Logic Expressions (Cambridge (CIE) O Level Computer Science)
Revision Note
Written by: James Woodhouse
Reviewed by: Lucy Kirkham
Logic Expressions
A logic expression is a way of expressing a logic gate or logic circuit as an equation
The output appears on the left of the equals sign with the inputs and logic gates on the right
Gate | Symbol | Truth Table | Logic Expression | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
NOT |  |
| Z = NOT A | |||||||||||||||
AND |  |
| Z = A AND B | |||||||||||||||
OR |  |
| Z = A OR B | |||||||||||||||
NAND |  |
| Z = A NAND B | |||||||||||||||
NOR |  |
| Z = A NOR B | |||||||||||||||
XOR |  |
| Z = A XOR B |
Logic circuits containing multiple gates can also be expressed as logic expressions/statements
An example logic circuit containing two inputs
The logic circuit above can be expressed as the logic expression Q= NOT(A OR B)
An example logic circuit containing two inputs
The logic circuit above can be expressed as the logic expression Q= (NOT A) AND B
An example logic circuit containing three inputs
The logic circuit above can be expressed as the logic expression P = ((NOT A) OR B) NAND C
An example logic circuit containing three inputs
This logic circuit above can be expressed as X = NOT (A NAND B) OR (B NOR C)
Examiner Tips and Tricks
You may be required to write a logic expression/statement from a truth table or a logic circuit. You may also have to do the opposite - draw a logic circuit and complete a truth table for a logic expression
Worked Example
Consider the logic statement: X = (((A AND B) OR (C AND NOT B)) XOR NOT C)
a. Draw a logic circuit to represent the given logic statement. [6]
Answer
One mark per correct logic gate, with the correct input
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