Why not just one big fast store?: Because no such thing exists. Memory is either small and fast or big and slow — never both.
So a computer uses several, and keeps whatever it needs next in the fastest one.
Each bar's width is how much that store holds. Step through to see a cache hit, a cache miss, and what survives the power going off.
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The order to remember: Registers → cache → RAM → storage.
Each step down holds more and takes longer to reach.
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Registers
Inside the CPU, beside the ALU. Hold the handful of values being worked on right now — one value each.
Cache — L1, L2, L3
A small store of what was used recently, kept close by in case it is needed again. L1 is smallest and fastest, L3 largest and slowest.
RAM
Holds the programs and data in use now. Big enough for everything open, and cleared when the power goes off.
ROM
Read only and permanent. Holds the small program that starts the computer up, which must survive being switched off.
RAM and ROM are not a pair: The names look like opposites but the jobs are unrelated.
RAM is the big working space that empties. ROM is a tiny permanent store holding the start-up instructions.
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Look in the nearest place first: Before going to RAM, the CPU checks cache.
Cache hit — it is there, and the wait is short.
Cache miss — it is not, so the CPU fetches from RAM and copies it into cache on the way back.
A hit
- The value is already in cache
- No trip to RAM is needed
- The instruction finishes sooner
A miss
- The value is not in cache
- A slower trip to RAM is required
- The value is copied into cache, so next time hits
Why copying it back matters: Programs tend to use the same values again shortly afterwards. Copying a missed value into cache turns the next request into a hit — which is the whole reason cache works.
How this is tested — each store must be given a job NOTHING ELSE has, and hit/miss must be used correctly. It comes up two ways:
Paper 1 Section A
- Explain the purposes of the types of primary memory, 4-6 marks
- Define cache hit and cache miss
- Say why RAM is volatile and ROM is not
Paper 1 Section B — case study
- Explain why the case study's system needs more RAM or more cache
- Justify a storage choice from how that system is used
The classic trap: Writing that all of them "store data for the CPU". True, and worth one mark at most — the question asks what makes each one different.
Explain the purpose of registers, cache and RAM in a computer system, and explain what is meant by a cache miss.
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