What does 4.7 Computer organisation and architecture cover?

It covers the internal components of a computer and the system buses, the difference between RAM, ROM and cache, the stored program concept and the Von Neumann and Harvard architectures, the processor components (ALU, control unit, registers) and the fetch-decode-execute cycle, machine code structure and immediate and direct addressing modes, and the principles of magnetic, optical and solid state secondary storage.

What is the fetch-decode-execute cycle?

It is the cycle the processor repeats for every instruction. In fetch, the program counter's address goes to the memory address register, the instruction is read into the memory data register and copied to the current instruction register, and the program counter is incremented. In decode, the control unit works out the operation and operands. In execute, the operation is carried out, for example the ALU performs a calculation into the accumulator.

What is the difference between Von Neumann and Harvard architecture?

Von Neumann uses a single shared memory and a single bus for both instructions and data, which is simple but creates a bottleneck because instructions and data cannot be fetched at the same time. Harvard uses separate memories and buses for instructions and data, so both can be fetched simultaneously, which is faster but more complex. Many modern CPUs use a hybrid.

What is the difference between immediate and direct addressing?

In immediate addressing the operand is the actual value to be used, so no memory access is needed, but the value is fixed in the instruction. In direct addressing the operand is the memory address where the value is stored, so the processor must fetch from that address; it is more flexible because the stored value can change, but it needs an extra memory access.

How do the three types of secondary storage compare?

Magnetic storage (hard disk) stores bits as magnetised regions on spinning platters: high capacity and low cost but slow and fragile due to moving parts. Optical storage (CD, DVD, Blu-ray) uses pits read by a laser: cheap and portable but low capacity. Solid state storage (SSD, flash) uses transistors with no moving parts: fast and durable but more expensive per byte with limited write cycles.

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AQA A-Level Computer Science 4.7 Computer organisation and architecture: the processor, fetch-execute, addressing and storage

A deep-dive AQA A-Level Computer Science guide to 4.7 Computer organisation and architecture. Covers the internal hardware and buses, the stored program concept and Von Neumann and Harvard architectures, the processor components and fetch-execute cycle, addressing modes, and secondary storage.

Generated by Claude Opus 4.818 min read4.7Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What 4.7 actually demands
Internal hardware and memory
Stored program and architectures
The processor and the cycle
Addressing and storage
Check your knowledge

What 4.7 actually demands

This module is how the machine actually runs a program. AQA expects you to know the components and buses, explain the stored program concept and the main architectures, trace the fetch-decode-execute cycle naming the registers, distinguish addressing modes, and compare secondary storage types.

Internal hardware and memory

The core components, connected on the motherboard, are the processor, main memory, the system buses and secondary storage. The address bus (unidirectional) carries addresses, the data bus (bidirectional) carries data, and the control bus carries control signals. RAM is volatile working memory, ROM is non-volatile boot firmware, and cache is small fast memory near the processor.

Stored program and architectures

The stored program concept keeps instructions and data together in memory, so the processor fetches and executes them in turn. The Von Neumann architecture uses one shared memory and bus (simple, but with a bottleneck); the Harvard architecture uses separate memories and buses for instructions and data (faster, more complex).

The processor and the cycle

The processor contains the ALU, the control unit and registers (PC, MAR, MDR, CIR, accumulator). The fetch-decode-execute cycle fetches the instruction at the PC's address, increments the PC, decodes it, then executes it. Performance depends on clock speed, cores, cache and word/bus width.

Addressing and storage

A machine code instruction has an opcode and an operand; immediate addressing uses the operand as the value, direct addressing uses it as an address. Secondary storage is needed because RAM is volatile, and comes in magnetic, optical and solid state forms, each with its own trade-offs.

Check your knowledge

State the difference between RAM and ROM. (2 marks)
State the purpose of the address bus. (1 mark)
State the stored program concept. (2 marks)
State the main advantage of the Harvard architecture over Von Neumann. (1 mark)
State the role of the program counter. (1 mark)
Describe the fetch stage of the fetch-decode-execute cycle. (3 marks)
Explain the difference between immediate and direct addressing. (2 marks)
Give one advantage and one disadvantage of solid state storage versus magnetic. (2 marks)

Solutions

Q1: RAM is volatile working memory that loses its contents when power is off; ROM is non-volatile and holds the boot firmware.
Q2: It carries the memory address that the processor wants to read from or write to.
Q3: Program instructions are stored in main memory alongside the data, and the processor fetches and executes them one at a time.
Q4: Separate buses for instructions and data let both be fetched at the same time, avoiding the Von Neumann bottleneck.
Q5: It holds the address of the next instruction to be fetched.
Q6: The PC's address is copied to the MAR, the instruction is read from memory into the MDR and copied to the CIR, and the PC is incremented to point to the next instruction.
Q7: In immediate addressing the operand is the value itself; in direct addressing the operand is the memory address of the value, requiring an extra memory access.
Q8: Advantage: faster and more durable with no moving parts. Disadvantage: more expensive per byte and a limited number of write cycles.

Sources & how we know this

AQA A-level Computer Science (7517) specification — AQA (2015)