3.6 Primary Storage Trainer
RAM vs ROM — volatile vs non-volatile, what each stores, and why it matters. Clean marks every session.
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What do I need to know before I start?
Primary storage is the memory the CPU can access directly. There are two types: RAM, which holds the programs and data currently in use and loses everything when the power goes off, and ROM, which permanently holds the start-up instructions. The exam tests the exact differences — volatile vs non-volatile, what each stores, and why the CPU needs both. Get RAM and ROM straight and this topic is quick marks.
By the end of 3.6 you can…
| Objective | Why it matters |
|---|---|
| Define primary storage and say why the CPU needs it | Primary storage is directly accessed by the CPU — the defining feature versus secondary storage. |
| State what RAM does and that it is volatile | RAM holds currently running programs, data and parts of the OS; contents are lost when power is off. |
| State what ROM does and that it is non-volatile | ROM holds the boot/start-up instructions (BIOS, bootstrap); contents are kept when power is off. |
| Compare RAM and ROM across the key features | Volatility, whether contents change, and what each stores are all commonly tested. |
| Explain why a computer needs both | ROM starts the machine; RAM runs everything after start-up. |
Cambridge-approved terms
Memory that the CPU can access directly. Includes RAM and ROM.
Random Access Memory. Holds currently running programs and data. Volatile — contents lost when power is off.
Read Only Memory. Holds start-up instructions. Non-volatile — contents kept when power is off.
Loses its contents when power is removed (RAM).
Keeps its contents when power is removed (ROM).
The start-up instructions stored in ROM that begin loading the operating system when the computer is switched on.
RAM vs ROM
| Feature | RAM | ROM |
|---|---|---|
| Volatile? | Yes — data lost when power off | No — non-volatile, data kept |
| Directly accessed by CPU? | Yes | Yes |
| Do contents change? | Constantly, as data is processed | Fixed — not normally changed |
| Stores | Currently running programs, data, parts of the OS | Boot/start-up instructions (BIOS, bootstrap) |
Traps that cost marks every session
RAM is volatile — its contents are lost when the power is turned off. Only ROM keeps its contents.
Both RAM and ROM are primary storage and are directly accessed by the CPU.
ROM is primary storage for start-up instructions — not a hard drive or SSD.
RAM holds the currently running programs, data and parts of the OS — not permanent files.
Different things. RAM is capacity for running programs; clock speed is FDE cycles per second (3.3).
Volatile = loses data (RAM). Non-volatile = keeps data (ROM). Learn the pair together.
Check yourself — tap to reveal
Answer in your head, then tap to see the model answer. Aim for 5 of 6 before moving to Learn.
✅ Ready for Learn?
Before moving on, make sure you can answer these without notes:
- What makes storage "primary"?
- Is RAM volatile or non-volatile — and what does it store?
- Is ROM volatile or non-volatile — and what does it store?
- Give three differences between RAM and ROM.
- Why does a computer need both?
What is primary storage?
Primary storage
Storage that is directly accessed by the CPU. Connected to the CPU via buses. Two types: RAM and ROM.
RAM vs ROM
💙 RAM (Random Access Memory)
Volatile — data is lost when power is turned off.
Stores data and programs currently in use.
Contents constantly change as data is processed.
Can be increased by adding more RAM.
Read and write — CPU can read from and write to RAM.
💚 ROM (Read Only Memory)
Non-volatile — data is kept when power is turned off.
Stores the bootstrap / BIOS (start-up instructions).
Contents are fixed and do not normally change.
Not easy to alter — reprogramming is complex and rare.
Read only — CPU can read from ROM but not normally write.
| RAM | ROM | |
|---|---|---|
| Volatile? | Yes — data lost without power | No — data kept without power |
| Stores | Programs/data currently in use | Bootstrap / BIOS |
| Contents | Constantly changing | Fixed / rarely changed |
| Expandable? | Yes — can add more | No — difficult to alter |
| Read/Write | Read AND write | Read only |
Why does the computer need both?
The computer needs ROM to start up — without it, the computer wouldn't know how to boot. The bootstrap in ROM loads the operating system from secondary storage into RAM. Once running, RAM stores everything currently being used. When you turn the computer off, RAM empties (volatile) but ROM keeps its start-up instructions (non-volatile) ready for next time.
Quick check
RAM is volatile. This means:
Activity 1 — RAM or ROM?
Press New Question.
Activity 2 — True or False?
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Activity 3 — Spot the Mistake
Press New Question.
Adaptive practice
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Cambridge-style questions
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Review quiz
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Exam traps
Memory triggers
🧠 "RAM = Running And Moving"
RAM stores what's currently running. Its contents are always moving/changing.
🧠 "ROM = Read Only, Mostly permanent"
ROM is read only. Its contents rarely change. It holds the boot instructions.
🧠 "Volatile = Vanishes"
Volatile starts with V, Vanishes starts with V. Power off = data vanishes.