← IGCSE Computer Science Paper 1

Writing an exam answer about software and interrupts

A full-mark answer uses the correct terms (system software, firmware, bootloader, ISR), gives points in the right order, and adds a reason for each point.

Part of: IGCSE Computer Science 0478 — 4.1 Types of Software and Interrupts — Module 62

Understanding Writing an exam answer about software and interrupts in IGCSE Computer Science Paper 1

What it does: This answer names the correct term (system software), gives an example (OS) and adds a reason after 'because'. A full-mark layers answer lists each layer in order, starting with the applications the user sees and ending with the hardware. Give one sentence that says what the function does, plus a reason. A vague phrase such as 'does memory' gains no mark. Calling an interrupt a crash is a common mistake. An interrupt is a normal signal, and the computer carries on afterwards. Describe interrupt handling in the order it happens. The state must be saved before the ISR runs so the CPU can resume later.

Common mistakes: After saving its state, the CPU runs the interrupt service routine (ISR), not the application. Line 3 should be '3 CPU runs the ISR'. An operating system is system software, not application software. Line 1 should be 'System software: the OS'.

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This module's key terms are also in a printable crossword (Modules 409–418) and word search (Modules 390–418).

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Learning
Duo
Which answer to 'State one difference between system and application software' earns the mark?
System software manages the computer (e.g. an operating system), but application software lets the user do a task (e.g. a word processor).System software is the important software, but application software is the less important software.
Matches
An interrupt is when the computer crashes.
↓
An interrupt is not a crash. It is a signal to the CPU that something needs attention, and the CPU runs an ISR and then carries on.
Applying
Meanings
Term
point and reason
Meaning
A full-mark answer uses the correct terms (system software, firmware, bootloader, ISR), gives points in the right order, and adds a reason for each point.
Practice
An interrupt is a signal that tells the CPU to stop and deal with an event.
Testing
Gaps
The OS is
Check Answer
Spots
22 CPU saves its registers
33 CPU runs the application

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Quick Questions

1. Which answer to 'State one difference between system and application software' earns the mark?
A. System software manages the computer (e.g. an operating system), but application software lets the user do a task (e.g. a word processor). ✓
B. System software is the important software, but application software is the less important software.
A full-mark answer uses the correct terms and gives examples. Say what each type does, not how important it is.
2. Which is the better way to describe an OS function in an exam?
A. The OS does memory.
B. The OS manages memory by deciding where each program is stored in RAM, so programs do not overwrite each other. ✓
Give one sentence that says what the function does, plus a reason. A vague phrase such as 'does memory' gains no mark.
3. The question asks you to describe the layers of a computer from the user's side. Which answer gives them in the correct order?
A. Hardware, then applications, then firmware, then operating system.
B. Applications, then operating system, then firmware, then hardware. ✓
Order matters in a 'describe' answer. The layers run from applications at the top, through the OS and firmware, down to the hardware.
4. Which statement uses the correct term for what an interrupt is?
A. An interrupt is a signal sent to the CPU asking it to stop its current task and deal with an event. ✓
B. An interrupt is a crash that happens when the computer breaks.
Calling an interrupt a crash is a common mistake. An interrupt is a normal signal, and the computer carries on afterwards.
5. Which answer correctly describes the first steps of interrupt handling?
A. The interrupt is generated, the CPU finishes the cycle and checks for it, then saves its state and runs the ISR. ✓
B. The CPU runs the ISR, then the interrupt is generated, then the CPU saves its state.
Describe interrupt handling in the order it happens. The state must be saved before the ISR runs so the CPU can resume later.

Match the Pairs

State the point, then add 'because...' → A full-mark structure: give one correct point and follow it with a reason that explains why it is true
1. Save state 2. Run ISR 3. Restore state → Describing interrupt handling in the correct order, using the correct term ISR
Use the correct term for software that manages the computer, with an example and a reason → The OS is system software because it manages hardware and files.
Describe the layers in the correct order from the user down to the hardware → Applications > Operating system > Firmware > Hardware
An interrupt is when the computer crashes. → An interrupt is not a crash. It is a signal to the CPU that something needs attention, and the CPU runs an ISR and then carries on.
Firmware is an application the user installs. → Firmware is not an application. It is permanent software stored in ROM that starts up and controls the hardware, such as the bootloader.

Key Term

point and reason
A full-mark answer uses the correct terms (system software, firmware, bootloader, ISR), gives points in the right order, and adds a reason for each point.

Fill in the Blanks

1. The OS is system software because it manages the hardware.
Hint: Type the category of software that the operating system belongs to.
2. 1. The CPU checks for an interrupt 2. The ISR is run to handle it
Hint: Type the abbreviation for the routine that handles an interrupt.
3. Applications > OS > firmware > hardware
Hint: Type the name of the layer that sits between the operating system and the hardware.
4. The bootloader loads the OS into RAM because the OS must be running to use apps.
Hint: Type the word that introduces the reason in a full-mark answer.
5. An interrupt is a signal, not a crash.
Hint: Type the word for a program failure that students wrongly use to describe an interrupt.

Spot the Error

Question 1
A 1 A signal is sent to the CPU
B 2 CPU saves its registers
C 3 CPU runs the application
D 4 CPU restores its registers
E 5 CPU resumes the old task
Line C — After saving its state, the CPU runs the interrupt service routine (ISR), not the application. Line 3 should be '3 CPU runs the ISR'.
Question 2
A Application software: the OS
B It manages the hardware
C Reason: apps need it to run
Line A — An operating system is system software, not application software. Line 1 should be 'System software: the OS'.
Question 3
A Layer 1: applications
B Layer 2: operating system
C Layer 3: firmware
D Layer 4: utility software
E Reason: each uses the one below
Line D — The bottom layer is the hardware. Utility software is system software and sits in the operating system layer. Line 4 should be 'Layer 4: hardware'.
Question 4
A Firmware is stored in ROM
B It contains the bootloader
C The bootloader loads the OS
D Reason: ROM is volatile
Line D — ROM is non-volatile, so firmware is kept when the power is off and is available at start-up. Line 4 should say 'non-volatile'.
Question 5
A An interrupt is a signal
B It means the computer crashed
C Reason: the CPU must respond
Line B — An interrupt is not a crash. It is a signal asking the CPU to deal with an event. Line 2 should be 'It asks the CPU for attention'.

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