Logic Gates (Cambridge (CIE) O Level Computer Science)
Revision Note
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 1 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) |  | Returns TRUE only if both inputs are TRUE TRUE AND TRUE = TRUE Otherwise = FALSE |
OR
Expression operator | Circuit symbol | Explanation |
(A OR B) |  | Returns TRUE if either input is TRUE TRUE OR FALSE = TRUE FALSE OR FALSE = FALSE |
NOT
Expression operator | Circuit symbol | Explanation |
(NOT A) |  | Reverses the input value NOT TRUE = FALSE NOT FALSE = TRUE |
XOR (exclusive OR)
Expression operator | Circuit symbol | Explanation |
(A XOR B) |  | 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) |  | 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) |  | 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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