AQA A Level Computer Science Paper 2: Written exam June 2017 question paper 7517/2

processor are not shown.

Describe how an instruction is fetched from main memory during the fetch stage of the fetch-execute cycle.

Your description should cover the use of registers and buses, together with the role of main memory.

During the decode and execute stages of the fetch-execute cycle the instruction that is being processed is stored in the CIR. Explain why the instruction could not be processed directly from the MBR.

The computer system shown in Figure 1 uses the von Neumann architecture. The Harvard architecture is an alternative to this.

Explain why the Harvard architecture is sometimes used in preference to the von Neumann architecture.

Decrypt the ciphertext "QGOZRKT" using the Caesar cipher with the settings shown in Figure 2.

The Vernam cipher is a more sophisticated cipher system that, under certain circumstances, offers perfect security.

State two conditions that must be met for the Vernam cipher to offer perfect security.

Condition 1

Condition 2

Both the Caesar and Vernam ciphers are symmetric ciphers, whereas a public and private key encryption system is an asymmetric cipher system.

Explain the difference between a symmetric and an asymmetric cipher system.

group of bits.

Given that eight different voltage levels are used, how many bits can be in a group that is encoded by a voltage level?

The baud rate for this system is 500 baud.

What is the system’s bit rate?

Figure 3 shows three suggested relationships between bandwidth and bit rate.

Shade one lozenge to indicate which of the lines, A, B or C in Figure 3, shows the correct relationship between bandwidth and bit rate.

The system sends the data over a long distance using serial communication.

Explain why serial communication is more appropriate in this instance than parallel communication.

channel D or both are available to use.

The states of processes and communication channels can be read using the following Boolean variables:

• A is set to TRUE if process A has completed and FALSE if process A is still running.

• B is set to TRUE if process B has completed and FALSE if process B is still running.

• C is set to TRUE if communication channel C is available and FALSE if it is not available.

D is set to TRUE if communication channel D is available and FALSE if it is not available.

The Boolean variable X should be set to TRUE if the values of the variables A, B, C and D indicate that process X can start and to FALSE if they indicate that process X cannot start yet.

In the space below, draw a logic circuit that will represent the logic of the system described above for the inputs A, B, C and D and the output X.

Write a Boolean expression to represent the logic used to start process X.

Using the rules of Boolean algebra, simplify the following Boolean expression.

You must show your working.

D-type flip-flops can be included in logic circuits.

Explain the general purpose of a D-type flip-flop.

One input to a D-type flip-flop is a data signal.

State what the other input to a D-type flip-flop is and what it is used for.

State the purpose of the operand part of an instruction and explain how the addressing mode is related to this.

Discuss the advantages and disadvantages of programming using a high-level language compared to programming using assembly language.

What is the list or value that is the result of applying the functions head(tail(tail b)) ?

Calculate the results of making the function calls listed in Table 2 with the lists a, b and c above.

map is an example of a higher-order function.

Explain what a higher-order function is.

some information that was being transmitted on personal WiFi networks. The company apologised for doing this and an investigation found that a small number of software developers had been responsible for adding this functionality to the mapping system data collection software.

In the context of this example, discuss:

• how it was possible for this data to be collected.

• what steps the owners of the networks could have taken to prevent the data from being collected.

• what legal and ethical issues might have arisen as a result of collecting this data.

• what lessons the company might have learnt from the incident and how their practices might have changed as a result of it.

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

Sound is being sampled using a 16-bit sample resolution and a sample rate of 20 000 Hz.

1 Hz is one sample per second.

Calculate the amount of storage space that will be required to store 30 seconds of recorded sound. Express your answer in kilobytes.

You must show your working.

The highest frequency component in the sound that is being sampled is 14 500 Hz. The sample rate of 20 000 Hz is not high enough to enable a faithful reproduction of the original sound from the sample.

Explain why this is the case, justifying your response.

MIDI is a system that can be used to enable musical devices to communicate and to represent music on a computer.

Explain how MIDI represents music and the advantages of using MIDI for representing music instead of using sampled sound.

Explain the difference between a physical topology and a logical topology.

Explain the differences between client-server and peer-to-peer networking.

jobs that have been completed and jobs that are waiting to be done.

The details of the jobs that the garage does, together with the parts that it stocks and uses are stored in the database using the four relations shown in Figure 7.

• Each car has a unique CarRegNo.

• A type of car can be uniquely identified by the combination of its Make and Model. Different Makes may use the same Model name and a particular manufacturer (Make) will produce several different car Models.

• A booking made for a car on a particular date counts as one job, regardless of how many different tasks are completed upon it.

• A job might require the use of any number of parts, including zero.

• Some of the details are stored in the database as soon as a booking is made and others are only added when a job has been completed.

The attribute JobID is the Entity Identifier (Primary Key) of the Job relation.

If the JobID attribute were not included in the Job relation, which other attribute or attributes that are currently in the relation could probably be used as an Entity Identifier (Primary Key) instead?

It has been suggested that the owner details (OwnerName, OwnerEmail, OwnerTelNo) should not be stored in the Car relation and that a new relation should be created to store owner details separately from car details.

Explain why storing the owner details separately would improve the design of the database.

When an appointment is made for a job, this is represented in the Job relation. At the time of booking, the InGarage attribute is set to False and the JobDuration attribute is set to 0:00. When the car arrives at the garage the value of the InGarage attribute is changed to True. When the job is finished the value of the JobDuration attribute is updated to indicate how long the job took and details of the parts used are recorded in the database.

The Job with JobID 206 has been completed. The job took 1 hour 30 minutes (1:30) and used two of the parts with PartID 12.

Write the SQL commands that are required to record the amount of time that the job took in the database.

Write the SQL commands that are required to record in the database the fact that two of the parts with PartID 12 were used.

Figure 7 is repeated below.

A mechanic needs to produce a list of all of the parts used on the job with JobID 93 for a customer.

This list must include the PartID, Description, Price (each) and QuantityUsed of each part, and no other details. The parts in the list should be ordered by PartID with the parts with the lowest PartIDs nearest to the top of the list.

Write an SQL query to produce the list.

There are restrictions on which parts can be fitted to which cars. For example:

• The driver’s door mirror with PartID 104 can only be fitted to one particular make and model of car.

The ignition switch with PartID 27 can be fitted to any model of car for one particular make as the maker uses the same ignition switch in all models.

The tyre with PartID 97 can be fitted to a wide range of cars of different makes and models as it is a standard size.

If the information about which parts could be fitted to which makes and models of cars were represented in the database, it could be used to help a mechanic identify the correct parts to use for a job.

Explain how the database design could be modified to represent which makes and models of car a part can be fitted to.

Four bit patterns that are stored in this computer’s memory are listed in Figure 8 and are labelled A, B, C and D. Some of the bit patterns are valid normalised floating point numbers.

Figure 8

Shade one lozenge to indicate which bit pattern (A – D) in Figure 8 represents a negative normalised value.

Shade one lozenge to indicate which bit pattern (A – D) in Figure 8 represents the smallest positive normalised value.

The following is a floating point representation of a number:

Calculate the decimal equivalent of the number. You must show your working.

There can be a loss of precision when a decimal number is stored when using a floating point system.

The closest possible representation of the decimal number 13.8 is shown below.

By converting this bit pattern back into denary it can be seen that the actual number stored is 13.75, not 13.8.

Calculate the absolute error that has occurred.

Calculate the relative error that has occurred. Express your answer as a percentage to two decimal places.