A Level Computer Science OCR Exam Questions (Practice)

Database of OCR questions and their answers.

Give one use a firm might have for GPUs. (1 mark)

To render models of proposed buildings. (1)
Run CAD software. (1)
Run modelling calculations. (1)
Any example sensible to scenario. (1)

A gaming company decides to release a new video games console. The console will use a modified version of an operating system called Linux. As well as a CPU the console contains a GPU for 3D graphics. Explain why a GPU is more suitable than a CPU for this task. (3 marks)

CPUs are general purpose processors (1)
whereas GPUs are designed specifically for graphics (1).
And so likely to have built in circuitry / instructions for common graphics operations (1).
GPUs are able to perform an instruction on multiple pieces of data at one time (1)
often we want to do this when processing graphics (e.g. transforming points in a polygon or shading pixels) (1)
which means it can perform transformations to onscreen graphics quicker than a CPU (1).

*Some problems require a large amount of computing power that goes well beyond a single CPU. Discuss the different approaches that can be taken to provide increasingly larger amounts of computing power and the types of problem they are suited to. (12 marks)

Processors have increasingly large clock speeds and can be overclocked (1)
Processors can have multiple cores. Super computers can have multiple processors (and GPUs). (1) GPUs can be applied to problems other than graphics processing. Problems can be distributed across a number of computers working together.(1)
(1-4 marks)
Having multiple cores can speed up smaller problems but this will not be enough for larger problems. Supercomputers are prohibitively exceptionally expensive to buy and run for all but large organisations. GPUs are becoming a cost efficient way of tackling problems. GPUs tend to have large number of cores so can run on highly parallelisable problems... ...but only where the same instruction is being applied to multiple pieces of data (SIMD) (5-8 marks)
Increased clock speed is limited to smaller problems. Even doubling the clock speed would only halve the time taken. Parallel processing isn't suited to all problems. Most problems are only partially parallelisable. Writing algorithms for parallel processing is more challenging than GPUs suited to a subset of science/ engineering problems where the same calculation is repeated on multiple data sets. (9-12 marks)

Explain the effect of using a co-processor system for each of the following applications. Complex calculations for scientific research. (2) Printing personalised letters to customers for an advertising campaign.(2)

Calculations are done by the maths coprocessor(1)... ...so processing is faster ...when using floating point arithmetic (1)
No increase in speed (1)...as co-processor not suitable for task / as there are no calculations(1)

Give 3 features of the Von Neumann Architecture (3 marks)

• Single control unit One instruction at a time (1)

• Uses fetch execute cycle (1)

• Program & data stored together / program & data in same format (1)

Describe one feature, not part of the standard Von Neumann Architecture, which contemporary CPUs may have in order to improve performance. (2)

• Two separate areas of memory one for instructions & one for data./instructions and data can be accessed concurrently.

• Different (sets of) buses one for instructions & one for data./ instructions and data can be accessed concurrently.

• Pipelining whilst an instruction is being executed the next can be decoded and the subsequent one fetched.

• Use of Cache A small amount of high performance memory is (next to the CPU) / which stores frequently used data/instructions Multiple Core Each core acts as a separate processing unit.

Explain, giving an example, how pipelining in a CPU could speed up the execution of this program- 3 Commands of program are LDA 1, LDA 0 and OUT. (3 Marks)

- An instruction can be fetched as the previous one is being decoded (1)
- and the one before that is being executed. (1)
- E.g. LDA 0 can be fetched, while OUT is being decoded and start LDA 1 is being executed. (1)

Explain how pipelining would help a CPU execute the code in Fig. 3.1 more quickly. (Fig 3.1. Has NO BRA commands and has OUT as a command use in Your Answer) (3 marks)Explain how pipelining would help a CPU execute the code in Fig. 3.1 more quickly. (Fig 3.1. Has NO BRA commands and has OUT as a command use in Your Answer) (3 marks)

Pipelining would allow one instruction to be fetched as the previous one is being decoded and the one before that is being executed.(1) For example OUT could be fetched (1).
As there are no jump/branch instructions it pipelines well (as there is no need to flush the pipeline). (1)

Describe one issue the line BRP start may cause for a CPU using pipelining. (2 marks)

- BRP could be followed by one of two possible instructions, which one will only be determined at execution (1)
- Meaning the wrong one may be fetched / decoded (1)

Describe 2 ways in which the Accumulator is used. (4 marks)

Temporary storage for data being processed (1)
during calculations (1)
I/O in processor (1)
used as a buffer / gateway(1)

State what is meant by the term real-time (1 mark)

The system gives a response within a guaranteed time frame (1).

Explain why a real-time operating system would be suitable for Intensive Care Units. (2 marks)

If something happens to a patient, a response must be immediate (1).

Other types of system may have delays in response (1).

This could result in a patient not receiving treatment in time (1).

(i) Explain why the first come, first served scheduling method may not be efficient.

Once a job starts it prevents other jobs from being processed.

A job using a slow resource (eg printer) wastes processor time.

Describe one other scheduling method. (2 marks)

Round robin

Time slice to each user in turn

Or Length of job

Shortest job first

Explain the necessity of scheduling (2 marks)

Processes many jobs as possible in the least amount of time.(2)

Maximises number of interactive users in real time(2)

Efficiently uses resources.(2)

Explain why Memory management is necessary (2 marks)

Organises the use of RAM by converting logical addresses into physical addresses in RAM.

Allows programs to share and allocate memory and protects data from each other.

Allows programs to large for RAM to run (Through use of Virtual memory).

Describe paging (3 marks)

Partitioning memory, into physical divisions of fixed sizes. Used for virtual memory.

What is meant by "paging”? (2 marks)

A program is partitioned into equal fixed sizes using physical divisions.

Explain what is meant by ‘segmentation’ (2 marks)

A program is partitioned into unequal sizes using logical divisions.

State two similarities between paging and segmentation. (2 marks)

Both ways of partitioning memory use virtual memory to handle programs to large for RAM using virtual memory.

A developer is responsible for writing the code for what happens when the CPU receives an interrupt. Outline what the code must do. (6 marks)

When the CPU receives an interrupt the program must:

First, Complete the current FDE cycle.

Secondly, check the priority of the incoming interrupt.

Thirdly, if the priority is greater than the other instructions stored in the stack…

Then, the relevant interrupt service routine is loaded by loading the correct value for the routine into the PC.

Finally, when the ISR is completer the previous state is taken from the stack and loaded back into the register.

A duplicate file finder is an example of a utility. Describe what is meant by a utility. (2 marks)

A utility performs a specific task (1) and is usually related to the upkeep of the system (1).

Examples of a utility include a virus checker (1) / disk defragmenter (1)

The program above is supposed to take 2 numbers, compare them and output the higher. What type of translator would be needed to convert the code above to machine code. (1)

An assembler (Assembler converts assembly language to machine code).

Explain how Linux being open-source would benefit a games company. (2 marks)

This means that a lot of the core functionality they need is already available (1) so the company just has to make amendments / additions specific to their system (1) saving time and money (1).

The owner of a small shop has bought some new stock-handling software and is setting up a computer system in order to run it. The owner will use a number of pieces of utility software.

State the purpose of a File handler and how the owner would use it. (3 marks)

A file handler Manages the storage of a computer. (1)

It is used for deleting/ sorting/ moving/ copying/ creating files or folders. (1)

It can also manage the storage of software. (1)

The owner of a small shop has bought some new stock-handling software and is setting up a computer system in order to run it. The owner will use a number of pieces of utility software.

State the purpose of a Hardware driver and how the owner would use it. (3 marks)

A driver enables peripheral and OS to communicate. (1)

It is used to configure hardware or install new output systems to be used by the OS. (1)

The owner of a small shop has bought some new stock-handling software and is setting up a computer system in order to run it. The owner will use a number of pieces of utility software.

State the purpose of a Backup utility and how the owner would use it. (3 marks)

A backup utility sysem automatically makes copies of files storing in different memory locations in case the original file is corrupted or lost. (2)

It is used to make incremental backups(1)

Ensure that stock data is backed up.(1)

When translating computer languages, intermediate code may be produced. Explain the need for intermediate code and its purpose in a virtual machine. (8 marks)

Intermediate code is code that acts between a high level programming language and a low level programming language.(1)

It is produced by a compiler and is able to run on any computer allowing portability between machines and allowing sections of code to be written in different languages. (3)

A virtual machine is a theoretical computer that provides an environment in which a translator is available. (2) It uses an interpreter to run intermediate code.(1)

A translator is used to convert the code from one language to another from source code to object code. (2)

Describe what is meant by the term ‘open source software’. (2 marks)

Open source software is where the original source code is made freely available and may be redistributed and modified. This means that anyone can view, access and modify the source code.

Describe what is meant by the term ‘assembler‘. (2 marks)

the advantages and disadvantages of writing programs in assembly code rather than a high-level language (3)

Assembly code uses mnemonics to represent machine code and therefore 1 machine instruction equals 1 assembly instruction. (1)

This makes it architecture specific (1). Though because of this it takes more lines of code to perform the same task as a higher level language.(1)

Alternative answers:

It allows direct control of how memory is used via different addressing modes; This gives it direct control of hardware (1). Though High level languages also have optimisers that attempt to achieve the same thing.(1)

High level languages are more intuitive making it easier to follow and debug. This can reduce the development time of programs (1). It also can be recompiled to fit different architectures(1).