Compare RAM and ROM as primary storage
Explain why a computer uses both RAM and ROM and distinguish their roles and persistence.
Section 3 · Hardware
This sequence follows Cambridge IGCSE Computer Science 0478, 2026–2028 syllabus Version 6. It covers subject content sections 3.1–3.3 only. The separately published 2029–2031 syllabus is not mapped here. Practice is original, self-checked and does not award an official Cambridge grade. Full timed papers and human subject-teacher review remain outstanding.
Before you begin
- No programming language is required. Be comfortable with place value and multiplication; each lesson introduces the computer-science notation it uses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does primary storage mean in the syllabus?
What you will learn
- Define primary storage
- Compare RAM with ROM
- Explain why the computer needs both
Primary storage is directly accessed by the CPU
Primary storage is accessed directly by the CPU. Random access memory (RAM) holds programs and data currently in use; it is normally volatile, so its contents are lost when power is removed.
Apply and check the rule
Read-only memory (ROM) stores instructions needed to start the computer and is non-volatile. A computer needs RAM for active work and ROM for persistent startup instructions. ROM is not a replacement for writable working memory.
Pause and explain
Why is ROM useful when a computer is switched on?
Worked example
A computer starts after being switched off and then opens a program. Which storage helps with each stage?
Show the worked solution
- ROM retains startup instructions without power.
- The operating system and program are loaded into RAM for current use.
- The CPU accesses primary storage while processing.
Answer ROM retains startup instructions; RAM holds the operating system and active program while they run.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the main difference between RAM and ROM when power is removed.
Give me a hint
Compare whether their stored contents persist.
Compare my reasoning
- RAM is normally volatile.
- ROM is non-volatile and retains its contents.
RAM loses its contents without power; ROM retains stored startup instructions.
Look for these in your work
- RAM persistence
- ROM persistence
A running program is being edited. Where is its current working data held?
Give me a hint
Consider data the CPU needs during active processing.
Compare my reasoning
- The program is loaded for current use.
- RAM holds the active program and its working data.
RAM
Look for these in your work
- Current use recognized
- RAM selected
A student says ROM can replace RAM because both are primary storage. Explain why the computer needs both.
Give me a hint
Separate startup instructions from writable active work.
Compare my reasoning
- ROM retains startup instructions when power is off.
- RAM provides volatile working space for the operating system, programs and current data.
- Their different roles make them complementary rather than interchangeable.
ROM keeps startup instructions; RAM stores programs and data in use. The computer needs persistent startup code and writable working memory.
Look for these in your work
- ROM role
- RAM role
- Need for both explained
Common mistakes
- Saying RAM keeps its contents without power
- Describing ROM as the location for all active programs
What can you explain now?
Which primary storage is usually volatile?
Compare with the explanation
RAM
RAM's working contents are lost when power is removed.
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Make it stick
Which memory is volatile, and which retains startup instructions without power?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.