WJEC GCSE Computer Science Computer systems and hardware: a complete overview of the CPU, the fetch-decode-execute cycle, memory and storage

What the Computer systems and hardware content demands

This area is where WJEC checks that you understand what is inside a computer and how it processes instructions. The CPU and the fetch-decode-execute cycle are heavily examined, and the memory and storage topics are reliable sources of marks because the distinctions (RAM versus ROM, the three storage types) come up year after year. The content links closely to data representation (everything stored is binary) and to the operating system (which manages memory and devices).

This guide walks through the Computer systems and hardware content and ties together the matching dot-point pages, each of which has its own worked examples and practice questions.

The CPU and von Neumann architecture

The CPU fetches, decodes and executes instructions. Inside it, the ALU does arithmetic and logic, the control unit coordinates the processor and sends control signals, and registers are tiny, very fast stores for values in immediate use. The von Neumann (stored-program) architecture stores instructions and data together in the same main memory, fetching instructions one at a time, which makes the computer a flexible, general-purpose machine.

The fetch-decode-execute cycle

The CPU runs programs by repeating the fetch-decode-execute cycle. Fetch copies the next instruction from memory using the program counter, which is then increased. Decode has the control unit work out the instruction. Execute carries it out, often using the ALU and the accumulator. The memory address register holds the address being accessed and the memory data register holds the data or instruction in transit. The cycle repeats billions of times a second.

Processor performance

Three factors mainly affect performance. Clock speed is cycles per second, so a higher clock speed does more per second. The number of cores lets the CPU process several instructions at once, but only if the software uses them. Cache is fast memory near the CPU holding likely-needed data, cutting waiting time for slower RAM. No single factor decides real-world speed, because RAM, storage speed and software all matter too.

Memory: RAM, ROM and virtual memory

RAM is the volatile working memory holding programs and data in use; its contents are lost without power. ROM is non-volatile and stores the fixed startup (BIOS) instructions. Volatile means lost without power; non-volatile means kept. Virtual memory uses slower secondary storage as extra RAM when RAM is full, letting larger or more programs run at the cost of speed.

Secondary storage

Secondary storage gives long-term, non-volatile storage because RAM is volatile. Magnetic storage (hard disks) is high-capacity and cheap per gigabyte but slower with moving parts. Optical storage (CDs, DVDs) is cheap and portable but low-capacity and easily scratched. Solid-state storage (SSDs, flash) is fast, durable and silent but costs more per gigabyte. To find how many files fit, convert to the same unit and divide capacity by file size.

Input and output devices

Input devices send data into the computer (keyboard, mouse, microphone, scanner, barcode reader, sensor); output devices present data to the user (monitor, printer, speakers). A computer is an input-process-output system, often with storage. Some devices, such as a touchscreen, are both input and output. When choosing devices for a task, pick ones suited to the purpose, environment and user, and justify the choice.

Check your knowledge

A mix of CPU, cycle, performance, memory, storage and device questions covering the Computer systems and hardware content. Attempt them under timed conditions, then check against the solutions.

1. State the role of the control unit (CU) in the CPU. (1 mark)
2. State one feature of the von Neumann architecture. (1 mark)
3. Name the three stages of the fetch-decode-execute cycle. (1 mark)
4. State what the program counter (PC) holds. (1 mark)
5. Explain how more cores can improve performance, and one limit. (2 marks)
6. State whether RAM is volatile or non-volatile, and what that means. (2 marks)
7. Give one advantage and one disadvantage of solid-state storage compared with a hard disk. (2 marks)
8. A $64\,\text{GB}$ card stores $8\,\text{MB}$ photos. How many photos fit? (2 marks)