How common characteristics of CPUs affect their performance Clock speed…

The idea

CPU performance is influenced by clock speed, cache size and number of cores. Clock speed, measured in hertz (Hz), is the number of clock cycles per second; a higher clock speed generally means more instructions can be processed per second, although a full fetch-decode-execute cycle may take multiple clock cycles. Cache is fast memory inside or close to the CPU that stores frequently used instructions and data; a larger cache reduces the need to access slower main memory, improving performance. More cores allow multiple instructions or programs to be processed simultaneously, which can improve performance for tasks that can be parallelised. However, performance gains are limited by factors such as cache misses, memory bandwidth, thermal limits and how well software uses multiple cores.

What earns the marks

• States that clock speed is measured in hertz (Hz) and indicates clock cycles per second.
• Explains that a higher clock speed generally increases the number of instructions processed per second, while noting that a full fetch-decode-execute cycle may take multiple clock cycles.
• Describes cache as fast memory that stores frequently used instructions and data close to the CPU.
• Explains that a larger cache can reduce access to slower main memory and improve performance.
• States that more cores allow multiple instructions or programs to be processed simultaneously.
• Explains that performance gains from multiple cores depend on software being able to use them in parallel.
• Recognises that other factors such as cache misses, memory bandwidth or thermal limits can affect performance.

Common mistakes and how to put them right

• Thinking that doubling the number of cores always doubles performance.

  Put it right: Performance depends on how well software can use parallel processing.

• Confusing clock speed with the number of cores.

  Put it right: Clock speed is clock cycles per second; cores are independent processing units.

• Believing cache stores all data permanently.

  Put it right: Cache is small, fast temporary storage for frequently used instructions and data.

• Assuming a higher clock speed always guarantees better real-world performance.

  Put it right: Other factors such as cache size, number of cores and memory speed also matter.

• Defining clock speed as fetch-decode-execute cycles per second.

  Put it right: Clock speed is clock cycles per second; one fetch-decode-execute cycle may require several clock cycles.

Exam tips

• When explaining an effect, link the characteristic to the fetch-decode-execute cycle or to memory access, for example 'a larger cache reduces the number of slow main memory accesses'.
• Use comparative language such as 'higher', 'larger' and 'more' and state the direction of the effect on performance.
• For questions about multiple cores, mention parallel processing and the need for suitable software.
• Avoid absolute claims; use 'generally' or 'can' where performance depends on other factors.