Syllabus & Goals 3 min
Cambridge 3.1 · Computer architecture Paper 1 · Computer Systems
By the end of this lesson you can:
- State the purpose of the CPU and the idea of von Neumann architecture.
- Describe the roles of the CU, ALU and the main registers.
- Describe the address, data and control buses.
Recap / Warm-Up 5 min
Units 1–2 covered how data is stored and moved. Unit 3 opens the box: the actual hardware. The CPU is the "brain" that runs every instruction.
Quick starter
A kitchen has a chef (who decides what to do) and a chopping station (which does the work). Which part of a CPU is like each one?
Reveal the answer
The chef = the Control Unit (it directs); the chopping station = the Arithmetic & Logic Unit (it does the calculations).
Key Concept 14 min
1 · The CPU and von Neumann architecture
The CPU (central processing unit) — also called the processor or microprocessor — executes all the instructions in a program. Modern computers use von Neumann architecture: a stored-program machine where program instructions and data share the same memory, and instructions run in sequence.
2 · CU, ALU and registers
- Control Unit (CU) — reads and interprets instructions and sends control signals to coordinate the whole computer.
- Arithmetic & Logic Unit (ALU) — carries out arithmetic (+, −, shifts) and logic (AND, OR) operations.
A register is a tiny, very fast store inside the CPU. The special-purpose registers are:
| Register | Purpose |
|---|---|
| PC — Program Counter | Holds the address of the next instruction. |
| MAR — Memory Address Register | Holds the address being read from / written to. |
| MDR — Memory Data Register | Holds data just read from, or about to be written to, memory. |
| CIR — Current Instruction Register | Holds the instruction currently being decoded/executed. |
| ACC — Accumulator | Holds data temporarily during ALU calculations. |
3 · The three buses
Worked Example 12 min
(a) Bus width and memory
How many memory locations can an address bus of each width reach?
| Address bus width | Locations (2ⁿ) |
|---|---|
| 8 bits | 256 |
| 16 bits | 65 536 |
| 32 bits | 4 294 967 296 |
(b) Which part does the job?
| Task | Part of the CPU |
|---|---|
| Add two numbers together | ALU |
| Send a signal telling memory to read | Control Unit (via the control bus) |
| Hold the address of the next instruction | Program Counter (PC) |
| Hold a running total during a calculation | Accumulator (ACC) |
Try It Yourself 12 min
Goal: Name the two main units inside the CPU and state what each does in one sentence.
Goal: State which bus carries each of: (a) a memory address, (b) an instruction, (c) a "read" signal.
Goal: Explain why the address bus is unidirectional but the data bus is bidirectional.
📝 Exam Practice 10 min
State the purpose of the Arithmetic & Logic Unit (ALU).
Mark scheme
- Carries out arithmetic and/or logic operations (1).
Describe the purpose of the MAR and the MDR.
Mark scheme
- MAR holds the address of the memory location to be read from / written to (1).
- MDR holds the data read from / to be written to that location (1).
Explain how the width of the address bus affects a computer.
Mark scheme
- A wider address bus has more lines / bits (1)…
- …so more memory locations can be addressed (1).
Recap & Key Terms 3 min
The CPU runs instructions using a CU (directs), an ALU(calculates) and fast registers (PC, MAR, MDR, CIR, ACC). It talks to memory over the address (one-way), data (two-way) and control (two-way) buses.
- CPU
- The processor that fetches, decodes and executes a program's instructions.
- Control Unit (CU)
- Reads instructions and sends control signals to coordinate the computer.
- ALU
- Carries out arithmetic and logic operations.
- Register
- A small, very fast store inside the CPU (e.g. PC, MAR, MDR, CIR, ACC).
- Address / data / control bus
- The three sets of wires connecting the CPU to memory and I/O.
Homework 1 min
Task (≤ 15 min): Draw the von Neumann CPU (CU, ALU, registers) connected to memory by the three buses, labelling the direction of each bus.
Model answer
- CU and ALU inside the CPU, plus the registers (PC, MAR, MDR, CIR, ACC).
- Address bus — one-way arrow CPU → memory.
- Data bus and control bus — two-way arrows.