Topic 4 · Software

System software, operating systems, and interrupts

The difference between system and application software, what an operating system does, and how interrupts work.

Lesson 1 of 2Syllabus 4.1updated October 31, 2026

System software and application software

Application software lets the user do a task: write an essay, edit a photo, browse the web, play a game.

System software runs and maintains the computer itself and provides a platform for applications to run on. It includes the operating system and utility software such as anti-virus, defragmentation, backup, and compression tools.

Layers of a computer

Each layer relies on the one beneath it. Applications ask the operating system for what they need; the operating system controls the hardware, starting from the firmware that runs when the power comes on.

User
- Applications
  - Operating system
    - Firmware
      - Hardware

Firmware is software stored in ROM, such as the bootloader, which loads the operating system when the computer starts.

What the operating system does

The operating system (OS), such as Windows, macOS, Android, or Linux, manages everything:

Function	What it means
Managing files	Create, name, open, move, copy, and delete files and folders.
Handling interrupts	Respond to signals that need the CPU's attention.
Providing a user interface	A graphical (GUI) or command-line interface so people can use the computer.
Managing peripherals and drivers	Communicate with devices like printers through driver software.
Managing memory	Decide which programs and data are in RAM, and stop programs using each other's memory.
Managing multitasking	Share CPU time between many programs so they appear to run at once.
Providing a platform for applications	Give applications a consistent way to use the hardware.
Providing system security	User accounts, passwords, access rights, and updates.
Managing user accounts	Separate settings and files for each user.

Interrupts

An interrupt is a signal sent to the CPU to say that something needs attention. A hardware interrupt comes from a device, such as a key being pressed or a printer running out of paper. A software interrupt comes from a program, such as a division by zero or a request for data.

At the end of each fetch-decode-execute cycle, the CPU checks for interrupts:

1
Interrupt sent
It is given a priority and queued.
2
Check at end of cycle
The CPU checks if an interrupt is higher priority than the current task.
3
Save and handle
The current task's state is saved and an interrupt service routine (ISR) runs.
4
Resume
The saved state is restored and the original task carries on.

Key terms

Application software: Software that lets the user perform a task.
System software: Software that runs and maintains the computer, such as the OS and utilities.
Firmware: Software stored in ROM that controls hardware, such as the bootloader.
Interrupt: A signal to the CPU that an event needs attention.

Check yourself

Is a web browser system software or application software?Show answer

Application software. It lets the user do a task.

Give three functions of an operating system.Show answer

Any three, for example: managing files, handling interrupts, providing a user interface, managing memory, multitasking, managing peripherals, security.

Give one example of a hardware interrupt and one of a software interrupt.Show answer

Hardware: a key pressed, printer out of paper. Software: division by zero, a program requesting input.

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