproblem.



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ADDITIONAL NOTES:

Real-time operating systems are characterised by their ability to:

deal with events which happen at unpredictable moments in time; deal with multiple events that occur simultaneously; support application programs which are non-sequential in nature i.e. programs which do

not have a START:PROCESS:END structure;

carry out processing and produce a response within a specified interval of time.

Note that the results of processing may be returned in milliseconds as in the guidance systems of a

cruise missile or if the processing is complex, it might take a couple of seconds as in a

temperature control system in a large greenhouse.

Examples of real-time operating systems:

Airline flight reservation; Missile guidance; Temperature/pressure control;

Process control.

Flight reservation

The booking needs to be processed quickly and confirmation given to the customer straight away.

When a seat has been booked on a flight, the system needs to be updated before the next transactionoccurs in order to avoid the possibility of double-booking.

Missile guidance

Various sensors that detect the altitude, latitude, speed etc. constantly provide up-to-date

information to the guidance system. If the missile is slightly off course then adjustments must be

made immediately a one second delay could result in the missile hitting the wrong target.

Temperature/pressure control

Sensors in a nuclear power station will be providing data on the current temperature of the reactor.

If the reactor starts to overheat, then initiating cooling will have to happen instantly to avoid apotentially dangerous situation or meltdown.

Sensors in a greenhouse will be providing data on the current temperature and humidity. If eithergoes outside the preset range then an action such as opening/closing the greenhouse windows will

need to occur straight away.

Process control

In manufacturing processes 1000s of signals per second can arrive from sensors attached to thesystem being controlled. Because such systems are extremely fast moving, the response time may

have to be less than one thousandth of a second.



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ADDITIONAL NOTES:

Distributed system

A distributed system is one in which file storage (or processing) is shared between differentcomputers at different locations.

Users of a distributed system will be unaware of the exact physical location of the data that they areaccessing it is likely that one set of records is retrieved from one remote system and the next setof records is retrieved from another!

An example is a banking system where the data for each customer is held on the server at their local

branch, but is accessible from any branch.

Network operating system

A network operating system (NOS) is one that is able to share its resources (such as hard disk orprinter) and use the resources of others.

Note that in sharing its resources the network operating system is able to control what other users

are permitted access to and whether that access should be read & write or read-only.

A NOS often consists of exactly the same files as a standard operating system, but has an extralayer of software. This software (known as the redirector) intercepts commands for file storage and

printing and, in a manner completely transparent to the user, redirects the requests to the appropriateserver. In this way files, printers and application programs resident on the server can be used by the

client exactly as if they were resident on the users own system.

Using a network printer

When a computer is set up to share its printer (known as a print server), the NOS will:

allow the computer directly attached to the printer to make its printer visible to the othercomputers;

allow the other computers to be aware of the existence of the shared printer; allow the print requests from the other computers to be redirected to the print servers

printer;

allow this printer server to control the printing requests of the other computers by putting all

the requests into aprint queue;

Note that it is usually possible for the print server to use access rights to control which users are

allowed to use the printer and to assign users priority in the queue.



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ADDITIONAL NOTES:

Human-computer interface (user interface)


23. Identify and describe the purpose of different types of userinterface: forms, menus, GUI, natural language and commandline, suggesting the characteristics of user interfaces thatmake them appropriate for different types of user.

The user interface is the hardware and software that provides the means for communication

between the user and the computer.

Different interfaces have been developed for different needs. Common types of HCI are:

Forms Menus; Graphical (GUI); Natural Language; Command Line.



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ADDITIONAL NOTES:

Forms

This kind of interface presents the user with an on-screen form into which they enter or view data.

The form will often be arranged into different sections and will consist of text-boxes, checkboxes,

radio buttons, drop down lists and other input controls to help the user enter data quickly and tohelp with validation.

Travel agents and other booking systems would typically use a form interface.



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ADDITIONAL NOTES:

Menu interface

A menu interface is one that provides a list of choices from which the user can choose by pointingto/clicking on. Each choice that the user selects will display a screen with other choices and

ultimately the desired choice.

ATMs and mobile phones often use menu driven interfaces and so do the ticket machines on the

London underground and many tourist information systems.

The benefits of a menu interface is:

less human error the user can only choose from the options available; user is restricted from accessing other parts of the system.

The drawbacks include:

there may be no shortcuts for accessing common choices.

Tourist information systems would typically use a menu interface with a touch-screen to act asboth an input and output device. This:

avoids the need for additional peripherals such as a mouse; allows ease of use by indicating possible choices with icons (not just text); provides an enclosed system with protection against vandals and the weather (can be used

outside).



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ADDITIONAL NOTES:

Graphical User Interface (GUI)

A graphical user interface (GUI) is one that provides a means of interaction using windows, icons,menus and a pointer to control the programs and operating system.

This kind of interface is sometimes called a WIMP and consists of the following:

Windows; Icons; Menus;

Pointer.



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ADDITIONAL NOTES:

Windows

A window is a bounded area of the screen within which a specific task is executing e.g. wordprocessing, web browsing, file management etc.

Icons

An icon is a small image that represents a program, folder, a device or a file.

Menus

A menu is a listing of options from which a user may choose menus in a GUI are usually pop-upor drop-down.

PointerA pointer is an on-screen arrow, usually controlled by mouse, used for navigation and to selectappropriate options by clicking a button.

Note that it is possible to set the pointer to be an image other than an arrow, but doing this oftenmakes the system harder to use.

Benefits and drawbacks of a GUI

Benefits Drawbacks

Easy for a novice because a GUI is usuallyintuitive the screen is arranged as ametaphor of a desktop with icons used torepresent familiar objects.

Powerful hardware is required fastprocessor, high quality graphics card and VDU,RAM and HDD with large capacity.

User does not have to remember the preciseformat of the instructions instructions areinitiated by selecting icons or menucommands.

Can be frustrating for an experienced user toperform certain tasks because a greaternumber of operations may be required.

There is likely to be comprehensive, easy tonavigate, on-line help available.

Not all instructions are available through theGUI the command line will still need to beused for many technical tasks.

Modern GUIs allow very easy execution ofsome commands by drag and drop.



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ADDITIONAL NOTES:

Natural Language

A natural language interface is one which allows a user to communicate with the computer byspeaking or typing in their normal way.

Ask Jeeveswas a natural language search engine it will allow you to type a question in the normalway and it will interpret the question and provide the answer if it can. Below is an example:

The image part with relationship ID rId47 was not found in the file.

The image part with relationship ID rId48 was not found in the file.

Type the question in yourusual language what is thecapital of France?

Jeeves will tell you theanswer if he knows it!



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ADDITIONAL NOTES:

Command Line interface

A command-line interface is one in which the operating system provides a prompt and the usertypes a command to start program execution or to perform a housekeeping task.

For example:

The computer might prompt as follows:

C:\>

The command promptis the > character and the C:\ is the pathname for the current directory.

The user typing the following command:

C:\>Del *.doc

will cause all files in the current directory with the extension .doc to be deleted.

Another command with MS DOS is:

C:\>copy MyFile.doc C:\Backups\MyFile.bak

This command copies the file called MyFile.doc into the directory called Backups and renames thefile MyFile.bak.

Sometimes switches can be used with commands:

C:\>xcopy C:\Backups A:\Backups /s /e

This command copies the directory named Backups from disk C: onto disk A: the /s means that

subdirectories are copied too, and the /e means that empty directories are also copied.

A command line interface is not for novice users, but is often used by an IT technician who needs to

perform tasks that are difficult when using a GUI.

When using a command line, the user must only type valid commands and they must be typed inthe correct format omitting a space or a \ will usually cause the command to fail.



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ADDITIONAL NOTES:

Benefits and drawbacks of a command line interface

Benefits Drawbacks

Only low specification hardware is required

monotone VDU, basic processor, small RAMand HDD.

Difficult for a novice because they have to

remember a large number of commands andtheir exact format.

Experts can perform complex tasks using asingle (although lengthy) command.

The syntax of the command is vital and so theinstruction will not execute if the command is

typed incorrectly

An instruction can be executed from any partof the system (do not have to have a certainwindow open).

Easy to make mistakes.



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ADDITIONAL NOTES:

1.5 DATA REPRESENTATION

Number representation


26. Express numbers in binary, binary-coded decimal (BCD), octaland hexadecimal.

Computers can only process binary data i.e. 1s and 0s. If numeric data is to be processed then itcannot be processed in its usual base10 form, it must be converted into its base2 form known as

binary.

There are two ways that can be used to represent numbers in binary:

pure binary; binary-coded decimal (BCD).

There are also two half-way stages that are sometimes used by programmers because they areeasier to understand than a string of 1s and 0s:

octal; hexadecimal.

Pure binary

Pure binary represents numbers using just two digits (0 and 1) and columns, which increase by afactor of two.

This is in contrast to our normal number system (denary), which uses ten digits (0-9) and columns,which increase by a factor of ten.

In denary the number one hundred and ninety seven is represented as:

100 10 1

1 9 7

In binary, it is represented as:

128 64 32 16 8 4 2 1

1 1 0 0 0 1 0 1



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ADDITIONAL NOTES:

Counting in binary

In binary, the first 15 numbers are as follows:

128 64 32 16 8 4 2 1

0 0 0 0 0 0 0 0 0

1 0 0 0 0 0 0 0 1

2 0 0 0 0 0 0 1 0

3 0 0 0 0 0 0 1 1

4 0 0 0 0 0 1 0 0

5 0 0 0 0 0 1 0 1

6 0 0 0 0 0 1 1 0

7 0 0 0 0 0 1 1 1

8 0 0 0 0 1 0 0 0

9 0 0 0 0 1 0 0 1

10 0 0 0 0 1 0 1 0

11 0 0 0 0 1 0 1 1

12 0 0 0 0 1 1 0 0

13 0 0 0 0 1 1 0 1

14 0 0 0 0 1 1 1 0

15 0 0 0 0 1 1 1 1

Can you see the pattern?

Converting from binary to denary

A binary number, such as 1001 0101, can be converted into its denary equivalent as follows:

1. Write the binary number with the appropriate column headings:

128 64 32 16 8 4 2 1

1 0 0 1 0 1 0 1

2. Add the column headings under which there is a binary 1:

= 128 + 16 + 4 + 1

= 149



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ADDITIONAL NOTES:

Converting from denary to binary

A denary number, such as 107, can be converted into binary as follows:

1. Write down the binary column headings:

128 64 32 16 8 4 2 1

2. Then, starting from the left, take out the values in the column headings, if possible:

128 cannot be taken out of 107 so that column contains a 0:

128 64 32 16 8 4 2 1

0

64 can be taken out of 107 so that column contains a 1; this leaves 107 64 = 43:

128 64 32 16 8 4 2 1

0 1

32 can be taken out of 43 so that column contains a 1; this leaves 43 32 = 11:

128 64 32 16 8 4 2 1

0 1 1

3. keep repeating the above process until the whole number has been converted to pure binary:

128 64 32 16 8 4 2 1

0 1 1 0 1 0 1 1



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ADDITIONAL NOTES:

Binary Coded Decimal (BCD)

Binary Coded Decimal is one of the early memory encodings. Rather than converting the entiredenary value into its pure binary form, it converts each digit, separately, into its 4-bit binaryequivalent. The table below shows the 4-bit BCD equivalents of the ten denary digits:

8 4 2 1

0 0 0 0 0

1 0 0 0 1

2 0 0 1 0

3 0 0 1 1

4 0 1 0 0

5 0 1 0 1

6 0 1 1 07 0 1 1 1

8 1 0 0 0

9 1 0 0 1

Note that the higher codes are not used in BCD because they do not represent a denary digit. Theseare:

1 0 1 0

1 0 1 1

1 1 0 0

1 1 0 1

1 1 1 0

1 1 1 1

Not used in BCD because theirdenary equivalents are higherthan 9.



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ADDITIONAL NOTES:

Converting from denary to BCD

Each digit is converted to its 4-bit BCD equivalent. Thus, the number 319 would be represented in12-bits as follows:

8 4 2 1 8 4 2 1 8 4 2 1

0 0 1 1 0 0 0 1 1 0 0 1

(3) (1) (9)

Converting from BCD to denary

Each group of 4-bits are converted into the equivalent denary digit. Thus, the 12-bit binary codeddecimal number0110 1000 0011 is denary 683 as shown:

8 4 2 1 8 4 2 1 8 4 2 1

0 01 1 0 1 0 0 0 0 0 1 1

6 8 3

Octal

The octal number system uses eight digits (0 to 7) to represent numbers, and columns which

increase by a factor of eight.

4096 512 64 8 1

Converting from octal to denary

The octal number 652 would be converted to denary as shown:

4096 512 64 8 1

6 5 2

(6 64) + (5 8) + (2 1) = 426



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ADDITIONAL NOTES:

Converting between octal and binary

The octal number 652 (426 in denary) is represented in binary as:

256 128 64 32 16 8 4 2 1

1 1 0 1 0 1 0 1 0

If we combine the bits in groups of three and label with the appropriate column headings

4 2 1 4 2 1 4 2 1

1 1 0 1 0 1 0 1 0

6 5 2

we can see that converting from octal to binary converts each digit into its 3-bit binaryequivalent (very similar as converting between denary and BCD).

Thus the octal equivalent of binary number 10011101 will be:

4 2 1 4 2 1 4 2 1

0 1 0 0 1 1 1 0 1

2 3 5

Note the extra 0 added at thefront because the originalbinary was only 8-bits.



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ADDITIONAL NOTES:

Hexadecimal

The hexadecimal number system uses 16 digits to represent numbers. The denary digits 0 9 areused together with the first six letters of the alphabet (A F).

With hexadecimal, instead of column headings doubling, as they do in binary, or increasing by afactor of 10 as they do in denary, each column heading in hexadecimal increases by a factor of 16.The column headings are:

65,536 4096 256 16 1

Examples of hexadecimal numbers include: 3FC2, CFF8, 92B0, EE4D, ACDC.

Note that the number 9375 could either be ordinary denary or hexadecimal to make it clear thesymbols h, # or & are often used. Thus, if the number was in hexadecimal, it would be writtenas 9375h, #9375 or &9375.

Converting from hexadecimal to denary

The hexadecimal number 2C5A can be converted into its denary equivalent as follows:

1. Write the hexadecimal number with the appropriate column headings:

4096 256 16 1

2 C 5 A

2. Noting that A 10 and C 12, convert in the same way as conversion from binary to denary:

= (4096 x 2) + (256 x 12) + (16 x 5) + (1 x 10)

= 11 354



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ADDITIONAL NOTES:

Converting from hexadecimal to binary

This uses the same method as octal to binary, except each hexadecimal digit is represented by 4-bits. Thus, the hexadecimal number B7C can be converted into a 12-bit binary as follows:

8 4 2 1 8 4 2 1 8 4 2 1

1 0 1 1 0 1 1 1 1 1 0 0

(B) (7) (C)

Converting from denary to hexadecimal

Convert to binary first and then to hexadecimal. For example, 462 can be converted as follows:

Convert to binary:

2048 1024 512 256 128 64 32 16 8 4 2 1

0 0 0 1 1 1 0 0 1 1 1 0

Convert each group of four into their hexadecimal equivalent:

8 4 2 1 8 4 2 1 8 4 2 1

0 0 0 1 1 1 0 0 1 1 1 0

1 12 (C) 14 (E)

= 1CE

Uses of hexadecimal

Hexadecimal is often used by Assembly language programmers to reference memory. It is also usedwithin HTML property values specifically background and font colours.

There are three advantages of using hexadecimal:

hexadecimal is quicker for a programmer to enter into a computer than binary; hexadecimal is easier for a programmer to understand and remember 8F8B is easier to

remember than 1000111110001011. it is very easy to convert between binary to hexadecimal.



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ADDITIONAL NOTES:

Summary

Expressing the denary number 195 as eight-bit binary, BCD, octal and hexadecimal:

Binary

128 64 32 16 8 4 2 1

1 1 0 0 0 0 1 1

BCD

8 4 2 1 8 4 2 1 8 4 2 1

0 0 0 1 1 0 0 1 0 1 0 1

Octal

512 64 8 1

0 3 0 3

or:

Pure Binary: 1 1 0 0 0 0 1 1

Group in threes: 0 1 1 0 0 0 0 1 1

Octal: 3 0 3

Hexadecimal

4096 256 16 1

C 3

or:

Pure Binary: 1 1 0 0 0 0 1 1

Group in fours: 1 1 0 0 0 0 1 1

Hexadecimal: C 3



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ADDITIONAL NOTES:

Negative binary numbers


27. Describe and use twos complement and sign and magnitudeto represent negative integers.

Twos compliment

Twos compliment is a method of representing negative numbers in binary, whereby the mostsignificant bit maintains its magnitude, but is made negative.

If, for example, one byte is used to represent a signed integer using the twos compliment method,the column headings would become:

128 64 32 16 8 4 2 1

Thus, the denary integer 18 would be represented as:

128 64 32 16 8 4 2 1

0 0 0 1 0 0 1 0

And the negative integer 18 would be represented as:

128 64 32 16 8 4 2 1

1 1 1 0 1 1 1 0

Notes:

negative numbers will always start with a 1 and positives will start will a 0; the range of integers that can be represented using one byte is from 128 up to + 127.

128 64 32 16 8 4 2 1

1 0 0 0 0 0 0 0 = 128

0 1 1 1 1 1 1 1 = + 127



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ADDITIONAL NOTES:

Converting a negative denary integer into twos complement

Due to the way that binary numbers work, there is an easy trick that allows very fast conversion.Taking the denary integer 52 as an example, you can use the three stages shown below:

Stage one

Convert the positive form of the number into unsigned binary:

128 64 32 16 8 4 2 1

0 0 1 1 0 1 0 0 = + 52

Stage two

Starting at the right hand side, copy each bit, up to and including the first 1:

128 64 32 16 8 4 2 1

0 0 1 1 0 1 0 0

1 0 0

Stage three

Reverse all the other bits:

128 64 32 16 8 4 2 1

0 0 1 1 0 1 0 0

1 1 0 0 1 1 0 0 = 52

This will always work even if you do not understand why!



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ADDITIONAL NOTES:

Converting a twos complement number into denary

This is the same as converting any binary number into denary, as long as you remember that themost significant bit is negative. For example the signed binary number 1 1 0 1 0 1 0 1 is convertedas follows:

128 64 32 16 8 4 2 1

1 1 0 1 0 1 0 1

= 128 + 64 + 16 + 4 + 1

= 43

Sign and magnitude

The alternative to using twos complement to represent negative numbers is to use the sign andmagnitude method here, the most significant bit is used as a sign bit without a numerical value.

64 32 16 8 4 2 1

1 1 0 0 1 1 0 0

= (64 + 8 + 4)

= 76

Notes: the range of integers that can be represented using one byte is from 127 up to + 127. although the sign and magnitude method is easier for humans it is much harder to use for

computers performing arithmetic.

SPOT CHECK

1. Assuming a single byte is used, convert the following numbers into twos compliment binary:

(a) 5 (b) 10 (c) 20

2. What is the denary value of 1010 1011 if the binary codes represent:

(a) a twos compliment number (b) a sign and magnitude number



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ADDITIONAL NOTES:

Binary arithmetic


28. Perform integer binary arithmetic, that is addition andsubtraction.

Addition

Computers will only ever add two numbers at a time if three numbers need to be added, acomputer will add the first two and then add the third number will be added to the result.

Since only two numbers are added at a time, there are limited outcomes:

0 + 0 = 0

0 + 1 = 1

1 + 0 = 1

1 + 1 = 2 (10 in binary this is 0 down and carry 1)

Note that when you add a carry to the next column, it is possible for:

1 + 1 + 1 (the carry) = 3 (11 in binary this is 1 down and carry 1)

This is better shown if we add 1010 1110 1100 and 0011 1010 1010:

1 0 1 0 1 1 1 0 1 1 0 0

0 0 1 1 1 0 1 0 1 0 1 0

Carried bits 1 1 1 1 1 1

1 1 1 0 1 0 0 1 0 1 1 0



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ADDITIONAL NOTES:

Subtraction

To perform subtraction, the number to be subtracted is converted into its twos complimentnegative and then added.

For example to subtract 12 from 25:

1. Convert the 12 into twos compliment 12

128 64 32 16 8 4 2 1

12 0 0 0 0 1 1 0 0

12 1 1 1 1 0 1 0 0

2. Now add this to the 25:

12 1 1 1 1 0 1 0 0

25 0 0 0 1 1 0 0 1

Carried bits 1 1 1 1

0 0 0 0 1 1 0 1

Note that there is still a carry bit, but this is ignored.

SPOT CHECK

1. Work out the following using binary addition and subtraction:

(a) 34 + 73 (b) 67 96



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ADDITIONAL NOTES:

Text (character) representation


29. Explain the use of code to represent a character set (ASCII,EBCDIC and UNICODE).

Character Set

A character set are the characters that can be recognised by a computer.

Character encoding

A computer is able to represent four types of characters:

alphanumeric characters lettersA Zand a zand the digits 0 9.

punctuation characters and other special symbols such as , . ; : ! @ $ % & * ( ) + < graphical characters such as , , , , ,,, ,, (and many more);

control characters [Return], [Esc], [Space], [Alt], etc.

Within a computer, each character is represented using a unique binary code. Although there aremany different methods of encoding the characters, three of the most common areASCII,EBCDICand Unicode.

ASCII

American Standard Code for Information Interchange (ASCII) is used for character encoding by

most Windows PCs. ASCII can be used to translate alphanumeric characters into a 7-bit binarycode that represents all the characters available from the keyboard including punctuation and somespecial symbols such as @, # and $:

A development of ASCII, known asExtended ASCII, uses an 8-bit code that also defines codes foradditional characters, including some graphical ones. Note that using an 8-bit code means themaximum number of characters that can be represented is 256.



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ADDITIONAL NOTES:

How character encoding works

The diagram below shows how the message Hello World is stored in the memory of a computerusing the ASCII codes:

The message is typed at the keyboard. Electronics in the keyboard convert the typed characters intoASCII binary codes that are sent from the keyboard along a cable to the computer. The computerstores these codes in its internal memory. The computer also provides a visual display of thecharacters as they are typed. To be able to do this, electronics inside the computer convert thestored binary codes back into their character equivalents.

EBCDIC

Extended Binary Coded Decimal Interchange Code (EBCDIC) was developed by IBM for use in

their mainframe systems. It has the same limitation as ASCII in that its 8-bit code can only define256 different characters.



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ADDITIONAL NOTES:

Notice how the EBCDIC codes are completely different to ASCII if a message was sent that hadbeen encoded using ASCII, but received by a system that used EBCDIC, then the resulting messagewould not make sense.

Unicode.

Unicode is an international system of representing characters using 16 bits. Using 16 bits meansthat 216 = 65 536 different characters can be represented (thus overcoming the limitation of ASCIIand EBCDIC).

Unicode allows every character from most alphabets to have a code of its own Chinese, Russian,Greek, Urdu etc, including Egyptian Hieroglyphics. Note that there are plenty of spare codes thatare used for mathematical symbols, common graphics and even the Braille symbols

Some of the Mongolian characters: Some of the Hebrew characters



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ADDITIONAL NOTES:

1.6 DATA CAPTURE

Standard input devices


30. Describe manual and automatic methods of gathering andinputting data into a system, including form design, keyboardentry, voice recognition, barcodes, optical mark recognition(OMR), optical character recognition (OCR), magnetic inkcharacter recognition (MICR), touch screens; image capture,chip and pin, sensors and remote data logging.

An input device is the hardware that is used to enter commands or data into a computer.

In many cases this requires the data to be converted into machine-readable form. Some input

devices such as a keyboard, mouse and flat-bed scanner require a human-user to be present to

control the input. Other devices such as sensors and optical mark readers can obtain data

automatically without the need for a human to be present. These latter devices are often referred toas data capture devices.

Keyboard

The keyboard remains the most common input device although in terms of speed, it is one of the

most limited. It is, however, suitable for entering a wide range of data and it is a device that is

familiar to every office worker. Every key on a keyboard has a switch underneath it when a key ispressed the switch is closed and a signal is sent to the computer. Keyboards are wired so that the

signal which is sent is determined by the row and the column in which the key is. Most keyboards

have the keys arranged in the same way as the once popular typewriter this arrangement is known

as QWERTY because it is the order of the first characters. This QWERTY layout was designed toreduce the risk of jamming on early mechanical typewriters by spreading the most commonly used

letters around the keyboard this effectively slowed down the typist. Improved layouts have beendesigned such as the Dvorak layout which rearranges the keys in an attempt to distribute typing

more evenly among the fingers of both hands. Using these improved layouts increases the speed at

which data can be entered. Such layouts have never become popular, however, because there has

been no general agreement on a standard layout and also because of the time that it takes to get used

to a new arrangement.Modern keyboards have in excess of 105 keys that

include 12 or more function keys, some of whichcan be programmed by the user. Other keys have

specialist functions that can be used for navigation

within a document, to copy an image of the screen,

to put the computer into energy saving mode or to



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ADDITIONAL NOTES:

display the Start Menu if using Microsoft Windows. Note that standard text entry using a

keyboard is usually between 20 and 60 words per minute (wpm), but a trained operator can reachspeeds in excess of 100 wpm.

Graphics Tablet

A graphics tablet is a specialist input device that is used to trace (or create original) drawings,pictures and designs into a computer. It consists of a flat tablet and a pen-like stylus. Software

detects the movement of the stylus on the tablets surface and the changes to the image areimmediately displayed upon the screen of the VDU. This provides an input device that is used in a

similar way to paper and pencil.

Light Pen

This is used in a similar way to a graphics tablet except the light pen is moved over the VDU screen

itself. The pen is triggered by the raster scan of the VDU it detects when the electron beam

building up the screen image has just passed the point where the pen is positioned. Knowing the

instant at which the beam passed the pen, the software can calculate whereabouts on the screen ithas been placed.

Touch Sensitive Screen

Touch sensitive screens are now often used as an alternative to a mouse. The user places their finger

(or a stylus device) on the screen and the position is detected the command/option that relates to

the screen display is then processed. One method of detecting the position of the figure is to have a

series of horizontal and vertical infrared beams directed across the screen. Position is calculated bydetecting which beams have been interrupted.

Touch sensitive screens are generally used for navigation in data display applications. Here the user

can select one of a limited number of menu/navigation choices by pointing at the screen.

These screens have started to become popular in restaurants, ATMs, London Underground ticketbooths and some British Telecom payphones. The Science Museum in London is now full of touch

screen terminals that are used to help visitors find the location of exhibits and also to provide them

with related information. They are particularly suited to this type of environment since they are

easily enclosed within a damage resistant casing.

Touch sensitive screens offer flexibility in that it is easy to redesign the screens whenimprovements or modifications are made for example an extra button can be added by modifying

the computer program that controls the interface. Systems can be made user-friendlier because thebuttons can contain relevant graphics as well as words.



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ADDITIONAL NOTES:

Scanner flatbed or hand-held

Scanners work by shinning a lightonto (or through) the material to be

scanned. The scanner then detects the

image by measuring the reflected (ortransmitted) light the most common

sensor used to detect the reflected light is the charge coupled

device (CCD) which is also used in digital cameras. When

using a flatbed scanner the image to be scanned is placed

upon a glass plate and a bright light source moves underneaththe glass. This is in essence the same as a photocopier. Some

flatbed scanners can be fitted with a transparency adapter sothat they can scan photographic negatives or slides.

Scanning technology has increased tremendously over the

last few years. A typical flatbed scanner can produce images

with a resolution of more than 1200 dpi. If a page of

Documents

57135443 a Level Computing 9691 Paper 1 Notes