The big idea: A logic gate takes one or two bits in and gives one bit out.
Which bit comes out depends only on which bits went in — the same inputs always give the same output, every time.
Why they matter: Gates are what the CPU is physically built from. Adding two numbers, comparing them, choosing between two paths — all of it is gates wired together.
Everything the CPU does sits on top of this.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
Each gate's symbol beside its truth table. Step through all seven.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
The three basic ones
- AND — 1 only when both inputs are 1
- OR — 1 when at least one input is 1
- NOT — flips the input; one input only
- Every other gate is built from these
The inverted ones
- NAND — AND with the output flipped
- NOR — OR with the output flipped
- Shown by a small circle on the output
- The circle always means "then invert"
The exclusive ones
- XOR — 1 when the inputs are different
- XNOR — 1 when the inputs are the same
- Shown by an extra curved line at the back
- XOR answers "do these disagree?"
Learn the circle, halve the work: NAND is not a new table to memorise — it is the AND table with every output flipped. Same for NOR and OR, and XNOR and XOR.
Three tables plus the circle gives you all seven.
Memorize terms 3x faster
Smart flashcards show you cards right before you forget them. Perfect for definitions and key concepts.
A gate is a condition in hardware: Every gate answers one yes-or-no question about its inputs. Wire a few together and you have a rule the machine obeys without any program running.
AND — every condition must hold
A microwave runs only if the door is closed AND the timer is running. Either input at 0 and the output is 0, so the machine stops.
Use AND whenever the rule says "both" or "all".
OR — any one condition is enough
A car alarm sounds if the driver's door opens OR the passenger's door opens OR the boot opens.
Use OR whenever the rule says "either" or "any".
XOR — the two must disagree
A landing light controlled by a switch at the top of the stairs and one at the bottom: flipping either switch changes the light.
XOR is also how a processor adds two bits: 1 + 1 gives 0 and carries.
NOT — turn a condition around
A fridge's cooling runs when the temperature is not below the target.
NOT is how a circuit acts on the absence of something rather than its presence.
How this is tested — you must name the right gate for a stated rule and justify it from the rule's wording. It comes up two ways:
Paper 1 Section A
- Describe what a named gate does, 1-2 marks
- Choose the gate that matches a described rule
- State the purpose of gates in a computer
Paper 1 Section B — case study
- Identify the logic behind a case study control rule
- Explain what happens when one input changes
The classic trap: Confusing OR with XOR. Plain OR is still 1 when both inputs are 1; XOR is 0 there. If the rule says "one or the other but not both", it is XOR.
A lift door may close only when the door sensor is clear and the close button has been pressed. It must not close if the emergency stop is active. Describe the gates needed and what each does.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.