Systems architecture: cache size.

The idea

Cache is small, very fast memory built into or close to the CPU that holds frequently used instructions and data so the processor does not have to wait for slower main memory (RAM). A larger cache can store more of this frequently used content, increasing the chance of a cache hit and reducing average access time, which can improve performance. Cache is much smaller than RAM but far quicker, and it sits between the CPU and RAM in the memory hierarchy. Increasing cache size helps only up to a point and depends on how the program uses memory; it is a separate factor from clock speed and number of cores.

What earns the marks

• States that cache is small, fast memory close to the CPU holding frequently used instructions and data.
• Explains that a larger cache can hold more frequently used content, raising the chance of a cache hit.
• Links larger cache size to reduced average time waiting for slower main memory and so improved performance.
• Distinguishes cache from RAM (larger but slower) and from clock speed and core count as separate performance factors.
• Recognises that the benefit of a larger cache is limited and depends on how software uses memory.

Common mistakes and how to put them right

• Confusing cache with RAM.

  Put it right: Cache is smaller and much faster and sits closer to the CPU, while RAM is larger main memory.

• Thinking cache size is measured in the same way as storage capacity and directly stores the whole program.

  Put it right: Cache holds frequently used instructions and data, not entire programs.

• Believing a bigger cache always gives a proportional speed increase.

  Put it right: Gains are limited and depend on memory access patterns.

Exam tips

• Look for the phrase about storing frequently used instructions and data when identifying the correct cache option.
• Reject options that describe clock speed or number of cores when the question is about cache size.
• Remember cache is faster but smaller than RAM, as distractors often swap these properties.