A1.1.1 · Computer hardware and operation
Inside the CPU
By the end you can
Describe what the control unit, ALU, registers and buses each do, and how they work together. Tell single-core, multi-core and co-processors apart.
Every program you have ever run, from a game to a spreadsheet, was carried out one instruction at a time by the . It is a chip the size of a postage stamp, but inside it is organised like a small, very fast factory: a manager that gives the orders, a workshop that does the calculations, a handful of workbenches for the values in use, and roads out to the warehouse (memory).
Selected
Control unit (CU)
Runs the fetch–decode–execute cycle. It decodes each instruction in the IR and sends control signals that tell every other part what to do and when, in step with the clock.
Connects: Reads the IR; drives the control bus; directs the ALU and the registers.
Click any box or bus on the diagram (or Tab to it and press Enter).
The control unit and the ALU#
The is the manager. It runs the fetch–decode–execute cycle (A1.1.5): it works out what each instruction means and then sends control signals, timed by the clock, that tell the other parts what to do, from “memory: read address 12” to “ALU: add these two”.
The is the workshop. It does arithmetic (add, subtract, and on most processors multiply and divide) and logic (AND, OR, NOT, and comparisons such as “is this zero?” or “is this bigger?”). Decisions in a program, like an IF statement, come down to the ALU comparing two values.
Registers#
are the fastest storage in the whole computer, built right into the CPU. Each holds one value. Five have special jobs that you need to know:
- : the address of the next instruction to fetch.
- : the address about to be read from or written to.
- : the value just read from memory, or about to be written to it.
- : the instruction being decoded and executed right now.
- : the value being worked on, and the results from the ALU.
Real CPUs also have many general-purpose registers for working values, but these five are the ones exam questions ask about.
How many bits a register holds is the CPU's . Almost every CPU today is 64-bit: it adds, compares and moves 64-bit values in one step, where an older 32-bit CPU would need two steps for the same number.
Buses#
A is a set of parallel wires, one bit per wire, connecting the CPU to memory and other devices. There are three:
- The says where: the memory address to use. It is one-way, from the CPU, because only the CPU chooses addresses.
- The carries what: the data or instruction itself. It is two-way: into the CPU when reading, out when writing.
- The carries when and how. It is two-way: the control unit sends read and write signals and the clock, and devices send signals back, such as interrupts (“I need attention”) and “ready” or “done” acknowledgements.
The width of a bus matters. A wider data bus moves more bits in one go (a 64-bit bus moves 8 bytes at once). The width of the address bus sets how much memory the CPU can reach: n wires can make 2ⁿ different addresses.
216 = 65,536 addresses
If each address holds one byte, that is 64 KiB of memory the CPU can reach directly.
Cores and co-processors#
A is one complete processing unit: its own control unit, ALU and registers. A single-core CPU can only execute one instruction stream at a time; it gives the impression of doing several things at once by switching between them very quickly. A multi-core CPU (dual-, quad-, octa-core ...) genuinely runs several streams at the same time, one per core, which is ideal for multitasking and for programs designed to share out their work.
A is an extra processor designed for one kind of work: graphics (the GPU, next section), sound, signal processing or encryption. The CPU hands that work over and gets on with everything else.
One core switches between the three jobs so quickly they seem to run together, but only one runs at any moment. All three finish after 12 time slots.
Check yourself
CPU components
Quick questions, made fresh every time and marked here. Do a few until they feel easy.
Exam practice
Paper 1-style questions
7 questions · 18 marks
Write your answer as you would in the exam, then show the mark points and tick each one your answer made. These are our own questions in the style of the exam, not past-paper questions.
1.Describe the function of the arithmetic logic unit (ALU).
[2]Describe: A detailed account: say what happens, step by step if it is a process.
2.Explain why the address bus carries signals in only one direction.
[2]Explain: Give reasons: say what happens AND why, or how one thing causes another.
3.Calculate the number of memory locations that can be addressed by a 32-bit address bus.
[1]Calculate: Work out the number, showing the working.
4.Explain why a quad-core processor may not run a program four times faster than a single-core processor.
[3]Explain: Give reasons: say what happens AND why, or how one thing causes another.
5.Outline the purpose of a co-processor, giving an example.
[2]Outline: A brief account: the main points, without much detail.
6.State two signals carried by the control bus, one sent by the CPU and one sent to it.
[2]State: Give a short answer, no explanation needed.
7.A school is choosing between a computer with a fast single-core processor and one with a slightly slower eight-core processor, for students who edit video while running other programs. Evaluate the eight-core option.
[6]Evaluate: Weigh the strengths against the weaknesses (or the benefits against the costs) and reach a judgement that you back up.
Past papers
Old exam questions on this topic
From HL Paper 1, May 2014 to November 2025. These papers were set on the previous syllabus, so the wording may differ from today's exam, but the topic is the same. Find the questions and markschemes in your school's past-paper pack.
| Paper | Question | Marks | About |
|---|---|---|---|
| November 2025 | 2 | 1 | Purpose of the MDR |
| May 2024 TZ1 | 6 | 2 | Purpose of the MDR (TZ2: the MAR) |
| November 2024 | 12(c)(i)-(iii) | 3 | Which register holds data on its way to memory; where calculations happen |
| November 2023 | 3 | 3 | Three jobs of the control unit |
| November 2020 | 6(b) | 2 | Function of the MAR |
| May 2019 | 3 | 2 | Purpose of the MAR |
| November 2017 | 7(b) | 2 | Roles of the data bus and address bus |
| May 2016 | 3 | 2 | What the control unit does |
| November 2015 | 8(c) | 4 | What the MAR and the MDR do |
| November 2014 | 2 | 1 | What the ALU does |
In a nutshell
- The CPU runs programs. Its main parts are the control unit (CU), the arithmetic logic unit (ALU) and the registers.
- The CU decodes instructions and sends control signals that coordinate everything. The ALU does arithmetic and logic.
- Registers are tiny, very fast stores inside the CPU: PC, MAR, MDR, IR and the accumulator each have a fixed job.
- Three buses connect the CPU to memory: address (one-way, CPU → memory), data (two-way) and control (two-way: read, write and clock out; interrupts and acknowledgements back).
- The width of the registers is the word size: 64 bits on most CPUs today.
- An n-bit address bus can address 2ⁿ locations; a wider data bus moves more bits at once.
- Multi-core CPUs run several instruction streams at once, but only software written for it gains. Co-processors take over specialist jobs.