Spec & Goals 3 min
AQA Spec 3.4.5 · Memory and storage
By the end of this lesson you can:
- Describe the purpose of RAM and ROM, and explain volatile vs non-volatile.
- Explain why a computer needs secondary storage.
- Compare magnetic, optical and solid-state storage on capacity, speed, cost and durability.
Warm-Up 5 min
Last lesson you measured CPU performance using clock speed, cores and cache. Now we look at where programs and data are held.
Quick starter
Aiman switches off his laptop while a document is open but unsaved. When he reboots, the work is gone. Why?
Reveal the answer
The open document was in RAM, which is volatile — it loses its contents when the power is removed. Only saved files on storage survive.
Key Concept — main memory and secondary storage 14 min
The computer keeps the programs and data it is using right now in main memory. There are two kinds: RAM and ROM.
Volatile vs non-volatile
This is the key difference the examiner tests.
Why secondary storage is needed
RAM is volatile and limited in size. Data that must be kept long-term cannot live there.
So files, applications and the operating system are stored on secondary storage and copied into RAM when needed.
Three types of secondary storage
AQA expects you to know three categories and an example of each.
Worked Example — comparing memory and storage 12 min
Two comparison tables that summarise everything the exam asks for.
1. RAM vs ROM
| Feature | RAM | ROM |
|---|---|---|
| Volatility | Volatile (lost on power-off) | Non-volatile (kept on power-off) |
| Read / write | Read and write | Read-only |
| What it holds | Programs & data currently in use | Start-up instructions (bootstrap / firmware) |
| Changes during use | Contents change constantly | Contents fixed |
2. Magnetic vs optical vs solid-state
| Property | Magnetic (HDD) | Optical (DVD) | Solid-state (SSD) |
|---|---|---|---|
| Capacity | Large | Low | Medium–large |
| Speed | Medium | Slow | Fast |
| Cost per GB | Cheap | Cheap | More expensive |
| Durability | Moving parts (fragile) | Scratches easily | No moving parts (durable) |
| Portability | Heavier | Portable | Light, portable |
Try It Yourself 12 min
Goal: State whether each is volatile or non-volatile: RAM, ROM, an SSD.
Hint: only RAM loses its contents when the power is removed.
Goal: Explain why a computer needs secondary storage as well as RAM.
Hint: think about what happens to RAM when the power is switched off.
Goal: A Penang film studio must archive 50 TB of footage cheaply for many years. Recommend a storage type and justify your choice.
Hint: weigh up capacity and cost per GB against speed.
📝 Exam Practice 10 min
Answer the way the examiner expects — the command word and the marks tell you how much to write.
State one difference between RAM and ROM.
Mark scheme
- RAM is volatile / read-write / holds programs & data in use (1).
- ROM is non-volatile / read-only / holds the start-up instructions (1).
Define the term volatile.
Mark scheme
- Loses its contents when the power is switched off (1).
A school needs fast, durable storage for its laptops. Recommend a storage type and justify your choice.
Mark scheme
- Solid-state / SSD (1).
- Justified: no moving parts so durable / fast read-write speed (1).
Give one suitable use of optical storage.
Mark scheme
- Distributing films / music / software on disc, or a cheap portable backup (1).
Recap & Key Terms 3 min
Main memory is RAM (volatile, read-write) and ROM (non-volatile, read-only). Because RAM is volatile and limited, long-term data lives on non-volatile secondary storage — magnetic, optical or solid-state — each with its own trade-offs.
- RAM
- Random access memory — volatile, read-write memory holding programs and data in use.
- ROM
- Read-only memory — non-volatile memory holding the start-up (bootstrap) instructions.
- Volatile
- Loses its contents when the power is switched off.
- Non-volatile
- Keeps its contents when the power is switched off.
- Secondary storage
- Non-volatile storage used to keep programs and data permanently.
- Magnetic
- Storage using magnetised patterns on spinning platters (e.g. HDD); large and cheap, with moving parts.
- Optical
- Storage read and written by a laser on a disc (e.g. CD/DVD/Blu-ray); cheap and portable, low capacity.
- Solid-state
- Flash storage with no moving parts (e.g. SSD/USB); fast and durable, more expensive per GB.
Homework 1 min
Task (≤ 15 min): Explain why RAM and ROM are both needed in a computer, and name one type of secondary storage that would suit a lightweight school laptop. Justify your choice.
Model answer
RAM holds the programs and data currently in use but is volatile, so its contents are lost on power-off. ROM is non-volatile and holds the start-up instructions that boot the machine.
A solid-state drive suits a laptop: it has no moving parts (durable when carried) and is fast.
Award marks for: RAM purpose / volatile (1), ROM purpose / non-volatile (1), solid-state named with a valid justification (1).