← IGCSE Computer Science Paper 1

The start-up sequence of a computer

Power on, the bootloader in firmware runs and loads the OS from storage into RAM (which is empty at power-on). Once the OS is ready, the user can open applications.

Part of: IGCSE Computer Science 0478 — 4.1 Types of Software and Interrupts — Module 38

Understanding The start-up sequence of a computer in IGCSE Computer Science Paper 1

What it does: RAM is volatile and loses its contents when switched off, so the bootloader copies the OS from storage into RAM. Applications need the OS to run, so the user can only open them once the boot sequence has finished and the OS is ready. The OS is kept permanently on storage. During boot it is copied into RAM, which the CPU can read and write quickly. Applications rely on the operating system to manage RAM, the CPU and the hardware, so the OS must be ready first. The boot sequence is the name for the whole start-up process, from switching on to a working OS.

Common mistakes: Once the bootloader has loaded the OS into RAM, the OS is ready, so it should be OS_READY = TRUE. Only then can the user open applications. RAM is empty at power-on, so the bootloader cannot run from it. It is stored in firmware and runs from there: RUN bootloader FROM firmware.

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This module's key terms are also in a printable crossword (Modules 390–408) and word search (Modules 390–418).

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Learning

Duo

What runs first when a computer is switched on?

The bootloader, stored in firmwareThe web browser, stored in RAM

Matches

power_on -> open(app) -> load(OS)

↓

The application is opened before the OS has been loaded. The OS must be loaded into RAM first, then the application opened.

Applying

Meanings

Term

boot sequence

Meaning

Power on, the bootloader in firmware runs and loads the OS from storage into RAM (which is empty at power-on). Once the OS is ready, the user can open applications.

Practice

POWER_ON

Testing

Gaps

runBootloader()

Check Answer

Spots

3LOAD OS FROM storage TO RAM

4OS_READY = FALSE

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Quick Questions

1. What runs first when a computer is switched on?

A. The bootloader, stored in firmware ✓

B. The web browser, stored in RAM

The bootloader is stored in firmware (ROM) so it is available at power-on. Applications can only run later, once the OS is loaded.

2. Which statement about RAM at power-on is correct?

A. RAM already holds the operating system because it is non-volatile

B. RAM is empty because it is volatile and loses its contents when power is off ✓

RAM is volatile, so everything in it is lost when the power goes off. The OS has to be loaded into RAM again at every start-up.

3. What is the job of the bootloader?

A. To load the operating system from storage into RAM ✓

B. To load the user's applications into the CPU's registers

The bootloader's job is to find the operating system on storage and copy it into RAM so the computer can start working.

4. Where is the operating system loaded from during the boot sequence?

A. From RAM into secondary storage (e.g. an SSD)

B. From secondary storage (e.g. an SSD) into RAM ✓

The OS is kept permanently on storage. During boot it is copied into RAM, which the CPU can read and write quickly.

5. Which order of the boot sequence is correct?

A. Power on, bootloader runs, OS loaded into RAM, user opens applications ✓

B. Power on, user opens applications, bootloader runs, OS loaded into RAM

Applications need the operating system to run, so the OS must be loaded and ready before the user can open any application.

Match the Pairs

boot sequence → The ordered steps from power on: the bootloader runs, loads the OS into RAM, and the OS is then ready for applications

bootloader.run() → The bootloader in firmware starts first and begins loading the OS from storage into RAM

RAM is empty at power-on, so the OS must be copied into it from storage → load(OS, storage, RAM)

Applications can only be opened after the OS is loaded and ready → if OS_ready: open(app)

power_on -> open(app) -> load(OS) → The application is opened before the OS has been loaded. The OS must be loaded into RAM first, then the application opened.

power_on -> RAM holds OS -> bootloader → RAM is empty at power-on and does not hold the OS. The bootloader in firmware runs first and loads the OS into RAM.

Key Term

boot sequence

Power on, the bootloader in firmware runs and loads the OS from storage into RAM (which is empty at power-on). Once the OS is ready, the user can open applications.

Fill in the Blanks

1. powerOn() runBootloader(firmware) loadOS(storage, RAM)

Hint: Type the name of the place where the bootloader is stored and runs from

2. powerOn() runBootloader(firmware) loadOS(storage, RAM)

Hint: Type where the operating system is loaded from, because it is not in RAM at power-on

3. powerOn() ramUsed ← 0 runBootloader(firmware)

Hint: Type the number that shows how much RAM is in use at the moment of power-on

4. osReady ← TRUE IF osReady THEN openApplication("browser")

Hint: Type the variable that must be true before the user can open an application

5. powerOn() runBootloader(firmware) loadOS(storage, RAM)

Hint: Type the name of the step that copies the operating system into RAM

Spot the Error

Question 1

A POWER_ON

B RUN bootloader FROM firmware

C LOAD OS FROM storage TO RAM

D OS_READY = FALSE

E OPEN applications

Line D — Once the bootloader has loaded the OS into RAM, the OS is ready, so it should be OS_READY = TRUE. Only then can the user open applications.

Question 2

A POWER_ON

B RUN bootloader FROM RAM

C LOAD OS FROM storage TO RAM

Line B — RAM is empty at power-on, so the bootloader cannot run from it. It is stored in firmware and runs from there: RUN bootloader FROM firmware.

Question 3

A POWER_ON

B RUN bootloader FROM firmware

C LOAD OS FROM RAM TO storage

D OPEN applications

Line C — The direction is reversed. RAM is empty at power-on, so the bootloader copies the OS from storage into RAM: LOAD OS FROM storage TO RAM.

Question 4

A POWER_ON

B RAM = FULL

C RUN bootloader

D LOAD OS INTO RAM

Line B — RAM is volatile and loses its contents when power is off, so it is empty at power-on. It should be RAM = EMPTY.

Question 5

A OPEN applications

B RUN bootloader FROM firmware

C LOAD OS FROM storage TO RAM

Line A — The start-up sequence begins with power on. Applications cannot be opened until the bootloader has loaded the OS, so line 1 should be POWER_ON.

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