AND, OR and NOT

Simple logic diagrams using the operators AND, OR and NOT

2.4.1 Boolean Logic

Boolean Logic — Note 1 of 4

Recognition of each gate symbol;

What you need to know

Logic gates are components inside a processor that perform logical operations on one or more Boolean inputs to produce a single output. The three gates you need to know are AND (output is 1 only when both inputs are 1), OR (output is 1 when at least one input is 1), and NOT (reverses the input). Each gate has a standard symbol used in logic diagrams.

What to know

AND gate: output is 1 only if BOTH inputs are 1. Symbol is a flat-ended D shape with 2 inputs.
OR gate: output is 1 if EITHER input (or both) is 1. Symbol is a curved/pointed D shape with 2 inputs.
NOT gate: reverses the input (0 becomes 1, 1 becomes 0). Symbol is a triangle with a CIRCLE on the output. Only 1 input.
Logic gates have inputs on the left and outputs on the right.

Key terms

Boolean Logic: A system of logic where values are either True (1) or False (0), used to control decision making in programs and logic circuits.

Exam technique

Tip:You MUST draw the correct gate SHAPES — labelling gates with names (e.g. a rectangle with 'AND' written in) is NOT accepted. Examiners mark the shape, not the label.
Mistake:Forgetting the CIRCLE on the NOT gate turns it into a buffer, not a NOT gate. Also do NOT add circles to AND/OR gates (that would make them NAND/NOR).
Tip:AND and OR gates must have exactly 2 inputs. NOT gates must have exactly 1 input and 1 output. Wrong number of inputs loses marks.
Tip:If an expression has 3 inputs with AND (e.g. A AND NOT B AND C), you need TWO AND gates since standard AND gates only take 2 inputs.

Ready to test yourself?

Practise 2.4.1 questions

← Previous topicRefining algorithms2.3.2 Testing Next note →Truth tables2.4.1 Boolean Logic

Back to Boolean Logic

Revise another OCR GCSE topic: Programming Fundamentals