What factors affect how fast a processor performs, and how do clock speed, cores and cache change performance?
The factors affecting processor performance: clock speed, the number of cores, and the amount and use of cache memory.
A focused answer to the WJEC GCSE Computer Science Unit 1 content on processor performance, covering how clock speed, the number of cores and the amount of cache memory each affect how fast a CPU can process instructions, and why simply increasing one factor does not always make a computer faster.
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What this topic is asking
WJEC wants you to know the factors that affect how fast a processor performs: clock speed, the number of cores, and cache memory, and to explain how each changes performance. This is part of the Hardware content in Unit 1 of WJEC GCSE Computer Science (3500).
Clock speed
Number of cores
Cache memory
Why one factor is not enough
A faster processor is only part of the story: performance also depends on the amount of RAM, the speed of secondary storage and how well the software is written. Doubling the clock speed or cores does not double real-world speed if another part of the system is the bottleneck.
Try this
Q1. State what is meant by the clock speed of a CPU. [1 mark]
- Cue. The number of fetch-decode-execute cycles the CPU carries out each second (in hertz).
Q2. Give one reason why adding more cores might not speed up a particular program. [1 mark]
- Cue. The program may not be written to split its work across multiple cores.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC-style Unit 14 marksDescribe how clock speed and the number of cores each affect the performance of a processor.Show worked answer →
A Unit 1 performance question. The clock speed is the number of fetch-decode-execute cycles the CPU carries out each second, measured in hertz (1 mark); a higher clock speed means more instructions can be processed per second, so the computer is generally faster (1 mark). The number of cores is the number of separate processing units in the CPU; each core can carry out its own fetch-decode-execute cycle, so a multi-core CPU can process several instructions at the same time (1 mark), which speeds up tasks that can be split across cores or several programs running at once (1 mark). Markers reward cycles per second for clock speed and parallel processing for cores. A common error is to say more cores always doubles the speed, which is not true if the software cannot use them.
WJEC-style Unit 13 marksExplain how cache memory improves processor performance.Show worked answer →
A Unit 1 explain question. Cache is a small amount of very fast memory located in or very close to the CPU (1 mark). It stores copies of the instructions and data the CPU is most likely to need next, so the CPU can fetch them from cache instead of the much slower main memory (1 mark). This reduces the time the CPU spends waiting for data, so more cycles are spent doing useful work and performance improves (1 mark). Markers reward fast memory near the CPU and reduced waiting for main memory. A common error is to confuse cache with RAM, or to say cache stores all the computer's data.
Related dot points
- The purpose and main components of the CPU (the ALU, the control unit and registers) and the von Neumann stored-program architecture.
A focused answer to the WJEC GCSE Computer Science Unit 1 content on the CPU and von Neumann architecture, covering the purpose of the CPU, the roles of the arithmetic logic unit, the control unit and registers, and the von Neumann stored-program model where instructions and data share the same memory.
- The fetch-decode-execute cycle and the roles of the registers used in it (the program counter, memory address register, memory data register and accumulator).
A focused answer to the WJEC GCSE Computer Science Unit 1 content on the fetch-decode-execute cycle, covering the three stages of fetch, decode and execute, the roles of the program counter, memory address register, memory data register and accumulator, and how the cycle repeats to run a program.
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- The need for secondary storage, the characteristics of magnetic, optical and solid-state storage, and calculating storage requirements and capacity.
A focused answer to the WJEC GCSE Computer Science Unit 1 content on secondary storage, covering why secondary storage is needed, the characteristics, advantages and disadvantages of magnetic, optical and solid-state storage, and calculating how many files of a given size fit in a given storage capacity.
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A focused answer to the WJEC GCSE Computer Science Unit 1 content on input and output devices, covering the purpose of input devices and output devices, common examples of each, the idea of the computer as an input-process-output system, and how to choose suitable devices for a given situation.