The big idea: A CPU has two working parts:
- the control unit (CU) — decides what happens next - the arithmetic logic unit (ALU) — does the maths
The CU never calculates. The ALU never decides.
Control unit (CU)
- Fetches the next instruction
- Decodes it — works out what it means
- Sends signals telling everything else what to do
Arithmetic logic unit (ALU)
- Arithmetic: + − × ÷
- Logic: AND, OR, NOT
- Compares: is this one bigger?
First — what memory looks like: Memory is one long row of numbered boxes. Step through the diagram below to see it.
A window onto memory. Each box holds one thing — and an instruction like ADD 5 counts as one, with an operation and a value inside it.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Our example: You are playing a game. Your score is 12. You grab a coin worth 5 points.
The instruction that adds it on is ADD 5, and it is stored in box 20.
So which part does what?
The CU decides
It goes to box 20, collects ADD 5, and works out what that means: an addition. It has not added anything — it has worked out what needs doing, and tells the ALU to do it.
The ALU calculates
It does the sum it was told to do: 12 + 5 = 17. Your new score. It never decided anything — it was handed a job and did it.
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.
Why registers exist: Memory is slow to reach — like digging something out of your school bag.
A register sits right next to the ALU, so reading it is instant. Each one holds exactly one value: your score of 12 sits in one, the instruction ADD 5 in another.
So yes — same idea as the memory boxes you just stepped through. But not the same thing:
Memory boxes
- Millions of them
- Outside the CPU
- Slower to reach
- Hold the whole program and its data
Registers
- Just five to know
- Inside the CPU, beside the ALU
- Read instantly
- Hold only what is being used right now
The same value, fetched two ways — already beside the ALU, or all the way out to memory and back. The three wires are named in the next section.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
PC — program counter
A bookmark: which box the CPU has got up to. Ours is in box 20, so PC = 20. The moment that instruction is collected, the PC moves on to 21.
MAR — memory address register
The box the CPU is asking for right now. 20 is copied here from the PC — so for a moment both hold 20. Then the PC moves to 21 and the MAR still points at 20, because that is the box being fetched.
MDR — memory data register
The value travelling to or from memory. ADD 5 lands here on its way in.
IR — instruction register
The instruction being carried out now. ADD 5 moves here from the MDR.
AC — accumulator
The number the ALU is working on. ADD 5 means add 5 to whatever is in here — so the 12 becomes 17, in this same register. It is both what goes in and what comes out.
The ones that get confused: PC and MAR both held 20 — so why two? The PC is a bookmark for instructions only. The MAR is used for every trip to memory, data included. They match for an instant, then part ways.
MAR = address. MDR = data. One says which box, the other carries what was in it.
Know your predicted grade
Take timed mock exams and get detailed feedback on every answer. See exactly where you're losing marks.
How the CPU talks to memory: The CPU and memory are joined by bundles of wires called buses.
There are three, one for each kind of traffic.
Address bus
Which box? It carries the number of the box the CPU wants.
One way only — the CPU asks, memory never asks back
Data bus
The value itself. It comes in when the CPU reads something, and goes out when the CPU saves something.
Both ways — a wider bus moves data faster
Control bus
Read or write? The CPU's orders and timing, and memory's replies.
Both ways — without it memory would not know what to do
Reading box 20 uses all three: the address goes out, the READ signal goes out, the value comes back.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Cores: Everything so far is one core — one CU, one ALU, one set of registers.
- single-core — one job at a time - multi-core — several cores, really running at once - co-processor — a separate chip built for one job, like graphics
The same four jobs on one core, on four cores, and on four cores when each job needs the one before it.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
The classic trap: Four cores do not make everything four times faster.
If each step needs the answer from the step before, the work cannot be split — so three cores sit idle.
Get feedback like a real examiner
Submit your answers and get instant feedback — what you did well, what's missing, and exactly what to write to score full marks.
How this is tested — you must give each part's JOB, not just the words behind the initials. It comes up two ways:
Paper 1 Section A
- Describe or explain the CPU components, 2-4 marks
- Label a CPU diagram, with arrowheads on the buses
- Trace a named instruction through the registers
Paper 1 Section B — case study
- Apply the same parts to the system in the case study
- Explain why that system needs a particular component
The classic trap: Expanding the abbreviation is not describing the part. "ALU — arithmetic logic unit" earns nothing; "the ALU carries out the arithmetic and logic" earns the mark.
The whole CPU with ADD 5 moving through it — step through before you answer.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Describe the functions of the control unit and the arithmetic logic unit, and explain how the accumulator is involved when the instruction ADD 5 is carried out.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.