Logic Gates and Circuits

Boolean logic is a system in which every value is either TRUE or FALSE, written as 1 or 0. A logic gate takes one or more Boolean inputs and produces a single Boolean output by applying a fixed rule.

Inside every digital computer the underlying signals are voltages. A high voltage stands for 1 (TRUE) and a low voltage stands for 0 (FALSE). Every arithmetic, control or decision step in the processor is built from logic gates that combine these 1s and 0s.

There are exactly six logic gates on the IGCSE syllabus: AND, OR, NOT, XOR, NAND and NOR. Each gate has:

• a rule that says what its output is for any combination of inputs,
• a circuit symbol used in logic diagrams,
• a Boolean notation used inside expressions,
• a truth table that lists the output for every possible input combination.

Inputs are usually labelled A, B, C; outputs are usually labelled Q, X or Z.