Logic Gates (Cambridge (CIE) IGCSE Computer Science)

Revision Note

James Woodhouse

Written by: James Woodhouse

Reviewed by: Lucy Kirkham

Logic gates

What is Boolean logic?

  • Boolean logic is used in computer science and electronics to make logical decisions

  • Boolean operators are either TRUE or FALSE, often represented as or 0

  • Inputs and outputs are given letters to represent them

  • To define Boolean logic we use special symbols to make writing expressions much easier 

What are logic gates?

  • Logic gates are a visual way of representing a Boolean expression

  • The logic gates covered in this course are:

    • AND

    • OR

    • NOT

    • XOR

    • NAND

    • NOR

AND

Expression operator

Circuit symbol

Explanation

(A AND B)

and-gate

Returns TRUE only if both inputs are TRUE 

TRUE AND TRUE = TRUE

Otherwise = FALSE

OR

Expression operator

Circuit symbol

Explanation

(A OR B)

or-gate

Returns TRUE if either input is TRUE 

TRUE OR FALSE = TRUE 

FALSE OR FALSE = FALSE

NOT

Expression operator

Circuit symbol

Explanation

(NOT A)

not-gate

Reverses the input value

NOT TRUE = FALSE

NOT FALSE = TRUE

XOR (exclusive OR)

Expression operator

Circuit symbol

Explanation

(A XOR B)

xor

Returns TRUE if either input is TRUE but NOT both

TRUE OR FALSE = TRUE 

FALSE OR FALSE = FALSE

TRUE OR TRUE = FALSE

NAND (not and)

Expression operator

Circuit symbol

Explanation

(A NAND B)

screenshot-2023-05-30-at-08-25-01

Returns TRUE if either or both inputs are FALSE

TRUE OR FALSE = TRUE

FALSE OR FALSE = TRUE

TRUE OR TRUE = FALSE

NOR (not OR)

Expression operator

Circuit symbol

Explanation

(A NOR B)

screenshot-2023-05-30-at-08-25-22

Returns TRUE if both inputs are FALSE

TRUE OR FALSE = FALSE

FALSE OR FALSE = TRUE

TRUE OR TRUE = FALSE

Examiner Tips and Tricks

You will need to either draw a diagram of a logic circuit using these symbols, or you will have to write the boolean expression from an existing diagram.

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James Woodhouse

Author: James Woodhouse

Expertise: Computer Science

James graduated from the University of Sunderland with a degree in ICT and Computing education. He has over 14 years of experience both teaching and leading in Computer Science, specialising in teaching GCSE and A-level. James has held various leadership roles, including Head of Computer Science and coordinator positions for Key Stage 3 and Key Stage 4. James has a keen interest in networking security and technologies aimed at preventing security breaches.

Lucy Kirkham

Author: Lucy Kirkham

Expertise: Head of STEM

Lucy has been a passionate Maths teacher for over 12 years, teaching maths across the UK and abroad helping to engage, interest and develop confidence in the subject at all levels.Working as a Head of Department and then Director of Maths, Lucy has advised schools and academy trusts in both Scotland and the East Midlands, where her role was to support and coach teachers to improve Maths teaching for all.