AQA AS Computer Science Past Papers & Mark Schemes

The on-screen exam tests programming under pressure, and the most efficient preparation is practising with past paper skeleton code. Download the supplementary files, open them in your IDE, read through the existing code carefully, and then complete the tasks within the time allocation. The examiners design skeleton code with deliberate structure — understanding what the existing code does before you start modifying it is essential. Students who dive straight into writing without reading the context consistently introduce bugs or duplicate functionality. Code tracing questions carry significant marks and require systematic discipline. Create a trace table with one column per variable, step through each line of execution, and update values methodically. Pay particular attention to loop boundaries (off-by-one errors are deliberately tested), the scope of variables (local vs global), and recursive calls (track the call stack). Tracing is tedious but mechanical — if you follow the process systematically, you will get the correct answer. Binary arithmetic is examined every session. Practise converting between denary, binary, and hexadecimal until it is automatic. Two's complement representation requires careful attention to the sign bit — the range of an 8-bit two's complement number is −128 to +127, not −128 to +128. Floating-point representation divides the binary number into mantissa and exponent; normalisation requires the mantissa to start with either 0.1 (positive) or 1.0 (negative in two's complement). When analysing algorithms, Big-O notation describes how execution time grows as input size increases, not the actual execution time. Linear search is O(n) because in the worst case you check every element. Binary search is O(log n) because each comparison halves the remaining search space. Bubble sort is O(n²) because each pass through the list is O(n) and you may need n passes. The examiners want you to explain why an algorithm has its particular time complexity, not just state it.