Spec & Goals 3 min
AQA Spec 3.4.5 · Systems architecture — the CPU and von Neumann
By the end of this lesson you can:
- State what the CPU does and what the von Neumann architecture means.
- Describe the role of the ALU, the control unit, registers and cache.
- Label a diagram of the CPU and main memory, including the buses.
Warm-Up 5 min
Last lesson you saw how code is translated into machine code. That machine code is run by the CPU — the part that does the actual processing.
Quick starter
A computer holds both your program's instructions and its data. Where do you think they are stored while the program runs?
Reveal the idea
In the same place — main memory. Storing instructions and data together in one memory is the heart of the von Neumann architecture.
Key Concept — inside the processor 14 min
The CPU (central processing unit) processes the instructions that make a computer work.
The von Neumann architecture
In the von Neumann model, program instructions and data are stored together in the same main memory. The CPU fetches both from there.
The main CPU components
| Component | Role |
|---|---|
| ALU (arithmetic logic unit) | Carries out calculations and logic / comparisons. |
| Control unit (CU) | Decodes instructions and coordinates / controls the rest of the CPU. |
| Registers | Tiny, very fast stores inside the CPU holding data being worked on. |
| Cache | Fast memory close to the CPU storing frequently used data and instructions. |
Buses
Signals travel between the CPU and main memory along buses: the address bus carries memory locations, the data bus carries the data, and the control bus carries control signals.
Worked Example — labelling the CPU 12 min
Problem: Label the parts of the CPU and show how it links to main memory in a von Neumann computer.
The control unit decodes each instruction and directs the ALU, registers and cache. Main memory holds both the instructions and the data they work on.
Try It Yourself 12 min
Goal: State which CPU component carries out calculations.
Hint: its name includes the words "arithmetic" and "logic".
Goal: Describe the difference between a register and cache.
Hint: both are fast, but think about where each sits and how much it holds.
Goal: Explain why the von Neumann design storing instructions and data together is useful, then name the three buses.
📝 Exam Practice 10 min
Answer the way the examiner expects — the command word and the marks tell you how much to write.
State what the CPU does.
Mark scheme
- Processes / executes the instructions (that make the computer work) (1).
Describe the role of the ALU.
Mark scheme
- Carries out calculations / arithmetic and logic / comparison operations (1).
Describe the role of the control unit.
Mark scheme
- Decodes instructions / coordinates / controls the activity of the CPU (1).
State what is meant by the von Neumann architecture.
Mark scheme
- Instructions and data are stored together (1).
- In the same main memory (1).
Recap & Key Terms 3 min
The CPU processes instructions. In the von Neumann design, instructions and data share one main memory. Inside the CPU, the ALU does maths and logic, the control unit decodes and coordinates, registers are tiny fast stores, and cache is fast memory close to the CPU. Buses carry signals between the CPU and memory.
- CPU
- The central processing unit — processes the instructions that make a computer work.
- Von Neumann architecture
- A design in which program instructions and data are stored together in the same main memory.
- ALU
- The arithmetic logic unit — performs calculations and logic / comparison operations.
- Control unit
- The part of the CPU that decodes instructions and coordinates its activity.
- Register
- A tiny, very fast store inside the CPU that holds data currently being processed.
- Cache
- Fast memory close to the CPU that stores frequently used data and instructions.
Homework 1 min
Task (≤ 15 min): A laptop sold in Kuala Lumpur lists a CPU with several cores and a large cache. Describe the role of the control unit and the role of the cache in that CPU.
Model answer
The control unit decodes instructions and coordinates the rest of the CPU. The cache is fast memory close to the CPU that stores frequently used data and instructions, so the CPU does not have to fetch them from slower main memory as often.
Award marks for: correct control-unit role — decodes / coordinates (1); correct cache role — fast memory storing frequently used data/instructions (1).