Primary storage (memory): Cache

The idea

OCR J277 explicitly lists Cache as a standalone item in 1.2.1 Primary storage (memory), while also naming cache as a CPU component in 1.1.1 and cache size as a performance factor in 1.1.2. Cache is small, fast, volatile memory close to or inside the CPU. It keeps copies of frequently used instructions and data so the processor waits less often for slower RAM. A cache hit supplies the requested item quickly; a miss requires a RAM fetch. Larger cache can improve average access time by retaining more useful items, although clock speed and core count also affect performance. J277 is untiered, so this teaching applies to every candidate.

What earns the marks

• Cache is a small, fast type of volatile memory located within the CPU architecture, used to store copies of frequently used instructions and data.
• Cache size is a key factor affecting CPU performance; a larger cache reduces the average time the CPU waits for data by storing more frequently used instructions.
• A cache hit occurs when requested data is found in the cache, which is much quicker than fetching it from main memory (RAM).
• A cache miss causes the CPU to fetch the required instruction or data from RAM and copy it into the cache for future use.
• Cache sits between registers and RAM in the memory hierarchy, being faster but smaller and more expensive per unit of storage than RAM.

Common mistakes and how to put them right

• Thinking cache is a type of secondary storage.

  Put it right: Cache is volatile memory located inside the CPU architecture, unlike a hard disk or SSD.

• Confusing cache with RAM.

  Put it right: RAM is larger and slower main memory, while cache is smaller, faster and holds copies of data that RAM also stores.

• Assuming a bigger cache always means a faster computer.

  Put it right: Performance also depends on other factors such as CPU clock speed and the number of cores.

Exam tips

• Explicitly link cache to CPU performance: explain that a larger cache size means more frequently used data can be stored, reducing slower RAM accesses.
• When asked to compare memory types, give a clear point plus a reason, for example cache is faster than RAM because it is closer to the CPU and built from faster memory.