← IGCSE Computer Science Paper 1

Why a computer needs both RAM and ROM

ROM holds the start-up instructions that survive power-off. RAM holds the changing operating system, programs and data in use. A computer needs both to start up and to run.

Part of: IGCSE Computer Science 0478 — 3.3 Data Storage — Module 6

Understanding Why a computer needs both RAM and ROM in IGCSE Computer Science Paper 1

What it does: The bootstrap in ROM runs first and loads the operating system into RAM, which shows why both are needed. ROM cannot be easily changed, so a computer with only ROM would have nowhere to keep programs and data in use. ROM is non-volatile, so the start-up instructions are still there after the power has been off. Programs and data in use change all the time, so they need RAM, which can be read and written to. ROM is non-volatile, so the bootstrap survives power-off and runs when the computer is switched on. RAM is fast and can be written to, so it holds the changing software and data in use.

Common mistakes: The operating system is copied into RAM, not ROM. ROM is read-only and holds only the start-up instructions. It should say 'bootloader loads OS into RAM'. ROM is non-volatile, so it keeps its contents when the power is off. That is why it can hold the start-up instructions. It should say 'ROM keeps data without power'.

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Learning
Duo
Which memory holds the start-up instructions (bootstrap) that run when the computer is switched on?
RAMROM
Matches
A computer works with only RAM, because RAM keeps the start-up instructions when the power is off
↓
RAM is volatile and loses its contents when the power is off, so it cannot keep the start-up instructions. These are stored in ROM, so the computer needs ROM as well.
Applying
Meanings
Term
RAM and ROM
Meaning
ROM holds the start-up instructions that survive power-off. RAM holds the changing operating system, programs and data in use. A computer needs both to start up and to run.
Practice
power ← ON
Testing
Gaps
READ bootstrap FROM
Check Answer
Spots
2ROM runs the bootloader
3bootloader loads OS into ROM

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Quick Questions

1. Which memory holds the start-up instructions (bootstrap) that run when the computer is switched on?
A. RAM
B. ROM ✓
ROM is non-volatile, so the start-up instructions are still there after the power has been off.
2. Which memory holds the operating system and programs while the computer is running?
A. ROM
B. RAM ✓
Once the computer has started, the operating system is loaded into RAM, where the CPU can access it quickly and it can change.
3. A computer has ROM but no RAM. What would happen?
A. It could start up but could not run programs or hold data in use ✓
B. It would run perfectly well as ROM can store everything
ROM is usually read-only and small, so there would be nowhere to hold changing programs and data while they are used.
4. A computer has RAM but no ROM. What would happen?
A. It would not know how to start up when switched on ✓
B. It would start up normally using the RAM
RAM is volatile and loses its contents when the power is off, so the start-up instructions would be lost.
5. Why can't RAM store the start-up instructions?
A. RAM is volatile, so it loses its contents when the power is off ✓
B. RAM is too slow to store instructions
The start-up instructions must survive power-off. Only non-volatile ROM can keep them.

Match the Pairs

ROM holds the bootstrap → The start-up instructions are stored in ROM so they are still there when the power is switched off and the computer can start.
RAM holds the OS and open programs → Once the computer is running, the operating system, programs and data in use are loaded into RAM, where they can change quickly.
Switch-on order: ROM runs the start-up instructions first, then the operating system is loaded into RAM → Power on -> ROM bootstrap -> OS loaded into RAM
A computer with only ROM could start up but could not hold changing programs and data to run them → ROM only: boots, but cannot store open programs and data
A computer works with only RAM, because RAM keeps the start-up instructions when the power is off → RAM is volatile and loses its contents when the power is off, so it cannot keep the start-up instructions. These are stored in ROM, so the computer needs ROM as well.
The operating system and open programs run from ROM, so a computer does not need RAM → ROM holds fixed start-up instructions and is not used for the changing programs and data in use. These are held in RAM, so the computer needs RAM as well.

Key Term

RAM and ROM
ROM holds the start-up instructions that survive power-off. RAM holds the changing operating system, programs and data in use. A computer needs both to start up and to run.

Fill in the Blanks

1. POWER ON READ bootstrap FROM ROM
Hint: Type the memory that permanently stores the start-up instructions.
2. LOAD operatingSystem INTO RAM
Hint: Type the memory that holds the operating system and programs while the computer is running.
3. IF power = OFF THEN RAM = EMPTY ENDIF
Hint: Type the power state that makes the volatile memory lose its contents.
4. IF ROM = EMPTY THEN OUTPUT "Cannot start up" ENDIF
Hint: Type the memory that must hold the bootstrap for the computer to start.
5. IF hasROM AND hasRAM THEN computerWorks = TRUE ENDIF
Hint: Type the logical operator that means both memory types must be present.

Spot the Error

Question 1
A POWER ON
B ROM runs the bootloader
C bootloader loads OS into ROM
D OS starts the programs
Line C — The operating system is copied into RAM, not ROM. ROM is read-only and holds only the start-up instructions. It should say 'bootloader loads OS into RAM'.
Question 2
A ROM loses data without power
B RAM loses data without power
C So ROM holds the boot program
Line A — ROM is non-volatile, so it keeps its contents when the power is off. That is why it can hold the start-up instructions. It should say 'ROM keeps data without power'.
Question 3
A IF power is switched on THEN
B run start-up from RAM
C load the OS into RAM
D ENDIF
Line B — RAM is empty when the computer is first switched on, so the start-up instructions must come from ROM. It should say 'run start-up from ROM'.
Question 4
A power <- ON
B CPU reads ROM
C OS copied to RAM
D CPU runs programs from ROM
Line D — Once the computer is running, the OS and programs in use are held in RAM. ROM only holds the fixed start-up instructions. It should say 'CPU runs programs from RAM'.
Question 5
A IF computer has only ROM THEN
B it can start up
C it can hold changing data
D ENDIF
Line C — ROM is read-only, so a computer with only ROM could not store changing programs and data in use. It should say 'it cannot hold changing data'.

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