How Computers Make Decisions

Mastering Boolean Logic: The simple rules behind every processor.

The "Can I Play?" Rule

Computers don't "think" like humans. They follow strict True/False rules, just like strict parents.

Rule 1: The Strict Parent (AND)

"You can only play if you have finished homework AND tidied your room."

Homework + Room = Play
Homework + Room = No Play

Rule 2: The Relaxed Parent (OR)

"You can play if it is the weekend OR if it is a holiday."

Weekend = Play
Holiday = Play

We call this Boolean Logic. Let's see how it looks inside a chip.

1. The Three Logic Gates

Tap cards to flip

Computers use digital switches called "gates". There are three you must know for the exam.

AND

Tap for definition

AND Gate

Output is 1 only if BOTH inputs are 1.

Also called: Conjunction

OR

Tap for definition

OR Gate

Output is 1 if EITHER (or both) inputs are 1.

Also called: Disjunction

NOT

Tap for definition

NOT Gate

Output is the INVERSE (opposite) of the input.

Warning: Don't forget the circle!

Logic Lab

Choose Gate:

Input A 0

Input B 0

AND Gate

Output (Q)

Try it: Turn both switches ON to light the bulb.

2. Mastering Truth Tables

A truth table lists every possible combination of inputs to show what the logic gate will do. You need to know how to fill these in for the exam.

The AND Rule

The output is 1 only if input A AND input B are both 1. Otherwise, it is 0.

Exam Tip: Notice how the inputs count up in binary?
00, 01, 10, 11. This is the standard order.

Input A	Input B	Output Q
0	0	?
0	1	?
1	0	?
1	1	?

Click the '?' cells to fill in the table.

The OR Rule

The output is 1 if A is 1, OR if B is 1, OR if both are 1.

Basically: If there is a 1 anywhere in the inputs, the answer is 1.

Input A	Input B	Output Q
0	0	?
0	1	?
1	0	?
1	1	?

Click the '?' cells to fill in the table.

The NOT Rule

This table is smaller because NOT only has one input. It simply flips the value.

If input is 0, output is 1.
If input is 1, output is 0.

Input A	Output Q
0	?
1	?

Click the '?' cells to fill in the table.

Logic: NOT (A AND B)

In exams, you often combine gates. The trick is to do the bit inside the brackets first!

Step 1 (Work it out): Calculate A AND B.
Step 2 (Result): Use the NOT gate to flip that answer.

Tip: Drawing an extra "Working Out" column helps you avoid mistakes.

A	B	Work Out (A AND B)	Q NOT(Work Out)
0	0	0	?
0	1	0	?
1	0	0	?
1	1	1	?

Click the '?' cells. Hint: Flip the "Work Out" column!

Extension: Logic Scenarios

In exams, you often have to combine gates. Let's look at the "Garden Floodlight" scenario.

Problem

"The light (Q) turns on if the Switch is ON (C) AND motion is detected (A) at Night (NOT B)."

1. NOT B converts "Daytime" into "Nighttime".

2. First AND checks: Is it Night AND Motion?

3. Second AND checks: Is step 2 true AND Switch ON?

How Computers Make Decisions

The "Can I Play?" Rule

Rule 1: The Strict Parent (AND)

Rule 2: The Relaxed Parent (OR)

1. The Three Logic Gates

AND

AND Gate

OR

OR Gate

NOT

NOT Gate

Logic Lab

2. Mastering Truth Tables

The AND Rule

The OR Rule

The NOT Rule

Logic: NOT (A AND B)

Extension: Logic Scenarios

Final Challenge

1. Which symbol represents a NOT gate?

2. If Input A = 1 and Input B = 0, which gate outputs 1?

3. How many rows does a Truth Table need for 3 inputs?