Unit 3 exam technique

The paper

• 1 hour 30 minutes, 80 marks, all questions compulsory.
• No calculator: practise binary, floating point, RSA, checksums and traversals by hand.
• The assembly language instruction set is provided in the paper's resource material.
• Question types: multiple choice, short and medium open response, extended open response, and tabular and diagrammatic items (complete a pipeline table, a class diagram, a trace table, a Dijkstra table).
• Unit 3 is worth 25% of the full International A level. It is available in January and June, with the first assessment in June 2028.

Sample paper structure

The sample assessment groups questions by topic area:

Total 80 marks

Command words and technique

• State / give / identify: a short answer, no explanation needed.
• Describe: say what something is or how it works, step by step.
• Explain: give a point and a reason or effect ('because ... so ...'). Two-mark explain questions usually want a point plus its expansion.
• Complete: fill in the table or diagram exactly; label everything.
• Trace: show every change in a table; record values only when they change.
• In 'why does a battery device use RISC?' style questions, link a feature to a consequence in the context given.
• Use the marks as a guide: 2 marks usually means two linked points.
• Show working in calculations; method marks are often available even if the final answer is wrong.

Common traps

• Floating point: forgetting the bias of 15 or storing the hidden 1.
• Pipelines: forgetting the first instruction needs all five cycles (k + n - 1).
• Dijkstra: not updating a distance when a shorter route appears, or not tracing the path back.
• BST deletion: using the wrong replacement node (use the in-order successor).
• RSA: confusing which key encrypts for confidentiality (receiver's public) and which signs (sender's private).
• K-maps: groups that are not powers of 2, or forgetting wrap-around.
• Hash tables: forgetting to wrap round to slot 0 when probing.

Worked example

A 2-mark question asks: 'Explain one advantage of a Harvard architecture over a von Neumann architecture.' Write a full-mark answer.

1. Point: instructions and data have separate memories and buses.
2. Expansion: so an instruction can be fetched at the same time as data is read or written, which increases performance.

Answer: Separate buses allow simultaneous instruction fetch and data access, so the processor runs faster.

Check yourself

1. How many marks is the Unit 3 paper worth?
   Show answer

   80. The paper is out of 80 marks.

2. How long is the Unit 3 exam, in minutes?
   Show answer

   90 minutes. 1 hour 30 minutes = 90 minutes.

3. Roughly how many marks per minute should you aim for on the Unit 3 paper? Give your answer to 2 decimal places.
   Show answer

   0.89. 80 marks in 90 minutes is about 0.89 marks per minute, so a 6-mark question deserves about 7 minutes.

4. Which equipment rule applies to the Unit 3 exam?
   • A calculator is required
   • Calculators are not allowed
   • A programmable calculator is allowed
   • Internet access is allowed
   Show answer

   Calculators are not allowed. Practise all calculations by hand.

5. What does an 'explain' question usually require?
   • A single word
   • A labelled diagram only
   • A list of definitions
   • A point together with a reason or consequence
   Show answer

   A point together with a reason or consequence. Explain means saying why or how, not just what.

6. What support is provided for assembly language questions in the exam?
   • A working assembler on the computer
   • The assembly language instruction set
   • Pre-written programs to copy
   • Nothing: you must memorise every instruction
   Show answer

   The assembly language instruction set. The instruction set is given; you must know how to use it.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Subsidiary and Advanced Level in Computer Science specification (2026), Issue 1
• Pearson Edexcel IAL Computer Science sample assessment materials (Unit 3 paper, resource booklet and mark scheme)
• Pearson Edexcel IAL Computer Science guide (2026)

Last updated 8 November 2026. Spotted a mistake? Tell us.