OCR GCSE Computer Science 1.1 Systems architecture: the CPU, fetch-decode-execute, performance and embedded systems

What topic 1.1 actually demands

Systems architecture explains how the processor runs a program. It is a definition-heavy topic examined in Paper J277/01, where the marks reward precise wording and clear reasoning rather than calculation. You need to know the purpose and components of the CPU, the fetch-decode-execute cycle in order, the von Neumann architecture, the factors that affect performance, and what embedded systems are.

This guide ties together the three dot-point pages for the topic.

The CPU and the fetch-decode-execute cycle

The CPU processes instructions using its arithmetic logic unit (calculations and logic), control unit (decoding and control signals), cache (fast nearby memory) and registers (tiny, very fast stores). It runs the fetch-decode-execute cycle continuously: fetch the next instruction from memory, decode it, then execute it. The four named registers are the Program Counter (address of the next instruction), the MAR (address being accessed), the MDR (data or instruction in transit) and the Accumulator (result of ALU calculations).

The von Neumann architecture

The von Neumann architecture stores instructions and data together in one memory, reached over a single shared bus (the stored-program concept). It is simple and cheap, but the single shared path can limit speed.

What affects performance

Three factors improve performance. A higher clock speed runs more cycles per second. More cores let several instruction streams run in parallel, though sequential programs and shared resources limit the gain. A larger or faster cache holds more frequently used data close to the CPU, so more requests are cache hits and the CPU waits less for slow RAM. A balanced processor improves all three.

Embedded systems

An embedded system is a computer built into a larger device to do one dedicated task (a washing machine, a car engine controller, traffic lights). It is typically small, cheap, low-power and reliable, with its program fixed in ROM, unlike a general-purpose computer that runs many user-installed programs.

Check your knowledge

A mix of recall and applied questions covering topic 1.1. Attempt them, then check against the solutions.

1. Name the three stages of the fetch-decode-execute cycle in order. (3 marks)
2. State the function of the control unit. (1 mark)
3. State which register holds the address of the next instruction. (1 mark)
4. State one feature of the von Neumann architecture. (1 mark)
5. Explain how a higher clock speed improves performance. (2 marks)
6. Explain why doubling the number of cores does not always double speed. (2 marks)
7. State what a cache hit is. (1 mark)
8. Define the term embedded system and give one example. (2 marks)