Computer Systems Logic Gates

1
Computer Systems Logic Gates

• Introduce the basic logic gates
• Introduce truth tables
• Introduce Boolean algebra (dont panic!)
• Examples of combining gates

2
AND gate
A
B
A
B
All inputs have to be true ( i.e 1) for the
output of the gate to be high, in all other cases
the output is false.
3
OR gate
A
A
B
B
If any of the inputs are true then the output is
true
4
NOT gate
_
A
The output of a NOT gate is the opposite of the
input, in other words the gate inverts the input,
so is often called an inverter.
5
NAND gate
The opposite of an AND gate when any of the
inputs are false the output is true
6
NOR gate
The opposite of the OR gate, the gate is only
true when none of the inputs are true.
7
Exclusive OR (XOR)
A
In a 2-input the gate is only true when the
inputs are different.
B
8
Boolean Algebra
9
Examples
10
Summary

• Introduced
• Basic logic gates
• Combined gates
• Boolean Logic
• Truth Tables

11
Sources for further reading

• Chalk et al (2004) pages 26-27.
• Dick (2002) pages 98-100
• Burrel (2004) Chapter 3 - pages 43 -47
• Tanenbaum (2005) pages 135-140