COMPUTER SCIENCE SPECIMEN ASSESSMENT MATERIALS

GCE AS/A LEVEL

WJEC GCE AS/A LEVEL in

COMPUTER SCIENCE

ACCREDITED BY WELSH GOVERNMENT

This Welsh Government regulated qualiﬁcation is not available to centres in England.

Teaching from 2015

SPECIMEN ASSESSMENT

MATERIALS

GCE AS & A LEVEL COMPUTER SCIENCE Specimen Assessment Materials 1

GCE AS & A LEVEL COMPUTER SCIENCE Specimen Assessment Materials 3

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 5

Candidate Name

Centre Number

Candidate Number

0

AS COMPUTER SCIENCE

UNIT 1

FUNDAMENTALS OF COMPUTER SCIENCE

SPECIMEN PAPER

2 hours

ADDITIONAL MATERIALS

The use of a calculator is permitted in this examination.

INSTRUCTIONS TO CANDIDATES

Answer ALL question(s).

Write your name, centre number and candidate number in the spaces at the top of this

page.

Write your answers in the spaces provided in this booklet.

INFORMATION FOR CANDIDATES

The number of marks is given in brackets at the end of each question or part-question.

The total number of marks available is 100.

Assessment will take into account the quality of written communication used in your

answers.

No certificate will be awarded to a candidate detected in any unfair practice during the

examination.

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 6

Unit 1

Answer all questions

1. Discuss the differences between RAM and Cache memory. [6]

…………………………………………………………………………………………………

2. Describe the functional characteristics of two contemporary secondary storage

devices. [6]

…………………………………………………………………………………………………

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 7

3. (a) Describe the use of the DHCP and HTTP protocols. [2]

…………………………………………………………………………………………………

(b) Explain why the FTP protocol is unsuitable for streaming a live video feed.

Suggest a more suitable protocol for this purpose, justifying your choice. [6]

…………………………………………………………………………………………………

(c) Giving an example, explain the role of handshaking. [2]

…………………………………………………………………………………………………

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 8

4. The 8 bit binary number 00000001

2

is used in a masking process.

(a) State what logical operation is used during masking and draw a truth table for

this logical operation. [2]

…………………………………………………………………………………………………

(b) State the effect that the 8 bit binary number given in (4.) would have when

masked with any 8 bit binary number. [1]

…………………………………………………………………………………………………

5. Explain the terms serial transmission and parallel transmission in a computer system

and give one advantage of each type of transmission. [4]

…………………………………………………………………………………………………

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 9

6. Related data stored on a computer system can become fragmented over time.

(a) Explain what is meant by the term fragmentation, give one possible effect and

explain defragmentation. [4]

…………………………………………………………………………………………………

(b) Discuss the issues surrounding the defragmentation of a Solid State Drive

(SSD). [3]

…………………………………………………………………………………………………

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 10

7. Explain file backup, generations of files and transaction logs. [6]

…………………………………………………………………………………………………

8. (a) Using binary addition, calculate the number that would result from adding

00110110

2

and 00101110

2

.

Convert the result into hexadecimal. [2]

…………………………………………………………………………………………………

(b) Assuming that 1 is used to indicate a negative number, show how the

negative number -12

10

will be represented using sign/magnitude in an 8 bit

register. [1]

…………………………………………………………………………………………………

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 11

(c) Integers can also be represented using two’s complementation.

Using the binary number 00001011

2

as an example, explain how two’s

complement is derived. [3]

…………………………………………………………………………………………………

(d) (i) Give a disadvantage of using floating point form rather than integer

form. [1]

………………………………………………............………………………………

(ii) Real numbers stored in floating point form can be stored using 16 bits

as shown below:

Mantissa

(12 bits in two’s complement form. The binary

point in the mantissa is immediately after the

left bit.)

Exponent

(4 bits in two’s

complement form.)

Convert the number 23.75

10

into this floating point form. [2]

………………………………………………............………………………………

(iii) In a different computer system, the following is a floating point

representation of a number, using an 8 bit mantissa and a 4 bit

exponent:

0•1011000

2

0101

2

Calculate the mantissa, exponent and decimal equivalent of the

number. [3]

………………………………………………............………………………………

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 12

9. (a) Complete the following Boolean expression to illustrate the distributive law.

[1]

   

…………………………………………………………………………………………………

(b) Clearly showing each step, simplify the following Boolean expression. [5]

   



  



    

…………………………………………………………………………………………………

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 13

10. Below is an algorithm that determines whether a positive whole number greater than

2, input by a user, is a prime number or not.

1 Num is Integer

2 Divisor is Integer

3 Prime is Boolean

4 startmainprog

5 set Prime = TRUE {initialise variables}

6 set Divisor = 2

7 output “type in a number”

8 input Num

9

10 repeat

11 if Num MOD Divisor = 0 then

12 set Prime = FALSE

13 endif

14 set Divisor = Divisor + 1

15 until (Prime = FALSE) OR (Divisor = Num)

16

17 if Prime = TRUE then

18 output Num, “is a prime number”

19 else

20 output Num, “is NOT a prime number”

21 Endif

22

23 Endmainprog

(a) Name the logical operator used in the algorithm: [1]

…………………………………………………………………………………………………

(b) Give an example of selection from the algorithm above and explain its

purpose. [2]

…………………………………………………………………………………………………

(c) Give an example of repetition from the algorithm above and explain its

purpose. [2]

…………………………………………………………………………………………………

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 14

(d) Give two examples of test data that would test that the algorithm on page 13

works as intended. [2]

…………………………………………………………………………………………………

11. Customers can obtain quotations for car insurance via email by completing an on-line

application form. Verification and validation checks are carried out on the data input.

(a) The customer has to create an account so that the insurance quotation can

be stored and retrieved at a later date. As part of this process, the customer

has to input a password which is verified.

Describe one method of verification that could be applied to the password.

[1]

…………………………………………………………………………………………………

(b) The number of whole years since the driver made an insurance claim is

validated. Excluding a presence check, describe a suitable validation check

that would be carried out on the number of whole years giving an example of

invalid data that would be detected by this check. [2]

…………………………………………………………………………………………………

(c) Describe a different suitable validation check that could be carried out on the

customer’s email address giving an example of invalid data that would be

detected by this check. [2]

…………………………………………………………………………………………………

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 15

12. A weather station records monthly rainfall figures in millimetres (mm) for a year.

Write an algorithm, using pseudo-code or a high level programming language, which

will use these twelve monthly rainfall figures as input. The program should output:

 the total rainfall for the year

 the mean monthly rainfall for the year

 the month numbers (1 for January, etc) where the rainfall was above the mean.

Your algorithm should contain meaningful identifiers. [6]

…………………………………………………………………………………………………

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 16

13. (a) Before new computer systems are introduced in an organisation, they are

tested to ensure they meet the intended requirements. Describe the different

types of system testing that will typically be carried out on the system.

[3]

…………………………………………………………………………………………………

(b) Following the installation of the new system, it will need to be maintained.

Giving suitable examples, discuss the different types of system maintenance

that will need to be carried out on the system. [3]

…………………………………………………………………………………………………

14. Identify and describe the principal stages involved in the compilation process. [8]

…………………………………………………………………………………………………

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 17

15. Explain lossy and lossless data compression techniques. [8]

…………………………………………………………………………………………………

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 18

Continuation page

…………………………………………………………………………………………………

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 19

Continuation page

…………………………………………………………………………………………………

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 20

Candidate Name

Centre Number

Candidate Number

AS COMPUTER SCIENCE

UNIT 2

Practical Programming to Solve Problems

SPECIMEN PAPER

2 hours

ADDITIONAL MATERIALS

In addition to this examination paper, you will need a 16 page answer book.

INSTRUCTIONS TO CANDIDATES

Answer ALL questions.

The question paper is divided into four main sections. Below are the recommended timings

for this assessment.

Section A

You are advised to spend no more than 60 minutes on this section.

You will be required to analyse and deconstruct the scenario so as to consider its

component parts in terms that can be addressed through automated computation.

Section B

You are advised to spend no more than 60 minutes on this section.

Questions will require you to develop programs that solve computing problems referring to

the initial scenario.

INFORMATION FOR CANDIDATES

The number of marks is given in brackets at the end of each question or part-question.

The total number of marks available is 60.

Assessment will take into account the quality of written communication used in your

answers.

You will need a computer with an installed functional copy of the Integrated Development

Environment (IDE) appropriate to your chosen programming language and word processing

software.

Remember to save your work regularly.

No certificate will be awarded to a candidate detected in any unfair practice during the

examination.

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 21

Scenario

FITNESS

LEISURE CENTRE

Fitness Leisure Centre is an established organisation based in Trefforest, South Wales.

The leisure centre has a large number of clients that take part in various activities on a

regular basis.

The leisure centre wishes to offer its clients the facility of being able to track their activities

using a computerised training log.

This new system will store the clients’ details along with details of the different activities that

are available, such as swimming, gym, aerobics, badminton, squash. When a client takes

part in one of the activities they will be able to keep a record of this in their training log on the

new system. The details stored in the training log include Activity ID, Client ID, Date, Time,

Duration, Number of calories burnt, etc.

The main requirements of the new computer system for Fitness Leisure Centre are:

 The ability to store and search clients details

 The ability to store activity details and search for activities using various preferences,

e.g. search for all “swimming classes”

 The ability to store a training log for each client

Fitness Leisure Centre has the following partially completed entity-relationship diagram as

an overview of the system they would like to create:

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 22

Section A: Analysis and Design (44 marks)

Answer all questions.

You have been asked to analyse the scenario as a preliminary step towards creating a

computer system for Fitness Leisure Centre.

Answer all questions. Present your answers as a single word-processed document

named Section A Analysis and Design.

1. Draw two data structure tables that will allow Fitness Leisure Centre to store client

and activity details. [10]

2. Copy and complete the entity relationship diagram provided in the scenario. [6]

3. Construct a flow chart for the process of searching for a specific activity, e.g.

Swimming classes. [6]

4. Select and fully justify your proposed method of solution for the three main

requirements listed in the scenario.

[6]

5. Consider the different methods of changeover available to Fitness Leisure Centre.

Your answer should be related to the impact of these methods on clients and staff.

[8]

6. Fitness Leisure Centre is concerned that their clients will be shown duplicate results

on screen when searching for activities.

Using a recognised convention, design and implement an algorithm that performs a

search for duplicate consecutive activities in an array. If a duplicate is found, the

algorithm should output the location of the duplicate and the word “TRUE”. If a

duplicate is not found, the algorithm should output the word “FALSE”. [8]

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 23

Section B: Develop programs (16 marks)

Select the programming language of your choice from section BI, BII or BIII and

answer all 3 questions within your chosen section.

BI Visual Basic

Fitness Leisure Centre wants a computer system to be developed using Visual Basic

that meets the requirements outlined in the scenario.

1. Open the file Activities.frm

 Read through the code and familiarise yourself with its contents

 The file contains incomplete code that displays the activities available to clients.

Complete this code.

Remember to save the changes made to the file Activities.frm [4]

2. Create a new form that will allow Fitness Leisure Centre to:

 Input customer details

 Validate customer details

 Store customer details on disc in a text file called customerdetails.txt

 Retrieve specified customer details from disc.

Save your new form as Customers.frm [8]

3. Using the internal facility of your chosen language, add annotated listings to your

code from question 2 that would clearly explain the design of your program to

another software developer.

Remember to save the changes made to the file Customers.frm [4]

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 24

BII Java

Fitness Leisure Centre wants a computer system to be developed using Java that

meets the requirements outlined in the scenario.

1. Open the file Activities.java

 Read through the code and familiarise yourself with its contents

 The file contains incomplete code that displays the activities available to clients.

Complete this code.

Remember to save the changes made to the file Activities.java [4]

2. Create a new form that will allow Fitness Leisure Centre to:

 Input customer details

 Validate customer details

 Store customer details on disc in a text file called customerdetails.txt

 Retrieve specified customer details from disc.

Save your new form as Customers.java [8]

3. Using the internal facility of your chosen language, add annotated listings to your

code from question 2 that would clearly explain the design of your program to

another software developer.

Remember to save the changes made to the file Customers.java [4]

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 25

BIII Python

Fitness Leisure Centre wants a computer system to be developed using Python that

meets the requirements outlined in the scenario.

1. Open the file Activities.py

 Read through the code and familiarise yourself with its contents

 The file contains incomplete code that displays the activities available to clients.

Complete this code.

Remember to save the changes made to the file Activities.py [4]

2. Create a new form that will allow Fitness Leisure Centre to:

 Input customer details

 Validate customer details

 Store customer details on disc in a text file called customerdetails.txt

 Retrieve specified customer details from disc.

Save your new form as Customers.py [8]

3. Using the internal facility of your chosen language, add annotated listings to your

code from question 2 that would clearly explain the design of your program to

another software developer.

Remember to save the changes made to the file Customers.py [4]

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 27

Candidate Name

Centre Number

Candidate Number

0

A LEVEL COMPUTER SCIENCE

UNIT 3

PROGRAMMING AND SYSTEM DEVELOPMENT

SPECIMEN PAPER

2 hours

ADDITIONAL MATERIALS

In addition to this examination paper, you will need a 16 page answer book..

The use of a calculator is permitted in this examination.

INSTRUCTIONS TO CANDIDATES

Answer ALL question(s).

Write your answers in the separate answer book provided.

Write your name, centre number and candidate number in the spaces at the top of this

page.

INFORMATION FOR CANDIDATES

The number of marks is given in brackets at the end of each question or part-question.

The total number of marks available is 100.

Assessment will take into account the quality of written communication used in your

answers.

No certificate will be awarded to a candidate detected in any unfair practice during the

examination.

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 29

Unit 3

Answer all questions

1. A binary tree can be constructed using the following rules:

Rule 1. The first item becomes the root node;

Rule 2. Items earlier or at the same position in the alphabet follow the left pointer;

Rule 3. Items later in the alphabet follow the right pointer.

(a) Draw a representation of a dynamic binary tree with pointers using the

following data:

Narberth, Cardiff, Pontypridd, Wrexham, Rhyl, Bangor, Denbigh [2]

(b) Show how the above tree could be represented using a two dimensional

array. [3]

(c) Make the following amendments to the original tree:

(i) Insert Pontardawe and Tonypandy into the tree; [2]

(ii) Delete Cardiff and Pontypridd from the tree. [3]

(d) Giving examples, compare a balanced and an un-balanced binary tree and

evaluate their effectiveness to solve problems by comparing the maximum

number of comparisons to locate an item in each of these trees. [4]

2. (a) Explain what is meant by the term linked list. Describe one benefit and one

drawback of using a linked list compared with using an array. [4]

(b) In a certain implementation, a linked list of integers is actually stored in a

table form as shown below. The integers are to be accessed in ascending

numerical order. A variable points to the address 752, which contains the

lowest integer, 2312.

Copy and complete the pointer column in the table below.

Address

Integer

Pointer

751

4811

752

2312

753

3599

754

4166

755

2567

756

5218

757

3100

[3]

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 30

3. In the programming industry, professional Codes of Conduct are used extensively

and they set rules that should be followed by programmers.

State six reasons why programmers are required to follow rules under a professional

Code of Conduct. Each reason must relate to a specific rule. [6]

4. (a) Using the laws of Boolean algebra, simplify the following Boolean expression.

[3]



  



   





(b) Using the laws of Boolean algebra and De Morgan’s theorem, simplify the

following expression. [3]

  



(c) Simplify the following Boolean expression. [3]

 



   





 



5. Two 8 bit numbers are:

X 10011011

2

Y 11010111

2

Explain how the XOR operation can be used for encrypting data, using the two

numbers above to illustrate your answer. You should also show how the original

data can be recovered. [3]

6. Write an algorithm that performs a binary search for SearchValue in an array of

size n called SearchArray. Your algorithm should output the position of the

SearchValue if found or an error message if the SearchValue is not found. [7]

7. Name the the type of algorithm shown below. Give two features of this type of

algorithm. Name a sort which uses this type of algorithm. [4]

function CalcF (Num: integer) :

integer

if Num = 1

then CalcF = 1

else CalcF = Num * CalcF(Num-1)

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 31

8. (a) Describe the purpose of Backus-Naur Form (BNF). [1]

(b) A warehouse stores a large number of mobile phone components. Each

component has a component code consisting of 6 digits and a description.

The description is made up of upper case letters only and may be of any

length.

Examples of valid codes are:

621402DIGITISER

506742USBPORT

(i) Produce an appropriate BNF definition for a component code. [4]

(ii) Produce an appropriate syntax diagram for a component code. [3]

9. Below is an algorithm that sorts data stored in an array.

Algorithm sort

Declare A(1 to n)

n = length(A)

for i = 1 to n

for j = 1 to n-1 inclusive do

if A[i-1] > A[i] then

swap( A[i-1], A[i] )

end if

next j

next i

(a) Determine how many comparisons are made when n is equal to:

10

100

1000 [1]

(b) Evaluate the efficiency of the algorithm and, using Big O notation, determine

the growth rate for the time performance. Your answer should refer to the

comparisons performed by the algorithm. [4]

(c) Using Big O notation, determine the growth rate of memory space used by

this algorithm. [2]

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 32

10. Describe the Waterfall and Agile approaches to systems analysis including their

advantages and disadvantages. [6]

11. A software development company specialises in developing computerised automatic

car braking systems.

(a) When the company develops such a system, a design validation is carried

out. Describe how a design validation should be carried out. [3]

(b) Explain why version control is important when developing computer

programs. [1]

12. (a) Describe the purpose of compilers, interpreters and assemblers. [3]

(b) Describe and give examples of three common translation and execution

errors.

[9]

13. Discuss the nature of different programming paradigms. [13]

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 33

Candidate Name

Centre Number

Candidate Number

0

A LEVEL COMPUTER SCIENCE

UNIT 4

COMPUTER ARCHITECTURE, DATA, COMMUNICATION AND

APPLICATIONS

SPECIMEN PAPER

2 hours

ADDITIONAL MATERIALS

In addition to this examination paper, you will need a 16 page answer book.

The use of a calculator is permitted in this examination.

INSTRUCTIONS TO CANDIDATES

Answer ALL question(s).

Write your answers in the separate answer book provided.

Write your name, centre number and candidate number in the spaces at the top of this

page.

INFORMATION FOR CANDIDATES

The number of marks is given in brackets at the end of each question or part-question.

The total number of marks available is 100.

Assessment will take into account the quality of written communication used in your

answers.

No certificate will be awarded to a candidate detected in any unfair practice during the

examination.

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 35

Unit 4

Answer all questions

1. (a) Explain the sequence of operations which will occur during the fetch phase of

the fetch execute cycle, making clear the role of the PC (program counter),

the MAR (memory address register) and the MDR (memory data register).

[3]

(b) In a certain computer, the following assembly language commands are

available.

Assembly

command

Description

STR R,X

Stores contents of register R in

address X

DEC R

Decrement register R

CLR R

Clears the contents of register

R

LSL R

Performs a logical shift left o

register R

LOD R,X

Loads register R with the

contents of address X

ADD R, S

Adds the contents of register R

to the contents of register S

Write a simple program using the assembly language commands in the table

above to demonstrate how two numbers can be added together and stored in

a register. [3]

2. Explain the terms indexed sequential file and multilevel index. Draw a diagram to

demonstrate the operation of a three-level index. [6]

3. Discuss random access files, making reference to:

 the purpose of a hashing algorithm

 the need for an overflow area

 the need for the random access file to be re-organised on occasions. [6]

4. Produce a forwarding table for node A, calculating the lowest cost routes on the

network below.

You may assume traversal of a node costs zero. [3]

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 36

5. Wales-Cycles owns a number of bicycle shops which sell bicycles, accessories, etc.

The table below shows some of items which it has for sale in its various shops,

together with other information such as manufacturer details.

Item

Code

Item Name

Shop

Code

Shop

Address

Shop

Manager

Number

In

Stock

Manuf

Code

Manuf

Name

Manuf

Contact

B2347

Smart Bike

S174

Manchester

Davies

4

M68

Dragon

Phillips

B2347

Smart Bike

S162

Bolton

Roggers

2

M68

Dragon

Phillips

A7219

Deluxe

Hemlet

S162

Bolton

Roggers

12

M17

Safe

Heads

Elhami

C2391

Peddles

S115

Stockport

Green

8

M68

Dragon

Phillips

C3844

Peddles

S162

Bolton

Roggers

5

M22

Fast

Bikes

Kazembe

A1955

Bright Jacket

S201

Rochdale

Williams

22

M61

Nite

Safe

Earley

A1955

Bright Jacket

S174

Manchester

Davies

16

M61

Nite

Safe

Earley

The data in the table above has not been normalised.

(a) Using examples from the table, describe two problems which result when

data has not been normalised. [4]

(b) Restructure the table shown above into third normal form.

There is no need to copy the data items. [6]

(c) Describe why distributed databases are often used and identify one difficulty

associated with using distributed databases. Explain what is actually

distributed in a distributed database. [3]

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 37

6. Two tables have been created in a database using SQL commands. They are:

PUPIL

PupNum

PupName

DateBirth

TeachNum

12675

Brown

02-Nov-96

307

13670

Jones

01-Jun-95

378

13777

Walker

23-Oct-95

307

14156

Vaas

29-Feb-96

307

14238

Thomas

18-Jan-95

378

TEACHER

TeachNum

TeachName

RoomNum

307

Kholi

106

345

Lester

113

378

Davies

113

(a) Write an SQL command to output the names and teacher numbers of all the

pupils. [1]

(b) Write an SQL command to output the names of pupils who have the teacher

with number 307. [1]

(c) Write an SQL command to change the teacher number of all those pupils

whose teacher number is currently 378. Their new teacher number should be

345. [2]

(d) Write an SQL command to output the names of all pupils who have the same

teacher as pupil 14238. [2]

(e) Write an SQL command to create a new table COMPUTER to contain the

serial numbers of each computer at the school (assume there is only one

computer per room). [2]

(f) Write an SQL command to enter the following data into the new table.

 Room 106 should have a computer with serial number 13457

 Room 113 should have a computer with serial number 66870 [2]

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 38

A certain computer uses two complementation and an 8 bit register.

7. (a) (i) Showing all your working, calculate the result of the binary addition of

–7

10

and 4

10.

[2]

(ii) Showing all your working, calculate the result of the binary subtraction

of 5

10

from 12

10.

[3]

(b) (i) Real numbers stored in floating point form can be stored using 16 bits

as shown below:

Mantissa

(12 bits in two’s complement form. The binary

point in the mantissa is immediately after the

left bit.)

Exponent

(4 bits in two’s

complement form.)

Convert the number 5.625

10

into this floating point form. Your final

answer should be normalised. [3]

(ii) In a different computer system, the following is a floating point

representation of a number, using an 8 bit mantissa and a 4 bit

exponent:

0•1111000

2

0011

2

Calculate the mantissa, exponent and decimal equivalent of the

number. [3]

(iii) In the same computer system as Question 7(b)(ii), the closest possible

representation of the denary number 6.9

10

is shown below.

0•1101110

2

0011

2

There can be a loss of accuracy when a denary number is stored

using floating point representation.

Calculate the absolute and relative error that has occurred above and

explain how the floating point system used could be modified to allow

a more accurate representation of 6.9

10

. [6]

(c) (i) Calculate the effect of carrying out an arithmetic shift left by two

places on the eight-bit positive integer 00001111

2

and state the effect

of this operation on the number. [2]

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 39

(ii) If an arithmetic shift left by two places was carried out on the eight-bit positive

integer 01001111

2

, a problem would arise. Name and describe the problem.

[2]

8. Explain what is meant by Big Data and data mining. Giving an example, explain

what is meant by predictive analytics. [4]

9. (a) An operating system on a large computer uses multiprogramming. Describe

the main features of a multiprogramming operating system. [8]

(b) (i) Explain what is meant by the term computer interrupt and describe

three conditions or events which could generate interrupts. [4]

(ii) Describe high priority interrupt handling. [3]

10. A new suspended railway system uses driverless trains. A computer system controls

the movement of trains between stations. Discuss the possible benefits of this

system. [3]

11. Explain the need for and the purpose of cryptography. [3]

12. Explain the limiting factors to parallelisation in parallel processing. [10]

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 41

UNIT 1

MARK SCHEME

Guidance for examiners

Positive marking

It should be remembered that learners are writing under examination conditions and credit

should be given for what the learner writes, rather than adopting the approach of penalising

him/her for any omissions. It should be possible for a very good response to achieve full

marks and a very poor one to achieve zero marks. Marks should not be deducted for a less

than perfect answer if it satisfies the criteria of the mark scheme.

For questions that are objective or points-based the mark scheme should be applied

precisely. Marks should be awarded as indicated and no further subdivision made.

For band marked questions mark schemes are in two parts.

Part 1 is advice on the indicative content that suggests the range of computer science

concepts, theory, issues and arguments which may be included in the learner's answers.

These can be used to assess the quality of the learner's response.

Part 2 is an assessment grid advising bands and associated marks that should be given to

responses which demonstrate the qualities needed in AO1, AO2 and AO3. Where a

response is not credit worthy or not attempted it is indicated on the grid as mark band zero.

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 42

Banded mark schemes

Banded mark schemes are divided so that each band has a relevant descriptor. The

descriptor for the band provides a description of the performance level for that band. Each

band contains marks.

Examiners should first read and annotate a learner’s answer to pick out the evidence that is

being assessed in that question. Once the annotation is complete, the mark scheme can be

applied.

This is done as a two stage process.

Stage 1 – Deciding on the band

When deciding on a band, the answer should be viewed holistically. Beginning at the lowest

band, examiners should look at the learner’s answer and check whether it matches the

descriptor for that band. Examiners should look at the descriptor for that band and see if it

matches the qualities shown in the learner’s answer. If the descriptor at the lowest band is

satisfied, examiners should move up to the next band and repeat this process for each band

until the descriptor matches the answer.

If an answer covers different aspects of different bands within the mark scheme, a ‘best fit’

approach should be adopted to decide on the band and then the learner’s response should

be used to decide on the mark within the band. For instance if a response is mainly in band

2 but with a limited amount of band 3 content, the answer would be placed in band 2, but the

mark awarded would be close to the top of band 2 as a result of the band 3 content.

Examiners should not seek to mark candidates down as a result of small omissions in minor

areas of an answer.

Stage 2 – Deciding on the mark

Once the band has been decided, examiners can then assign a mark. During standardising

(marking conference), detailed advice from the Principal Examiner on the qualities of each

mark band will be given. Examiners will then receive examples of answers in each mark

band that have been awarded a mark by the Principal Examiner. Examiners should mark the

examples and compare their marks with those of the Principal Examiner.

When marking, examiners can use these examples to decide whether a learner’s response

is of a superior, inferior or comparable standard to the example. Examiners are reminded of

the need to revisit the answer as they apply the mark scheme in order to confirm that the

band and the mark allocated is appropriate to the response provided.

Indicative content is also provided for banded mark schemes. Indicative content is not

exhaustive, and any other valid points must be credited. In order to reach the highest bands

of the mark scheme a learner need not cover all of the points mentioned in the indicative

content but must meet the requirements of the highest mark band. Where a response is not

creditworthy, that is contains nothing of any significance to the mark scheme, or where no

response has been provided, no marks should be awarded.

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 43

Q

Answer

Mark

AO1

AO2

AO3

Total

1

RAM (any three of)

 it has fast read and write access and is volatile

 used to store data and currently running programs

 RAM is needed because most data on computers is

stored in much slower "storage media" such as hard

disks, solid state drives or flash memory

 For the processor to be able to work on data or run

programs at any reasonable speed, the programs or

data need to be copied into RAM first.

Cache memory (any three of)

 is similar to RAM, except it resides on or close to the

CPU

 is faster than RAM and is also volatile

 used to store frequently used data from main memory

 used by the processor to avoid having to slow down to

the speed of the RAM all the time

 usually quite low-capacity (a few megabytes), so RAM

is still needed in order to avoid constantly accessing

things from slow storage media.

3

1.1b

6

2

3 marks for functional characteristics of each device x 2

External hard disc drive:

 Speed of access – Very fast data transfer, only flash

memory is faster

 Cost per unit of storage – external hard disc is quite

cheap per byte of storage

 Portability reason – external hard disc is physically

quite small and can be easily stored securely and

safely for example in a fire proof safe

Third party storage provider:

 Speed of access – very fast transfer achievable

(depending on network speed)

 Cost per unit of storage – could be cheaper or more

expensive than external disk – accept either with

justification

 Data is stored securely and safely on protected

servers

Flash memory drive:

 Speed of access – Very fast transfer which is

important for daily updates

 Cost per unit of storage – pen drive is quite cheap

 Portability reason – pen drive is physically small and

can be easily stored securely and safely for example

in a fire proof safe

Magnetic tape / tape streamer:

 Speed of access reason – Access to tape is serial and

can be slow but could only back up files amended that

day

 Cost per unit of storage reason – Tape is relatively

6

1.1b

6

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 44

Q

Answer

Mark

AO1

AO2

AO3

Total

(but drive can be expensive) cheap compared with

other secondary storage mediums

 Portability reason – Tape is physically small and can

be easily stored securely and safely for example in a

fire proof safe

3a

DHCP - assigning dynamic IP addresses to devices on a

network

HTTP - transferring multimedia web pages over the

Internet.

1

1.1b

2

3b

 The file transfer protocol, breaks data into packets and

can re-send lost or damaged packets

 it allows packets that have arrived in a random order

to be reassembled

 This is convenient for downloading files if network

traffic is slows or some of your packets are dropped /

arrive out of order

 However, the FTP protocol won't work as well for

streaming media as it is more important to continue to

receive new packets rather than retransmitting lost or

dropped packets

 Voice and video traffic is can be transmitted using

UDP

 Real-time video and audio streaming protocols are

designed to handle occasional lost packets, so only

slight degradation in quality occurs, rather than large

delays if lost packets were retransmitted

1

2.1a

6

3c

The exchange of signals between devices to establish

their readiness to communicate.

Example: Establishing a printers readiness to print

1

1.1b

2.1a

2

4a

AND

Input (A)

Input (B)

Output (A AND B)

0

1

0

1

0

1

1.1a

2.1a

2

4b

Any one of:

 Picks out / produces right bit / least significant bit

(which is 1)

 Determines whether right bit / least significant bit is a 0

or 1

1

2.1a

1

5

Serial transmission: data is sent one bit at a time along

the same data line

Advantage (any one of:)

 requires only two wires compared with 8 or 16 in

parallel

 serial can travel longer distances than parallel

 simpler interface / circuit board / fewer lines required

1

1.1a

1.1b

4

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 45

Q

Answer

Mark

AO1

AO2

AO3

Total

Parallel transmission: all bits in a byte are sent

simultaneously along separate lines

Advantage

 transmission is faster than serial transmission

1

1.1a

1.1b

6a

Fragmentation: related data is split and stored on different

parts of the disc.

If data is fragmented, it takes longer for the disc heads to

move between parts of the file, which slows the process of

loading it.

Defragmentation is the process where files are physically

re-arranged on disk so that they are no longer fragmented

and the parts of each file are stored together.

This improves the speed of accessing data from disk.

1

1.1b

4

6b

Any three of:

 SSD uses direct access to data (files) so there would

be no improvement in read times as there’s no

physical read-head to move

 Defragmentation may perform “trim” command which

may slightly improve the speed of future write

operations

 SSD is currently made out NAND based flash memory

 NAND based flash memory has a limited lifespan –

defragmentation process may shorten its lifespan.

3

1.1b

3

7

Backup

 Backup is a redundant copy of files, usually stored

separately from the original system

 It can be used to recover data in the event of

catastrophic failure of the original storage media

Generations of files

 A generation file backup system involves storage of

several of the most recent versions of a master file

Accept grandfather-father-son method

 Useful if one version is corrupted: the previous

version(s) is still available

Transaction logs

 A transaction log is used with on-line updating - stores

all the update data

 It can be used in case of failure - could restore data by

being combined with previous master/backup file, with

minimal data loss.

1

1.1b

6

8a

00110110

2

00101110

2

+

01100100

Hexadecimal value = 64

16

1

2.1a

2

8b

10001100

2

1

2.1a

1

8c

11110101

2

One method is:

 From RHS, rewrite it up to and including the first one

1

1.1b

2.1a

3

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 46

Q

Answer

Mark

AO1

AO2

AO3

Total

 Change other 1 digits to 0 and 0 digits to 1

Alternatively

 Flip the bits

 Add one

 (Ignore carry (ninth bit))

(Other methods equally acceptable)

1

1.1b

8di

Any one of:

 are not normally stored accurately

 require more complex processing

 no exact representation of zero

1

1.1b

1

8dii

000101111100

2

0101

2

1 for correct mantissa, 1 for correct exponent

2

2.1a

2

8diii

 Mantissa = 0.6875

10

or 11/16, Exponent = 5

10

 Answer = Mantissa x 2

exponent

 Answer = 22

10

1

2.1a

3

9a

A.(B + C)

(A.B) + (A.C)

1

2.1a

1

9b

A.B + A.(B + C) + B.(B + C)

A.B + A.B + A.C + B.B + B.C

A.B + A.B + A.C + B + B.C

A.B + A.C + B + B.C

A.B + A.C + B

B + A.C

1

2.1a

5

10a

OR

1

2.1a

1

10b

Example

24 if Num MOD Divisor = 0 then

25 set Prime = FALSE

26 endif

OR

17 if Prime = TRUE then

18 output Num, “is a prime number”

19 else

20 output Num, “is NOT a prime

number”

21 Endif

The purpose of selection is to execute code if a certain

condition is met.

1

1.1b

2.1a

2

10c

Example

10 repeat

11 if Num MOD Divisor = 0 then

12 set Prime = FALSE

13 endif

14 set Divisor = Divisor + 1

15 until (Prime = FALSE) OR (Divisor = Num)

The purpose of repetition is to repeatedly execute code

until a certain condition is met.

1

1.1b

2.1a

2

10d

 correct prime numbers and output “ “x is a prime

number” e.g. “3 is a prime number”

 correct non-prime number and output “x is NOT a

prime number” , e.g. “ 4 is not a prime number”

1

3.1c

2

11a

Ask customer to type password twice and compare both

1

2.1a

1

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 47

Q

Answer

Mark

AO1

AO2

AO3

Total

inputs to check that they are the same

11b

1 mark for check correctly described

1 mark for each example of invalid data that the check

described would detect

Suitable checks

Example of

invalid data

Range check – entries between sensible

limits, e.g. 0-60

-1 or 74

Type check – all entries should be

integer

B or #

NOTE - Do not accept length check, format check or look

up check and examples of invalid data must follow

check described

1

2.1a

2

11c

1 mark for check correctly described

1 mark for each example of invalid data that the check

described would detect

Suitable checks

Example of

invalid data

Format check - email address has a

string@string.string

abczyz$em

ail.co.uk

NOTE - Do not accept length check, type check or look up

check and examples of invalid data must follow

check described

1

2.1a

2

12

1 declare Rainfall array (1..12) of

integer

2 set Total = 0

3

4 for Count = 1 to 12

5 input Rainfall(Count)

6 set Total = Total + Rainfall(Count)

7 endfor

8

9 set Mean = Total / 12

10

11 output "Total = ", Total

12 ouput "Mean = ", Mean

13

14 output "Months above Mean = "

15 for Count = 1 to 12

16 if Rainfall(Count) > Mean then

17 output Count

18 Endfor

Marking

 Declare array and initialise variables

 Input loop structure + increment

 Calculate mean

 Output Total and Mean

 Output loop structures

 Detect and output above mean months

1

3.1b

6

13a

 Alpha testing – when software is issued to a restricted

1

1.1b

3

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 48

Q

Answer

Mark

AO1

AO2

AO3

Total

audience of testers within the developer's own

company

 Beta testing - when a version is released to a number

of people external to the company e.g. privileged

customers in exchange for their constructive

comments

 Acceptance testing - when testing is carried out to

prove to the customer / end user that the system

works correctly.

1

1.1b

13b

 Perfective - is when the performance/functionality of

the program has to be enhanced

 Adaptive – is when the program has to be altered e.g.

to run on a different operating system

 Corrective – is while the program is being used and an

error is discovered and corrected

1

1.1b

3

14

For each stage, 1 mark for each bullet point up to a

maximum of 2 marks

No marks for simply naming stages

Lexical analysis

 Comments and unneeded spaces are removed

 Keywords, constants and identifiers are replaced by

'tokens'

 A symbol table is created which holds the addresses

of variables, labels and subroutines

Syntax analysis

 Tokens are checked to see if they match the spelling

and grammar expected, using standard language

definitions. This is done by parsing each token to

determine if it uses the correct syntax for the

programming language.

 If syntax errors are found, error messages are

produced

Semantic analysis

 Variables are checked to ensure that they have been

properly declared and used

 Variables are checked to ensure they are of the

correct data type, e.g. real values are not being

assigned to integers

 Operations are checked to ensure that they are legal

for the type of variable being used e.g. you would not

try to store the result of a division operation as an

integer

Code generation

 Machine code is generated

 Code optimisation may be employed to make it more

efficient / faster / less resource intense

8

1.1b

8

15

Indicative content

 Data compression reduces the file size

Lossy data compression

 Compressed files can never be recovered exactly as

8

1.1b

8

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 49

Q

Answer

Mark

AO1

AO2

AO3

Total

they were before they were compressed

 When compressed files are decompressed they do not

give back the original data, i.e. data is lost

 Because lossy compression cannot be decompressed

to yield the exact original data, it is not a good method

of compression for critical data, such as textual data

 It is most useful for digitally sampled analogue data,

such as sound, video, graphics or images

 Algorithms for lossy compression vary, but many use

a threshold level truncation. This means that a level is

chosen past which all data is truncated, e.g. in a

sound file, the very high and low frequencies, which

the human ear can not hear, may be truncated from

the file

 Some examples of lossy data compression algorithms

are JPEG, MPEG, and MP3.

Lossless data compression

 The original message can be decompressed back to

its original form (recovers all original data)

 Lossless data compression works by finding repeated

patterns in data and compressing those patterns in an

efficient manner. For this reason, lossless data

compression is also referred to as redundancy

reduction. Becuase redundancy reduction is

dependent on patterns in the message, it does not

work well on random messages. Lossless data

compression is ideal for text. Most of the algorithms

for lossless compression are based on the LZ

compression method developed by Lempel and Ziv.

 One type of text encoding which is very effective for

files with long strings of repeating bits is RLE. RLE

stands for Run Length Encoding

 RLE uses a sliding dictionary method of the LZ

algorithm. The sliding dictionary method utilizes

pointers within the compressed file that point to

previously represented strings of bits within the file.

 Here is an example of a message which could be

effectively encoded with RLE:

o The word the, is the most frequently used word

in the English language. The string "the" could

be represented only once and could be pointed

to by all later calls to that string

 Huffman coding works by analyzing the frequency of

elements in data. The elements with the highest

frequency get assigned the shortest encoding (with

the fewest bits). Elements with lower frequencies get

assigned longer encodings (with more bits)

 Huffman coding could be used to compress sound

files, particulary recordings containing frequecies of

that heard in a human voice.

Other compression techniques accepted.

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 50

Band

AO1.1b

Max 8 marks

3

7 - 8 marks

The candidate has:

 written an extended response that has a sustained line of reasoning which is coherent, relevant, and

logically structured

 shown clear understanding of the requirements of the question and a clear knowledge of the indicative

content. Clear knowledge is defined as a response that provides seven to eight relevant detailed points

on lossy and lossless data compression techniques, which relate to an extensive amount of the indicative

content

 addressed the question appropriately with minimal repetition and no irrelevant material

 has presented a balanced discussion and justified their answer with examples

 used appropriate technical terminology referring to the indicative content confidently and accurately.

2

3 - 6 marks

The candidate has:

 written a response that has an adequate line of reasoning with elements of coherence, relevance, and

logical structure

 shown adequate understanding of the requirements of the question and a satisfactory knowledge of the

topic of changeover as specified in the indicative content. Satisfactory knowledge is defined as a

response that provides three to six points on lossy and lossless data compression techniques as

signalled in the indicative content. Up to five marks could be awarded to a response that provides

detailed points on one data compression techniques (lossy or lossless)

 has presented a discussion with limited examples

 used appropriate technical terminology referring to the indicative content.

1

1 - 2 marks

The candidate has:

 written a response that that lacks sufficient reasoning and structure

 produced a discussion which is not well developed

 attempted to address the question but has demonstrated superficial knowledge of the topics specified in

the indicative content. Superficial knowledge is defined as a response that provides one to two points on

lossy and lossless data compression techniques as signalled in the indicative content

 used limited technical terminology referring to the indicative content.

0

0 marks

Response not credit worthy or not attempted.

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 51

UNIT 2

MARK SCHEME

Guidance for examiners

Positive marking

It should be remembered that learners are writing under examination conditions and credit

should be given for what the learner writes, rather than adopting the approach of penalising

him/her for any omissions. It should be possible for a very good response to achieve full

marks and a very poor one to achieve zero marks. Marks should not be deducted for a less

than perfect answer if it satisfies the criteria of the mark scheme.

For questions that are objective or points-based the mark scheme should be applied

precisely. Marks should be awarded as indicated and no further subdivision made.

For band marked questions mark schemes are in two parts.

Part 1 is advice on the indicative content that suggests the range of computer science

concepts, theory, issues and arguments which may be included in the learner's answers.

These can be used to assess the quality of the learner's response.

Part 2 is an assessment grid advising bands and associated marks that should be given to

responses which demonstrate the qualities needed in AO1, AO2 and AO3. Where a

response is not credit worthy or not attempted it is indicated on the grid as mark band zero.

GCE AS & A LEVEL COMPUTER SCIENCE (WALES) Specimen Assessment Materials 52