AQA A Level Computer Science Paper 2: Written exam November 2020 question paper 7517/2

A sound has been sampled and recorded. The sound was sampled for 1 minute and 40 seconds at a sample rate of 8000 Hz with a 16-bit sample resolution.

A sample rate of 1 Hz means that one sample has been taken every second.

Calculate the minimum amount of storage space, in bytes, needed to store the sampled sound.

You should show your working.

An analogue to digital converter (ADC) was used during the sampling process.

Explain the principles of operation of an ADC.

Questions 02.2, 02.3, 02.4 and 02.5 use a normalised floating point representation with a 7-bit mantissa and a 5-bit exponent, both stored using two’s complement.

Non-integer values such as-1.65 and 23/1068 can be represented by a computer using a fixed point or a floating point system.

State one advantage of using a floating point system over a fixed point system and one advantage of using a fixed point system over a floating point system.

You should assume that the two systems use the same number of bits to store a value.

Advantage of floating point

Advantage of fixed point

The following is a floating point representation of a number:

Calculate the decimal equivalent of the number.

Express your answer to at least four decimal places or as a fraction. You should show your working.

Questions 02.2, 02.3, 02.4 and 02.5 use a normalised floating point representation with a 7-bit mantissa and a 5-bit exponent, both stored using two’s complement.

Write the normalised floating point representation of the decimal value 1632 in the boxes below.

You should show your working.

Questions 02.2, 02.3, 02.4 and 02.5 use a normalised floating point representation with a 7-bit mantissa and a 5-bit exponent, both stored using two’s complement.

State, in decimal, the highest (most positive) and lowest (most negative) values that could be represented by this floating point system.

You should show your working.

When the decimal value 28.25 is converted into binary using this floating point system, a rounding error occurs.

Explain:

• why a rounding error has occurred, and

• what the system might do when the value 28.25 is converted into binary.

The computer has 4 gibibytes of memory installed. How many kibibytes is this equivalent to?

A tablet computer uses the Harvard architecture. Describe two advantages of using the Harvard architecture compared to the von Neumann architecture.

Advantage 1

Advantage 2

The diagram in Figure 2 describes the fetch part of the Fetch-Execute cycle. Some of the names of registers have been omitted from the figure and replaced with the

State the full names of the registers that should appear in the diagram where the numbers are.

Interrupts can be generated by devices connected to the processor during the Fetch-Execute cycle.

Describe the role of interrupts.

Explain why the volatile environment (the contents of registers) must be saved before an interrupt is serviced.

Explain the relationship between hardware and software.

The list below contains four statements about the principles of CRUD and REST. One of these statements is false.

Shade one lozenge to indicate which statement is false.

An SQL query is executed to retrieve some details about properties with at least four bedrooms in a particular area. The following two records are found:

These records could be sent from the server to the client using XML or JSON.

Figure 4 shows the query results encoded using each of these methods.

Shade one lozenge to identify the method of encoding used by Representation 2.

State two reasons why it could be argued that JSON is better than XML.

Reason 1

Reason 2

A composite primary key has been selected for the Viewing relation. This consists of the attributes BuyerID, PropertyID and ViewingDate.

In selecting these attributes to form the primary key, what assumption has the database designer made about the behaviour of the buyers?

The computers on subnet 192.168.2.0 have been configured using the DHCP system.

State one advantage of using the DHCP system.

Computer A has the IP address 192.168.2.3

Many other computers connected to the Internet have the same IP address.

Explain how two or more computers connected to the Internet can have the same IP address and still communicate with each other.

In addition to routing, Router A3 also acts as a firewall to protect the computers on the LAN in the UK.

Explain four different ways that a firewall can protect computers on a LAN.

A packet of data is to be transmitted across the Internet from Computer A in the UK to Computer B in Belgium. A checksum will be used to attempt to detect if any errors have occurred during the transmission.

Explain how:

• Computer A will use a subnet mask to determine whether or not it can send the packet directly to Computer B across the LAN or if the packet must be sent via the Internet

• the packet will be routed across the Internet

• the checksum can be used to determine if the received packet has been changed during the transmission.

In your answer you will be assessed on your ability to follow a line of reasoning to produce a coherent, relevant and structured response.

A 7-segment display is a component used to display a digit on devices such as calculators. A 7-segment display consists of seven lights (called segments) which can be illuminated individually to make the shapes of digits. For example, the digit 3 could be displayed by illuminating five of the seven segments like this (black shading indicates an illuminated segment):

Figure 7 shows part of a logic circuit which is designed to take a binary representation of a single decimal digit as its input and light up the segments to display the decimal digit. The part of the circuit shown controls the lighting of just one segment of the display.

The inputs to the circuit are X3, X2, X1 and X0. Together these form the binary representation of the decimal digit to display. For example, if the inputs to the circuit were:

then the display would need to show this pattern:

as 0101 is the binary representation of the decimal digit 5

The output Q is connected to one segment of the display. When Q is 1 this segment lights up, when it is 0 the segment does not light up.

Complete every empty cell in the truth table below for the circuit in Figure 7

• The listed inputs to the table (X3, X2, X1, X0) represent the decimal digits 0–9 in binary, which are the only allowed inputs.

• The letters A–E have been used to label intermediate points on the circuit in Figure 7 to help you to work out the final output Q

• Some of the cells have been completed for you.

Figure 8 shows the patterns of segments that are illuminated for each of the decimal digits 0–9.

By considering the inputs and outputs of the circuit in Figure 7 and consulting your trace table, state which of the segments in the display (labelled a to g below) the output Q from the circuit is controlling.

One of the numbers listed below is a member of the set of integers, the set of rational numbers and the set of real numbers, but is not a member of either the set of irrational numbers or the set of natural numbers.

Shade one lozenge to indicate which number this is.

Shade one lozenge to indicate which type of number would be most appropriate to use to measure the length of an item, such as a piece of rope.

A warehouse stores products that are waiting to be delivered to supermarkets. The products are packed onto pallets. For example, a pallet might hold 120 boxes of washing powder. All of the products on one pallet are of the same type.

The individual products on the pallets could be identified by labelling them individually with barcodes or by attaching RFID (radio-frequency identification) tags to them.

Each barcode/RFID tag would store a representation of a ProductID number that uniquely identifies the type of product (eg 102546 might represent a specific brand and box size of washing powder) together with an ItemID number that is unique to the specific item (eg box number 1 of the washing powder, box number 2 of the washing powder etc).

Figure 9 shows an example of five boxes of washing powder loaded onto a pallet and their ProductID and ItemID values.

Figure 10 shows an excerpt from the simple database table that stores the details of the products that the warehouse has in stock.

Some pallets delivered to the warehouse will be of products that already exist in the database table. Other deliveries will be of pallets of items that don’t exist in the table because the warehouse has not stocked them before.

Explain why the warehouse owners might prefer the individual products to be identified using RFID tags.

Explain why the product manufacturers or supermarket owners might prefer the individual products to be identified using barcode labels.

Describe how an RFID reader would read the ProductID and ItemID values from RFID tags as pallets are delivered and explain how this data could be used to update the database table that stores details of the products that the warehouse has in stock.

You should include in your description references to the type(s) of SQL statements (eg INSERT, SELECT, UPDATE) that could be used and their purpose, but you do not need to write any SQL code.

Figure 11 shows the format of a machine code instruction for a particular processor and one instruction in that format.

If the operand can be used to refer to any location in the memory, how many memory locations can the processor address?

One of the two addressing modes that the processor supports is immediate addressing.

Explain what is meant by immediate addressing.

A company provides a social media service through which members can share information about themselves and view information and news from their friends.

The service also displays current affairs news stories to its members. The service does not have journalists who write the stories but instead it uses algorithms to select news stories written by other organisations and individuals and shows these. Different news stories may be shown to different members.

Discuss:

• how algorithms might determine which current affairs news stories to display to an individual member

• the moral, ethical and legal considerations that the developers of the system and its operators should consider in relation to how the algorithms work and which news stories are displayed.

The list towers is defined as: towers = ["Blackpool", "Paris", "New Brighton", "Toronto"]

What are the head and tail of this list?

Head

Tail

Functional programming languages support higher-order functions such as map and fold.

Explain what a higher-order function is.

What is the result of this application of the fold function? fold (*) 1 [2, 3, 2]

Result