OxfordAQA A-Level Computer Science: Computer systems (9645) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover topic 6, Computer systems, of the OxfordAQA International AS and A-level Computer Science (9645) specification, Version 1.1, for International AS exams May/June 2025 onwards and International A-level exams May/June 2026 onwards. They test sections 3.6.1 to 3.6.3.2, all International AS content listed under Unit 2: Concepts and principles of computer science. Learn the topic first with the study guide and the revision notes. Course links: course hub, printable checklist, free diagnostics.

Assembly language answers use the mnemonics in the specification’s appendix (Standard OxfordAQA assembly language instruction set).

Questions

1. Define the terms hardware and software. [2]

2. Explain what is meant by a utility program and give two examples. [3]

3. One role of an operating system is to hide the complexities of the hardware. Explain what this means, using an example. [2]

4. Three processes arrive at the same time in the order A, B, C. They need 4 ms, 7 ms and 2 ms of processor time. The scheduler gives each process a time slice of 3 ms in turn; a process that has not finished goes to the back of the queue.

(a) State the order in which the processes use the processor, with the time period for each turn. [2] (b) State the time at which each process finishes. [3]

5. Main memory has locations 0 to 127. The operating system occupies 0 to 23. The OS places each process in the first free block that is large enough. Processes P (40 locations), Q (30) and R (20) are loaded in that order.

(a) State the locations given to P, Q and R. [2] (b) Q then ends, and process S (25 locations) is loaded. State the locations given to S. [1] (c) Process T needs 16 locations. Explain why T cannot be loaded, even though 19 locations are free. [2]

6. Describe how an operating system carries out I/O device management and interrupt handling. [4]

7. A variable x is stored in memory location 150 and a variable y in location 151.

(a) Describe machine code and assembly language. [2] (b) Write assembly language instructions for the statement y ← x - 7. [3] (c) Location 150 holds 52. State the value in location 151 after your code runs. [1]

8.

(a) Explain the term “imperative high-level language”. [1] (b) State two differences between source code and object code. [2] (c) State the role of an assembler. [1]

9. A team is writing a large app in a high-level language.

(a) Explain two differences between compilation and interpretation. [4] (b) For each situation, state whether a compiler or an interpreter is more appropriate and justify your choice. (i) Trying out a new feature, one part at a time, during development. [2] (ii) Selling the finished app to customers without giving them the source code. [2]

10. Some compilers produce bytecode as their final output instead of machine code.

(a) Explain three reasons why an intermediate language such as bytecode is produced. [3] (b) Describe two ways in which bytecode can be executed. [2]

11. A college computer has these programs installed: a web browser, a backup program, a graphics library, a Python interpreter and an operating system.

(a) Classify each program as application software or system software. For each piece of system software, state which kind it is. [5] (b) Explain why the operating system must be running before the other programs can be used together. Refer to memory allocation and scheduling. [4]

12. A firmware engineer must write a small program that controls a temperature sensor in a kettle. Evaluate writing it in assembly language rather than a high-level language. [6]

Answers

1. Hardware: the electronic components of a computer system [1]. Software: the sequences of instructions that are executed using the hardware [1]. [2] Examiner insight: “Programs” alone is too vague for software; tie it to instructions executed using the hardware, as the specification does.

2. A utility program adds additional functionality to assist with the management of a computer system [1]. Examples (any two): virus checker [1], compression program [1]; also backup program, disk defragmenter. [3] Examiner insight: An example that is really application software, such as a spreadsheet, gains nothing; each example must manage or maintain the computer.

3. The OS gives users and programs a simple interface, so a program can request “save this file” [1] without knowing which storage device is fitted or how to control it; the OS handles those details [1]. [2] Examiner insight: Say what is hidden (hardware detail) and who it is hidden from (users and other software); “makes the computer easier to use” alone is too general.

4. (a) First round: A 0-3, B 3-6, C 6-8 [1]. Second round: A 8-9, then B 9-12 and B 12-13 (B runs on alone) [1]. (b) C finishes at 8 ms [1], A at 9 ms [1], B at 13 ms [1]. [5] Examiner insight: C needs only 2 ms, so it gives up the processor after 2 ms, not 3; keep a running clock and check that the last finish time equals 4 + 7 + 2 = 13.

5. (a) P: 24-63 [1]; Q: 64-93 and R: 94-113 [1]. (b) S: 64-88 (first free block large enough is the gap left by Q) [1]. (c) The free locations are split into two blocks, 89-93 (5) and 114-127 (14) [1]. A process needs one block large enough, and neither holds 16 [1]. [5] Examiner insight: Count inclusively: locations 24 to 63 are 40 locations, not 39; write start and end for each block.

6. I/O device management: the OS controls devices through device drivers, so programs use them through standard requests [1], and it manages requests from several programs for the same device, for example by queuing print jobs [1]. Interrupt handling: when an interrupt signal arrives, the OS runs the matching interrupt service routine (ISR) [1], then lets the interrupted process continue from where it stopped [1]. [4] Examiner insight: A “describe” question needs what the OS actually does; naming the two functions without explaining them earns little.

7. (a) Machine code: instructions in binary that the processor executes directly, specific to that processor [1]. Assembly language: the same instructions written as mnemonics, usually one per machine-code instruction, translated by an assembler [1]. (b)

LDR R2, 150       ; load x into register 2
SUB R3, R2, #7    ; subtract 7
STR R3, 151       ; store result in y

Load x from 150 [1]; subtract the immediate value 7 using #7 [1]; store the result in 151 [1]. (c) 52 - 7 = 45 [1]. [6] Examiner insight: The appendix only allows #value or a register as the last operand, so SUB R3, R2, 7 does not follow the instruction set; always write #7 for an immediate value.

8. (a) A high-level language in which the commands describe the process that should be followed to carry out a task [1]. (b) Source code is written by the programmer in a high-level or assembly language, while object code is machine code produced by a translator [1]; the processor can execute object code directly but not source code [1]. (c) It translates assembly language into machine code [1]. [4] Examiner insight: For (b), each difference should state both sides; “object code is machine code” alone is only half a comparison.

9. (a) A compiler translates the whole program before it runs [1], whereas an interpreter translates and executes it one statement at a time while it runs [1]. A compiler produces object code that can be run again without retranslating [1], whereas an interpreter produces no object code, so the source is translated on every run [1]. (b)(i) Interpreter [1]: parts can be run straight away and execution stops at the first error reached, which helps testing [1]. (ii) Compiler [1]: customers receive object code that runs without the source code, which keeps the source private and usually runs faster [1]. [8] Examiner insight: “Explain two differences” needs both translators in each point, linked by “whereas”; two facts about the compiler alone do not make a difference.

10. (a) Bytecode is more portable than machine code: it runs on any platform that can execute it [1]. Security checks can be performed on it before it is executed [1]. It can use less memory than equivalent machine code [1]. (b) A virtual machine interprets the bytecode to execute it [1]; or a just-in-time (JIT) compiler converts it into machine code suitable for the computer it is running on [1]. [5] Examiner insight: Do not say the virtual machine “compiles” bytecode; the specification describes it as interpreting, and compiling is the JIT compiler’s job.

11. (a) Web browser: application software [1]. Backup program: system software, utility program [1]. Graphics library: system software, library [1]. Python interpreter: system software, translator [1]. Operating system: system software, operating system [1]. (b) The OS allocates each program an area of main memory to load into [1] and keeps the areas separate so one program cannot overwrite another [1]. The scheduler shares processor time between the programs so they can all make progress [1]. The OS also gives the programs access to hardware, such as storage for the backup, through its own interface [1]. [9] Examiner insight: When a question says “refer to” named functions, each one must appear and be applied to the scenario; generic OS lists score poorly.

12. For assembly: it gives direct control of the hardware and registers, useful for reading the sensor [1]; the code can be compact and fast, which suits a small device with limited memory [1]. Against: it is slower to write and harder to read, debug and maintain [1]; it is tied to one processor’s instruction set, so it must be rewritten if the kettle’s processor changes [1]. High-level code would be quicker to write and portable but must be translated and may be larger [1]. Conclusion: assembly is justified only if memory or speed is tightly limited; otherwise a high-level language is the better choice for maintenance [1]. [6] Examiner insight: “Evaluate” needs points on both sides and a justified conclusion; a list of advantages alone cannot reach the top of the marks.

Where marks are usually lost

• Classifying utilities as application software.
• Forgetting that a process with less remaining time than the slice finishes early in a schedule trace.
• Counting memory blocks exclusively, so block sizes come out one short.
• Explaining memory allocation with virtual memory, which is outside this topic.
• Leaving out # for immediate values in assembly answers.
• Stating that an interpreter creates object code or machine code files.
• Giving one-sided “differences” that only describe a compiler.
• Saying a virtual machine compiles bytecode.
• Ending an “evaluate” answer without a conclusion.

Next steps

• Computer systems revision notes
• Computer systems study guide
• Exam preparation for this course
• Course hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

OxfordAQA International AS and A-level Computer Science (9645) specification, Version 1.1, for International AS exams May/June 2025 onwards and International A-level exams May/June 2026 onwards, published by OxfordAQA. Section 3.6 Computer systems.