Setting up a Truth Table — Logic Gates and Circuits | IGCSE Computer Science

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 9% of your exam marks.

stable

Medium

Stable9%

Truth tables and Boolean expressions from circuit diagrams appear in every paper. 4 to 6 marks.

A truth table lists every possible combination of inputs alongside the output the circuit produces.

Number of rows. A truth table for n inputs always has 2ⁿ rows: 4 rows for 2 inputs, 8 rows for 3 inputs, 16 rows for 4 inputs.

To fill in the input columns without missing any combination, count up in binary from 0:

For 2 inputs (A, B): 00, 01, 10, 11.
For 3 inputs (A, B, C): 000, 001, 010, 011, 100, 101, 110, 111.

A practical trick: the right-most column alternates every row (0, 1, 0, 1, …); the next column alternates every two rows (0, 0, 1, 1, …); the next alternates every four rows. That guarantees every combination appears exactly once.

On the IGCSE syllabus, exam circuits use at most three inputs and produce a single output, so the largest table you will be asked to fill has 8 rows.