📚 Knowledge Library — Topic 3.7A — Hardware

Primary Storage

Understand the storage the CPU can use directly, including RAM, ROM, cache and registers, without going too deep too soon.

Start LearningWorked Example
1. Invitation

The CPU needs somewhere close to work.

When a program is running, the CPU needs quick access to data and instructions.

Primary storage is storage that the CPU can access directly while the computer is working.

💡 Remember: primary storage is close to the CPU and used while programs are running.
Figure 1.1
Close to the CPU
CPU
↔
Primary Storage
↓
Working data
2. Big Idea

Primary storage is working storage.

Primary storage is not mainly for keeping files forever.

It helps the CPU run programs, hold instructions, store temporary results and access important start-up instructions.

💡 Big idea: primary storage supports the computer while it is working.
Figure 2.1
Working Storage
Open program
↓
Data in use
↓
CPU processes it
3. FutureLogic Bridge

Think of a desk, not a warehouse.

Your desk holds the work you are using right now. It is quick to reach, but it is not where you store everything permanently.

Primary storage is like the computer's desk: close, fast and used for current work.

💡 Bridge: primary storage is the desk. Secondary storage is the filing cabinet.
Figure 3.1
Desk Model
Desk
= current work

Filing cabinet
= long-term files
4. Worked Example

RAM stores what is running now.

RAM stands for Random Access Memory.

It stores programs, data and instructions that are currently in use. It is volatile, so its contents are lost when the power is turned off.

Example: opening a game

Game file
loaded
RAM
holds data
CPU
uses it
The game runs from data currently stored in RAM.
Figure 4.1
RAM
Power on
↓
RAM holds work

Power off
↓
Data lost
Volatile means lost without power.
5. ROM

ROM stores start-up instructions.

ROM stands for Read Only Memory.

It is non-volatile, so it keeps its contents without power. It commonly stores firmware, BIOS or bootstrap instructions needed when the computer starts.

💡 ROM keeps important start-up instructions even when the computer is switched off.
Figure 5.1
ROM
Power off
↓
Instructions stay

Power on
↓
Computer starts
6. Cache and Registers

Some storage is even closer to the CPU.

Cache is small, fast memory that stores frequently used data and instructions.

Registers are tiny, very fast storage locations inside the CPU. You have already met: PC, MAR, MDR, CIR and ACC.

🎯 Exam Tip: cache stores frequently used data and instructions. Registers are inside the CPU.
Figure 6.1
Close Storage
Registers
↓ fastest
Cache
↓
RAM
↓
Secondary storage
7. Volatile vs Non-Volatile

Power matters.

Volatile storage loses its contents when power is turned off.

Non-volatile storage keeps its contents without power.

Quick check

RAM
volatile
ROM
non-volatile
SSD/HDD
non-volatile
Figure 7.1
Power Test
Power off

RAM = gone
ROM = stays
This distinction appears often in exams.
8. Exam Tip

Do not mix up RAM and ROM.

RAM stores data and programs currently in use. ROM stores start-up instructions.

RAM is volatile. ROM is non-volatile.

🎯 Exam Tip: if the question says currently running programs, think RAM. If it says start-up instructions, think ROM.
Figure 8.1
RAM vs ROM
RAM
= running now

ROM
= start-up code
9. Common Mistake

Primary storage is not just storage inside the case.

Students sometimes think primary storage means any storage inside the computer.

The important point is that primary storage is directly accessible by the CPU.

⚠️ Common Mistake: an internal SSD is inside the computer, but it is still secondary storage, not primary storage.
Figure 9.1
Inside Does Not Mean Primary
RAM
= primary

Internal SSD
= secondary
10. Summary

Primary storage in one screen.

Primary storage is directly accessible by the CPU.

RAM stores currently running data and programs. ROM stores start-up instructions. Cache stores frequently used data. Registers hold tiny pieces of data inside the CPU.

💡 Key idea: primary storage helps the CPU work quickly with what it needs now.
Figure 10.1
Primary Storage
RAM
ROM
Cache
Registers
↓
CPU support