Syllabus & Goals 3 min
Cambridge 3.3 · Data storage Paper 1 · Computer Systems
By the end of this lesson you can:
- Compare RAM and ROM (primary memory).
- Describe magnetic, optical and solid-state secondary storage.
- Explain virtual memory and cloud storage.
Recap / Warm-Up 5 min
The CPU works with memory. But there are different kinds — some lose their contents at power-off, some keep them forever.
Quick starter
When you switch off a computer without saving, your unsaved work is lost. Which kind of memory was it in?
Reveal the answer
RAM — it is volatile, so its contents vanish when the power is cut. Saving copies the work to non-volatile secondary storage.
Key Concept 14 min
1 · Primary memory: RAM and ROM
Primary memory is directly addressable by the CPU.
| RAM | ROM |
|---|---|
| Temporary (volatile) | Permanent (non-volatile) |
| Can be read and written | Can only be read |
| Holds the OS/programs/data in use | Holds the BIOS / start-up instructions |
| Bigger RAM = faster computer | Fixed contents set by the manufacturer |
2 · Secondary storage
Secondary storage is non-volatile, larger and not directly addressable by the CPU. Three technologies:
3 · Virtual memory and cloud storage
Worked Example 12 min
(a) RAM or ROM?
| Job | RAM or ROM |
|---|---|
| Holds the program you are running right now | RAM |
| Holds the start-up (BIOS) instructions | ROM |
| Loses its contents at power-off | RAM |
| Cannot be changed by the user | ROM |
(b) Choosing secondary storage
| Scenario | Best choice |
|---|---|
| Fast boot drive for a thin, light laptop | SSD (no moving parts, fast, light) |
| Cheap, very large backup of old photos | Magnetic HDD (low cost per GiB) |
| Distributing a high-definition movie | Blu-ray (large capacity, blue laser) |
Try It Yourself 12 min
Goal: State whether RAM and ROM are each volatile or non-volatile.
Goal: Give two advantages of an SSD over an HDD.
Goal: Explain how virtual memory lets a computer keep running when RAM is full.
📝 Exam Practice 10 min
State two differences between RAM and ROM.
Mark scheme
- RAM is volatile / temporary; ROM is non-volatile / permanent (1).
- RAM can be read and written; ROM is read-only (1).
Describe how data is stored on a magnetic hard disk drive.
Mark scheme
- Data is stored magnetically on spinning platters (1)…
- …in sectors and tracks, read/written by read-write heads (1).
Explain one benefit and one drawback of using cloud storage.
Mark scheme
- Benefit: access from anywhere / automatic backup / no local hardware needed (1).
- Drawback: needs an internet connection / relies on a third party / possible security concerns (1).
Recap & Key Terms 3 min
Primary memory is RAM (volatile, in-use data) and ROM (non-volatile, BIOS). Secondary storage is non-volatile: magnetic (HDD), solid-state (SSD) and optical (CD/DVD/Blu-ray). Virtual memory borrows disk space when RAM is full; cloud storage keeps data on remote servers.
- RAM / ROM
- Volatile read-write primary memory / non-volatile read-only primary memory.
- Secondary storage
- Non-volatile storage not directly addressable by the CPU (HDD, SSD, optical).
- Virtual memory
- Using disk space as extra memory when RAM is full.
- Cloud storage
- Data stored on remote internet servers run by a third party.
Homework 1 min
Task (≤ 15 min): A remote-controlled toy car contains both RAM and ROM. State one thing stored in each.
Model answer
- ROM: the factory settings / start-up routines / how the buttons control the car.
- RAM: instructions received from the remote, or any user-programmed routines.