GCSE · Computer Science · AQA · Spec 8525

CPU performance: cache size

Cache is tiny but mighty — it's the secret reason your CPU doesn't grind to a halt.

What you need to know

  • Cache is faster than RAM and sits between the CPU and RAM in the memory hierarchy.
  • A larger cache means the CPU can store more frequently used data nearby, reducing slow RAM accesses.
  • When the CPU finds what it needs in cache it is called a cache hit; when it does not, it is a cache miss and the CPU must fetch from RAM.
  • More cache improves performance by reducing the average time the CPU spends waiting for data.

The big picture

Cache is a small, very fast type of memory built into or very close to the CPU. It stores frequently used data and instructions so the CPU can access them without waiting for the slower main memory (RAM). The more cache a CPU has, the more data it can hold close by, reducing the number of slower RAM accesses. This means a larger cache generally leads to better CPU performance — but cache is expensive to manufacture, so CPUs only have small amounts of it.

TONIGHT'S REVISION

CPU Performance: Cache Size

How the amount of fast memory near the CPU determines how often it has to wait for data

Computer Science · Structure

The Memory Hierarchy: CPU and Cache

The CPU always checks the fastest store first. A cache miss at any level means a slower journey down the chain. Tap each node to see its role.

Check cache firstNot found → check next levelNot found → check largest levelMiss → fetch from RAM (slow)CPU (Processor)Cache (nearest the core)Cache (next level)Cache (largest level)Main memory (RAM)

Tap any part of the diagram to see what it does.

What the CPU does every time it needs data

Follow one memory request through the hierarchy — notice where cache size makes the difference.

Predict, then check

Think about what 'more cache' actually changes before you reveal the answer.

A CPU is running a large video-editing program and spends a lot of time waiting for data. Explain how increasing the cache size could reduce that waiting.

Cache size and CPU speed

?

Reason it through

Why does increasing cache size make a CPU faster?

Link 1 of 4

First link · your turn

Where does the CPU look for data before it checks RAM?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Cache hit vs cache miss

Cache HitvsCache Miss

Understanding the contrast between these two outcomes is the core of why cache size matters.

Focus

What happens

Cache Hit

CPU finds the required data already in cache

Cache Miss

CPU does not find the data in cache

The insight

This is the defining split — the entire performance story of cache size hangs on which of these two outcomes occurs more often.

Where data comes from

Cache Hit

Retrieved directly from fast cache

Cache Miss

Must be fetched from much slower RAM

Effect on the CPU

Cache Hit

CPU continues executing with only a short delay

Cache Miss

CPU waits noticeably longer while the data arrives from main memory

How cache size helps

Cache Hit

More cache = more data held nearby = hit more likely

Cache Miss

More cache = misses are rarer, but each individual miss is still just as slow

Cross-subject · Exam skill

Command Word Coach

AQA 8525 — know what each command word demands on cache questions

ExplainBloom · Understand / Apply

01Definition

Give a reason AND develop it — name the effect and say WHY it happens using a causal chain.

02Mark-band signal

A strong answer links cause to effect: cache size → how often main memory is needed → waiting time → speed.

03Sentence stems

  • Explain how cache size affects the performance of a CPU.
  • Explain why a CPU with a larger cache is generally faster.

04Common mistake

Writing only 'more cache = faster' leaves the reasoning out — chain it: a larger cache holds more frequently used data → fewer trips to main memory → less waiting → faster performance.

Key points

1Cache is much faster to access than main memory, because it is small and sits close to the CPU.
2Larger cache = more data stored close to the CPU = fewer cache misses = faster execution.
3A cache miss forces the CPU to fetch data from RAM, which stalls the processor and slows it down.
4Cache stores frequently used data and instructions close to the CPU, so they do not have to be fetched from slower main memory.
5Doubling cache size does not double performance — the benefit shrinks once the most-used data already fits.

Worked example

Problem

A student is comparing two CPUs. CPU A has a smaller cache and CPU B has a larger cache. Both have identical clock speeds and the same number of cores. Explain why CPU B is likely to be faster when running large applications.

🧠

Memory hook

Think of the CPU as a chef and cache as the worktop: the bigger the worktop, the more ingredients (data) within arm's reach — no slow trips to the fridge (RAM) needed.

★ Exam tip

When a question asks you to explain how cache size affects performance, set out the causal chain rather than a single statement: a larger cache can hold more of the data and instructions the CPU uses most often, so the CPU needs to fetch from slower main memory less often, so less time is spent waiting. Explaining each link shows your reasoning.

⚠ Watch out

Treating clock speed as the only factor — a CPU that is waiting for data from main memory is idle however fast it is clocked.

✓

Check yourself

Without looking — what is the difference between a cache hit and a cache miss, and why does a cache miss slow the CPU down?

Flashcards

(22)
What is cache memory?
A small, very fast memory store built into or very close to the CPU that holds frequently used data and instructions.
Why is cache faster than RAM?
Cache sits close to the CPU and uses faster memory technology, so access is much quicker than main memory.
What is a cache hit?
When the CPU looks for data and finds it already stored in cache, so no slow RAM access is needed.
What is a cache miss?
When the CPU looks for data in cache and does not find it, forcing it to fetch the data from the slower RAM.
How does a larger cache improve CPU performance?
It can store more frequently used data close to the CPU, so fewer requests have to go to slower main memory.
What are the three main cache levels?
Going further (not required for AQA 8525): caches are often built in levels, with the level nearest the core being smallest and fastest.
Which cache level is fastest?
The level closest to the CPU core is the fastest — going further, this is beyond what AQA 8525 requires.
Which cache level is largest?
Of the levels a processor may have, the one furthest from the core is typically the largest. Beyond the specification: for AQA 8525 you need only the effect of cache size on CPU performance.
What happens to performance when a cache miss occurs?
The CPU has to wait while the data is fetched from main memory, which slows execution.
Does doubling cache size double CPU performance?
No — the benefit decreases as cache grows. Once the most-used data fits in cache, adding more has less impact.
Where does cache sit in the memory hierarchy?
Between the CPU registers and RAM — faster than RAM but much smaller.
Why do CPUs not have very large caches?
Cache (SRAM) is expensive to manufacture and takes up significant chip area, so only small amounts are practical.
What type of data is stored in cache?
Frequently used instructions and data — the CPU copies them from RAM based on recent and predicted usage.
What is the memory hierarchy?
The ordering of storage from fastest/smallest (registers, cache) to slowest/largest (RAM, secondary storage).
How does cache size affect a program running a large loop?
If the loop's instructions fit in cache, each iteration runs quickly. If they do not, the CPU has to fetch from slower main memory more often, which slows it down.
What does 'cache hit rate' mean?
The proportion of memory accesses satisfied by cache rather than RAM — a higher rate means better performance.
Give one advantage of a larger cache.
Fewer cache misses means the CPU spends less time waiting for data, improving overall execution speed.
Give one disadvantage of a larger cache.
It is more expensive to manufacture and increases the physical size and power consumption of the chip.
Why is a cache kept small?
Keeping it small helps keep it fast and close to the CPU; cache memory is also more expensive than main memory.
If two CPUs have the same clock speed but different cache sizes, which is likely faster for complex tasks?
The one with more cache, because it will have fewer cache misses and spend less time stalled waiting for RAM.
What term describes the CPU being forced to wait because data is not yet available from RAM?
The processor waits, doing no useful work, until the data arrives from main memory.
What is SRAM?
Static RAM — the fast, expensive memory technology used in cache. Much faster than the DRAM used in main RAM.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learn CPU performance: cache size properly — interactive practice, marked questions and flashcards.

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