Logic Expressions (Cambridge (CIE) O Level Computer Science)

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		A Z 0 1 1 0	Z = NOT A
AND		A B Z 0 0 0 0 1 0 1 0 0 1 1 1	Z = A AND B
OR		A B Z 0 0 0 0 1 1 1 0 1 1 1 1	Z = A OR B
NAND		A B Z 0 0 1 0 1 1 1 0 1 1 1 0	Z = A NAND B
NOR		A B Z 0 0 1 0 1 0 1 0 0 1 1 0	Z = A NOR B
XOR		A B Z 0 0 0 0 1 1 1 0 1 1 1 0	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)