Logic Gates (Cambridge International IGCSE Computer Science)
Spec point — "1 Identify and use the standard symbols for logic gates"
Logic gates
What is Boolean logic?
Exam Hint Cambridge IGCSE 0478 expects you to identify logic gates, write Boolean expressions, and interpret simple logic circuits. This page covers exactly the gates and formats examiners use—nothing more, nothing less.
- 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 |
Exam Hint The most common exam mistake is confusing
ORandXOR. Remember:XOR* is only true if exactly one input is true. It’s false if both are true or both are 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 |
Exam Hint “Draw the logic circuit for the expression (A AND B) OR (NOT C)” Tip: Draw from left to right, and label all inputs clearly. Mislabelled inputs = lost marks
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