A Level Computer Science: System Software (A Level) — Practice Questions (Cambridge 9618)

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 — Cambridge International holds copyright in its own papers. Use these alongside the official past papers available from your board.

Each question practises a skill tested in the June 2025 Paper 31. After each answer there is an examiner insight, a mark-scheme insight or a tip, and the real question to try next where one matches.

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Questions

1. A student runs a program written in a high-level language using an interpreter. Describe how the interpreter executes the program. [4]

2. A museum gives each visitor a badge code. The rules for a valid badge code are: it starts with exactly one letter, which must be K, M, R or T; this is followed by one or more further characters, each of which is either a digit from 1 to 4 or one of the symbols ? and ~. No other characters are allowed. For each of the following, state why it is not a valid badge code.

(a) M

(b) R26 [2]

3. Using the badge code rules from Question 2, and given that <digit> ::= 1 | 2 | 3 | 4 and <symbol> ::= ? | ~ have already been defined, write Backus-Naur Form (BNF) definitions for <letter>, <tail> (the part of the code after the letter) and <badge>. [4]

4. A laptop is playing music, downloading a file and running a virus scan while its user types a report. Explain the term multi-tasking and give one way it helps the operating system manage processes. [2]

5. An operating system uses the shortest remaining time (SRT) scheduling routine. Process Alpha arrives at time 0 and needs 7 units of processor time. Process Beta arrives at time 2 and needs 3 units. Process Gamma arrives at time 3 and needs 1 unit. Explain how SRT schedules these three processes, stating the time at which each one finishes, and give one benefit of this routine. [4]

6. An expression is written in infix form as (p + q) × (r − s).

(a) Write this expression in Reverse Polish Notation (RPN).

(b) Given p = 6, q = 2, r = 9 and s = 4, describe how a stack is used to evaluate your RPN expression, and state the result. [4]

7. A computer with little main memory uses virtual memory and paging. Explain what is meant by disk thrashing and why it happens. [3]

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Answers

1. The interpreter takes the source code and translates a single line (statement) at a time [1]. If the line has no errors it is executed immediately, before the next line is translated [1]. If an error is found, the interpreter stops and reports an error message for that line; the lines before it have already run [1]. No executable (object code) file is produced, so every line must be translated again each time it is run, including lines repeated inside a loop [1].

Examiner insight (June 2025): some candidates described compiling, or stages within compilation, instead of how an interpreter executes a program.

Try the real question next: Cambridge International AS & A Level Computer Science 9618, June 2025, Paper 31, Question 6.

2. (a) M has a valid first letter but nothing after it; at least one digit or symbol must follow the letter [1].

(b) R26 contains the digit 6, which is not an allowed digit (only 1 to 4 are allowed) [1].

Mark-scheme insight (Cambridge 9618 June 2025 mark scheme, Paper 31, Question 7(a)): each credited reason named the rule that the code breaks, such as what the first character must be or which kind of character is not allowed in a given position, so say exactly which rule fails rather than writing “wrong format”.

Source for the mark-scheme insights on this page: Cambridge International AS & A Level Computer Science 9618 June 2025 mark scheme for Paper 31 (9618/31), paraphrased. This mark scheme is not published on Cambridge’s 9618 past papers page (checked September 2026).

Try the real question next: Cambridge International AS & A Level Computer Science 9618, June 2025, Paper 31, Question 7.

3.

<letter> ::= K | M | R | T
<tail> ::= <digit> | <symbol> | <digit><tail> | <symbol><tail>
<badge> ::= <letter><tail>

<letter> lists exactly the four allowed letters separated by | [1]; <tail> includes the single-character options <digit> | <symbol> [1] and the recursive options <digit><tail> | <symbol><tail>, allowing one or more characters [1]; <badge> ::= <letter><tail> [1].

Examiner insight (June 2025): to earn full marks candidates had to use recursion to allow a repeated part of any length, and the list of allowed letters must not have a stray | at its start or end.

Try the real question next: Cambridge International AS & A Level Computer Science 9618, June 2025, Paper 31, Question 7.

4. Multi-tasking means several processes run concurrently: the operating system switches the processor between them so quickly (using scheduling and time slices) that they appear to run at the same time [1]. Benefit: the processor is kept busy, so more tasks are completed in a given time than if each had to finish before the next could start [1].

Examiner insight (June 2025): most candidates knew that multi-tasking allows processes to run concurrently; only those who also gave a benefit to process management earned both marks.

Try the real question next: Cambridge International AS & A Level Computer Science 9618, June 2025, Paper 31, Question 8.

5. SRT always runs the process with the shortest remaining time, and is pre-emptive: a newly arrived process with a shorter time replaces the running one. Alpha runs from 0 to 2; at time 2 Beta (3 units) is shorter than Alpha’s remaining 5 units, so Beta pre-empts Alpha [1]. At time 3 Gamma (1 unit) is shorter than Beta’s remaining 2 units, so Gamma runs from 3 to 4 [1]. Finishing times: Gamma at 4, Beta at 6 (running 4 to 6), Alpha at 11 (running 6 to 11) [1]. Benefit: short processes finish very quickly, so the average waiting time is minimised [1].

Examiner insight (June 2025): many candidates stated that the shortest job is run first; higher-scoring answers also explained how processes are ordered in the queue and that the routine is pre-emptive.

Try the real question next: Cambridge International AS & A Level Computer Science 9618, June 2025, Paper 31, Question 8.

6. (a) p q + r s − × [2] (one mark for each correct bracketed part, p q + and r s −, placed before the final ×).

(b) Push 6 and 2; at + pop both and push 6 + 2 = 8. Push 9 and 4; at − pop both and push 9 − 4 = 5 [1]. At × pop 5 and 8 and push 8 × 5; the single value left on the stack is the result, 40 [1].

Tip: when evaluating RPN with a stack, the second value popped is the left-hand operand, which matters for − and ÷.

7. In virtual memory, pages are swapped between main memory (RAM) and secondary storage when they are needed [1]. If memory is too small for the processes running, pages that are swapped out are needed again almost immediately and must be swapped straight back in [1]. The computer then spends more time swapping pages than executing instructions, so performance falls sharply: this is disk thrashing [1].

Tip: thrashing is about the amount of swapping, not just about using virtual memory; make it clear that the swapping takes over from useful processing.

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Where marks are usually lost

• Describing compilation when the question asks about an interpreter.
• Writing BNF without recursion, so only a fixed number of characters is allowed.
• Leaving a stray | at the start or end of a list of alternatives in BNF.
• Saying SRT runs the shortest job first without explaining pre-emption when a shorter process arrives.
• Stating that multi-tasking runs processes concurrently without giving the benefit that was asked for.