OCR A Level Computer Science Computer systems June 2018 question paper H446/01

A digital coffee making machine has a CPU that uses the Little Man Computer Instruction Set.

Little Man Computer operates on a computer system based on the Von Neumann Architecture.

State two features of the Von Neumann architecture.

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Describe one feature, not part of the standard Von Neumann Architecture, which contemporary CPUs may have in order to improve performance.

Part of the coffee making machine’s code asks the user to press a button to select strength. The code outputs 1 which will switch on a green light to indicate a valid selection or outputs 0 to indicate an invalid selection.

The code is shown below:

Tick the appropriate boxes below to indicate which inputs will result in a green light (i.e. code outputs 1) and which with a red light.

Explain which registers and buses are used, and the values they store/carry, when the line LDA redLight is executed (after it has been fetched and decoded). You should assume the address redLight refers to memory location 11.

Write code in a high‑level language or pseudocode that has the same functionality as the code in Fig. 1.

* Discuss the differences between assembly code and high‑level languages. You should refer to:

• the advantages and disadvantages of writing programs in assembly code rather than a high-level language

• when each approach might be used

• why the coffee machine was programmed in assembly code.

A software company decides to build an operating system for OCR smart watches.

Memory management is one of the functions of an operating system.

List three functions, other than memory management, of an operating system.

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Part of a computer’s memory is represented below (Fig. 2). The operating system divides the memory into equally sized chunks.

State the name of the type of memory management used in Fig. 2.

The operating system needs to load program C into memory but there is not enough space. Describe how the operating system would use virtual memory to load program C.

Write the HTML to produce the extract from the webpage above. You can assume it will be placed within the <body> tags of a pre-existing page. You do not need to specify the font.

Explain what happens when a search engine indexes the page. You do not need to discuss ranking.

Explain why using a RISC processor rather than a CISC processor is likely to result in increased battery life.

Describe what the SQL statement below does.

SELECT FlightNumber FROM Flight WHERE DestinationCode='JFK'

The airport cancels all its flights to Heathrow on 4^th July 2018.

The SQL statement below shows all the data for flights going to Halifax. Rewrite it so it instead removes all flights to Heathrow on 4^th July 2018.

SELECT * FROM Flight WHERE DestinationName='Halifax'

Tables often have primary and secondary keys.

State why DestinationCode would not be a suitable primary key for the Flight table.

State why DestinationCode would be a suitable secondary key for the Flight table.

The airline wishes to ensure the database is normalised.

Describe why the database can be considered to be in First Normal Form.

Describe why the database can be considered to be in Second Normal Form.

Describe why the database can not be considered to be in Third Normal form.

The airport wishes to allow airlines to be able to access the data it has on flights via the internet.

Describe one format or method the airport could use to provide the data to the airlines so they can use it in their own applications.

The internet can be considered an example of a WAN.

Describe what is meant by the term ‘WAN’.

The internet uses a set of protocols referred to as the TCP/IP stack. The TCP/IP stack consists of four different layers, each with its own set of protocols.

Explain why protocols are important on a network.

State the name of the four layers of the TCP/IP stack.

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A software company is producing software that allows users with severe mobility issues to input data into a computer.

The software flashes up letters on the screen one at a time. The user sends a signal to the computer when the letter they want appears on the screen.

State the name of an input device and describe how it could be used by a user with very limited mobility in their hands and arms to send a signal to the computer.

How it would be used:

Rather than displaying the whole alphabet, once the first letter has been entered, the program only shows letters that could be possible according to words in its dictionary. All possible words are stored in a tree data structure.

The program is tested on a sample dictionary of four words, represented as a tree in Fig. 3:

Annotate Fig. 3 to show how the word BELTS would be removed from the tree.

Annotate Fig. 3 to show how the words BEACH and BONE would be added to the tree.

The developer decides she wants to make the software program open source.

Explain the benefits to the users of the software being open source.

6* “Technology is changing too quickly for the law to keep up.”

Discuss to what extent you agree with the statement above. In your discussion you should explain which laws regulate the use of technology and how advancements in technology have made the laws difficult to enforce/implement.

A taxi firm is investigating replacing its drivers with self-driving cars.

Explain why the self-driving system will use a real-time operating system.

The code for the self-driving system has been written using an object-oriented programming language.

It recognises obstacles in the road and then classifies them. The class for Obstacle is shown below.

Write a line of code to create an object called bollard of type Obstacle which is not moving and is 7.8 metres away in a direction of 8 degrees.

Describe an example of encapsulation in the class definition code above.

Describe the advantages of using encapsulation.

The self-driving program recognises people as a special type of obstacle and the class Person should inherit the methods and attributes of Obstacle. People are treated like other obstacles except:

• when the updateDistance method is called, if the person is more than 2 metres away but is 5 metres (or less) away, the method Controls.beepHorn() is called.

• when the person is 2 metres away (or closer), the method Controls.applyBrakes() is called as well as Controls.beepHorn().

Complete the class Person.

Give one advantage and one disadvantage to the customers of the taxi using self-driving cars rather than drivers.

Advantage

Disadvantage

A student writes a program to apply a symmetric encryption algorithm to work on messages of up to 25 ASCII characters.

Describe what is meant by the term ‘ASCII’.

The encryption algorithm works in the following way. A message of up to 25 characters (spaces and punctuation are not included) is placed in a 5×5 array. Any leftover spaces are filled with random letters. The message I LOVE COMPUTER SCIENCE becomes:

The key is a sequence of ten numbers. In this example we will use 1 2 3 4 5 1 2 3 4 5. The first 5 numbers state how many spaces the rows 0 to 4 must be rotated right.

A key with the first 5 digits 1 2 3 4 5 would result in

The next 5 digits state how many spaces down the columns 0 to 4 should be rotated.

Applying the last 5 digits 1 2 3 4 5 to the grid above would give

Part of the pseudocode for the algorithm is written below.

Show the result of running the algorithm on the grid and key below.

KEY: 3 3 3 3 3 1 1 1 1 1

Grid after only the rows are shifted:

Grid after columns have also been shifted:

Write the procedure shiftRow.

*Modern encryption is much stronger than the method described in the first part of this question. Discuss the impact of modern encryption on society. You should refer to:

• The importance of asymmetric encryption and how it differs from symmetric encryption.

• Different circumstances in which symmetric and asymmetric encryption may be used.

Convert the binary number shown below to hexadecimal.

0011011100001111

The number below is represented in floating point format with a 5-bit mantissa in two’s complement followed by a 3-bit exponent in two’s complement. Calculate the denary value of the number, showing your working.

01001 010

The numbers below are represented in floating point format with a 5-bit mantissa in two’s complement followed by a 4-bit exponent in two’s complement. Normalise the numbers shown below, showing your working.

00011 0010

11100 0110

Show the byte below after having an AND applied with the masking byte.

Show the byte below after having an OR applied with the masking byte.

Draw a logic gate diagram to represent the Boolean expression

Q ≡ ¬A∨B