AQA A Level Computer Science Paper 2: Written exam June 2019 question paper 7517/2

Shade one lozenge to indicate to which category of system software a virus checker belongs.

The operating system is responsible for resource management.

Describe two different types of resource management that an operating system is responsible for.

Type 1:

Type 2:

A company is setting up a computer network to help manage its business.

The company sets up a computer that will act as a server. The server’s primary role will be to act as an email server. It will also allow technicians to remotely login so that the server can be managed from other computers.

State the names of two application layer protocols that the server must implement and explain what each will be used for.

Protocol 1:

Use:

Protocol 2:

Use:

Explain how the transport layer of the TCP/IP stack determines which application layer software on the server should deal with a received request.

Describe one function of the network layer of the TCP/IP stack.

The paragraph of text in Figure 1 is to be compressed using a dictionary-based compression method.

Figure 1

Unfortunately time after time it is the case that programmers fail to put enough effort into commenting their code. Effort put into commenting could make the code easier to maintain when the time comes to do this.

Dictionary-based compression is an example of a lossless encryption method.

Explain the key difference between lossless and lossy compression methods.

Explain how the paragraph of text in Figure 1 could be compressed using a dictionary-based method.

After the text in Figure 1 has been compressed it is to be transmitted across a computer network.

Explain why dictionary-based compression is not very effective for compressing small amounts of text for transmission.

A student has written a computer program using an imperative high-level programming language. The program could be translated using either a compiler or an interpreter.

Describe the steps that must be completed to translate and execute the program.

Your description should include:

• why translation is necessary

• the differences between how a compiler and an interpreter would translate the program

• how the machine code instructions that are used to carry out the program will be fetched and executed by the processor from main memory.

A veterinary practice with four different surgeries intends to use a relational database to store the data that it needs to manage its business.

Customers of the practice are pet owners who bring their pets to one of the surgeries for appointments. The surgeries are staffed by vets.

• Each customer is identified by a unique identity number and the customer’s forename, surname and telephone number are recorded.

• Each pet is identified by a unique identity number and the pet’s name, type and date of birth are recorded.

• Each surgery is identified uniquely by its name. The town in which it is located and the surgery’s telephone number are recorded.

• Each vet is identified by a unique identity number and the vet’s forename and surname are recorded.

A pet is owned by one or more customers and each customer may own any number of pets. Over their lifetimes, pets may attend many appointments.

To make an appointment for a pet, a customer contacts a surgery. The appointment is made for the pet to take place on a particular date and time at a specific surgery.

Each vet is associated with one surgery which they work at; each surgery is staffed by several vets.

Complete the entity-relationship diagram below for a fully normalised relational database to store the data required by the veterinary practice.

Some of the entities and relationships have been drawn for you. You need to draw the remaining three entities and clearly show the relationships between the entities and their degree.

Develop a fully normalised design for a relational database to store the information required by the veterinary practice. To help you, the Pet, Surgery and Vet relations have already been defined in Figure 3.

Figure 3

Pet PetName, Type, DateOfBirth)

Vet VetForename, VetSurname, SurgeryName)

Using the format shown in Figure 3 list all the other relations that will need to be created, together with the attributes that each will contain.

Underline the attribute(s) that will form the entity identifier (primary key) in each relation.

The SQL query in Figure 4 has been written to produce a list of all of the vets who work at the surgery in the town of Torquay. Some errors have been made in the query.

Figure 4

SELECT VetForename, VetSurname FROM Surgery, Vet WHERE Town = Torquay

Describe two errors that have been made in the query. You should not give the omission of a semi-colon (;) as one of the errors.

Error 1:

Error 2:

The database is stored at the practice’s head office. Staff at the individual surgeries access it using a client-server database system, which enables the management of concurrent access to the database.

Describe an example of a problem that could occur if no system were in place to manage concurrent access to the database.

Two methods that can be used to manage concurrent access are:

record locks

timestamp ordering.

Select one of these methods and describe how it manages concurrent access.

Method selected:

How it works:

Figure 5 shows three commonly used mathematical functions: add, square and pred.

Figure 5

add(x,y) = x + y

pred(x) = x - 1

For example:

add(3,2) evaluates to 5

square(2) evaluates to 4

pred(8) evaluates to 7

The domain of the functions square and pred in Figure 5 is the set of integers and the domain of the add function is

What is the co-domain of the pred function?

What is the result of applying square ° pred to the argument 3?

The add function takes two arguments. Describe how the add function could be partially applied to the arguments 4 and 6.

Complete the truth tables for the OR and NAND gates.

Identities are often applied to help simplify Boolean expressions. One such identity is:

Without using a truth table, explain why this identity is true.

A data communication system uses asynchronous serial communication.

Explain the difference between asynchronous and synchronous communication.

The ASCII code for the digit '0' is 48 in decimal. In ASCII, other digits follow on from this value in sequence.

The digit '4' is to be transmitted in ASCII using asynchronous serial transmission and even parity, with the parity bit stored in the most significant bit of the byte of data containing the ASCII code.

Complete Figure 6 below to show a valid bit pattern for transmitting the digit '4'

Figure 6

It is proposed that the communication system is modified so that:

a majority voting system is used instead of the parity bit

Unicode is used to encode the characters to be transmitted instead of ASCII.

Discuss the improvements that will occur in the communication system as a result of these changes and any disadvantages that will result from them.

Write a program using the AQA assembly language instruction set, shown on

• At the start, the positive integer A will be stored in memory location 102 and the positive integer B in memory location 103. Your program should use these values to find their greatest common divisor.

• When your program terminates it should store the greatest common divisor of these two numbers in memory location 104.

Questions 11.1, 11.2, 11.3 and 11.4 use a normalised floating point representation with an 8-bit mantissa and a 4-bit exponent, both stored using two’s complement.

Write the smallest positive number that can be represented by the floating point system in the boxes below.

The following is a floating point representation of a number:

Calculate the decimal equivalent of the number.

You must show your working.

Write the normalised floating point representation of the decimal value 0.15625 (5/32 as a fraction) in the boxes below.

You must show your working.

The two floating point numbers below are multiplied together.

A problem occurs as a result of the multiplication operation.

Explain what problem has occurred and how the floating point representation could be redesigned to avoid it.

A particular computer system uses a 32-bit address bus and a 32-bit data bus. Each addressed memory location can store one byte of data.

What is the maximum amount of memory, in bytes, that could be accessed?

A different computer system has a wider data bus; this will speed up the execution of programs.

Explain how the wider data bus has resulted in this effect.

Discuss the advantages and disadvantages of representing an image as a vector graphic instead of as a bitmap.

In your answer, include an example for which it would be most appropriate to use a vector graphic and an example for which it would be most appropriate to use a bitmap.

The ciphertext message "BVP" has been received. The message was encrypted using the Vernam cipher and the key "TIN".

Conversion between letters and their equivalent binary patterns was carried out using a special code called the Baudot-Murray code. A version of the Baudot-Murray codes for each letter is shown in Figure 8.

Figure 8

Decrypt the ciphertext to work out what the original plaintext message was.

Express the plaintext as letters.

You must show your working.

Plaintext

The Vernam cipher can offer perfect security. Most encrypted transmissions that are made by computers use ciphers that are computationally secure but not perfectly secure.

Explain what it means for a cipher to be described as being computationally secure.

Many computerised cipher systems use asymmetric encryption methods to resolve the key exchange problem that is associated with symmetric ciphers, such as the Vernam and Caesar ciphers.

Explain what the key exchange problem is, in relation to a symmetric cipher.

A message is to be transmitted from computer A to computer B. The message will be encrypted using asymmetric encryption. To enable computer B to authenticate that the message was sent by computer A, a digital signature will also be sent with the message.

Explain how computer B will decrypt the message and verify that it was sent by computer A.

In your response you should refer to the specific keys that will be used in this process.

You do not need to explain how computer A will encrypt the message or create the digital signature.