Reading a Boolean Expression — 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 logic expression is just an equation that describes a circuit. The output is on the left of =; the inputs and gate operators are on the right. Brackets control the order in which the gates act.

Order of evaluation (always evaluate inside out): innermost brackets first, then NOT, then AND, then OR.

Example — Q = (A AND B) OR (NOT C)

Working inside out:

Compute A AND B for the first bracket.
Compute NOT C for the second bracket.
Combine the two results with OR to get Q.

Example — P = ((NOT A) OR B) NAND C

Compute NOT A.
OR the result with B.
NAND the result with C to get P.

If a question asks "what does Q = NOT (A OR B) evaluate to when A = 1, B = 0?" the chain is: A OR B = 1, then NOT 1 = 0, so Q = 0.