Logic gates for GCSE Computer Science
GCSE Computer Science, Computer systems. Usually met in years 9 to 11.
Use AND, OR, NOT and XOR gates, read truth tables and work out the output of a logic expression.
What you practise
- What each gate does
- Truth tables for two and three inputs
- Expressions with brackets
Starts with single gates and builds to three-input expressions. Practice moves up a level when a round goes well and down when it does not.
Sample questions
Each sample comes with the method and the reason every wrong option is wrong. Real practice rounds are ten questions at your level.
Sample question, level 2 of 5
How many rows does a truth table with three inputs have?
- 3
- 6
- 9
- 4
- 8
Show the answer and method
Answer: E, 8
The method: Each input can be 0 or 1, so three inputs give 2 × 2 × 2 = 8 rows.
Why the other options are wrong:
- A, 3: That is the number of inputs, not combinations.
- B, 6: Each input doubles the rows: 2 × 2 × 2.
- C, 9: That is 3 × 3; each input has only two values.
- D, 4: That is for two inputs.
Sample question, level 4 of 5
P = (NOT (A OR B)) OR C. Which inputs make P = 0?
- A = 1, B = 1, C = 1
- A = 0, B = 1, C = 1
- A = 1, B = 1, C = 0
- A = 1, B = 0, C = 1
- A = 0, B = 0, C = 1
Show the answer and method
Answer: C, A = 1, B = 1, C = 0
The method: With A = 1, B = 1, C = 0, working from the inside brackets out gives P = 0.
Why the other options are wrong:
- A, A = 1, B = 1, C = 1: With these inputs P = 1. Work out the brackets first.
- B, A = 0, B = 1, C = 1: With these inputs P = 1. Work out the brackets first.
- D, A = 1, B = 0, C = 1: With these inputs P = 1. Work out the brackets first.
- E, A = 0, B = 0, C = 1: With these inputs P = 1. Work out the brackets first.
See where this fits: every GCSE option topic and Key Stage 3.
Related topics: CPU, memory and storage.
