Describe the von Neumann architecture, the components of the CPU, the fetch-execute cycle, memory and the storage hierarchy.

What this dot point is asking

WJEC wants you to describe the von Neumann architecture, name the components of the CPU and their registers, explain the fetch-execute cycle step by step, and describe the storage hierarchy from registers to secondary storage. Hardware is the foundation under the rest of Unit 1, and the fetch-execute cycle is one of the most reliably examined topics, usually as an extended-answer question worth several marks.

The answer

The von Neumann architecture

The key idea is the stored program: because instructions live in memory like data, a computer can run any program loaded into it, which is what makes it general-purpose rather than fixed-function.

The CPU and its registers

Registers are the fastest storage of all because they sit inside the CPU; the whole fetch-execute cycle is a choreography of moving values between these registers and memory.

The fetch-execute cycle

The processor repeats three phases. Fetch: the program counter's address goes to the MAR, the instruction is read into the MDR and copied to the CIR, and the program counter is incremented. Decode: the control unit interprets the instruction in the CIR. Execute: the operation is carried out, for example a calculation in the ALU with the result in the accumulator.

The storage hierarchy and performance

Performance depends on clock speed (cycles per second), number of cores (parallel instruction streams) and cache size (how much frequently used data stays close to the CPU).

Examples in context

Example 1. Why a stored-program machine is general-purpose

Because instructions sit in memory alongside data, loading a different program turns the same hardware into a calculator, a games console or a web server. This stored-program idea, central to the von Neumann model, is the reason one computer can do countless different jobs rather than one fixed task.

Example 2. Cache hits and misses in everyday use

When you scroll a document, the CPU repeatedly accesses the same nearby data. If that data is in cache, access is fast (a cache hit); if not, the CPU waits for slower RAM (a cache miss). A larger cache raises the hit rate, which is exactly why cache size is a stated performance factor.

Example 3. More cores versus higher clock speed

A video-editing task that can be split across cores benefits from a multi-core CPU, while a task that must run as one sequence benefits more from a higher clock speed. This illustrates that the right performance factor depends on whether the workload can be parallelised, a nuance examiners reward.

Try this

Q1. State what is stored in the program counter. [1 mark]

• Cue. The address of the next instruction to be fetched.

Q2. State one similarity and one difference between RAM and secondary storage. [2 marks]

• Cue. Both store data; RAM is fast and volatile, secondary storage is slower and non-volatile.