Types of software and interrupts

Computer Science · Software. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

System software runs and manages the computer itself, for example the operating system, utility programs and device drivers. Application software lets the user do a task, for example a word processor, a web browser or a game.

Software runs in layers. Applications run on the operating system. The operating system is loaded by the firmware (the bootloader). The firmware runs on the hardware.

An interrupt is a signal sent to the CPU by hardware or software to say that something needs attention. The CPU stops its current task for a moment, deals with the interrupt, then carries on from where it stopped.

Rules to remember

Operating system functions: managing files, memory, multitasking, peripherals and drivers, user accounts and security; handling interrupts; providing an interface; running applications
Device driver: software that lets the operating system communicate with a hardware device
Hardware interrupts: key pressed, mouse moved, printer out of paper
Software interrupts: division by zero, two processes trying to use the same memory location
Handling: the CPU finishes its current fetch-decode-execute cycle, checks the interrupt's priority, saves the current state (register contents), runs the interrupt service routine (ISR), then restores the state and continues
Multitasking: the CPU gives each task a short slice of time in turn and uses interrupts to switch

Common mistake

Students write that the CPU stops at once and the old task is lost. The current state is saved first, so the task continues afterwards.

Worked example

A printer runs out of paper while the user is watching a video. Describe what the CPU does.

The printer sends a hardware interrupt signal to the CPU.
The CPU finishes its current cycle, checks the priority, and saves the contents of its registers for the video task.
It runs the interrupt service routine for the printer, which shows an 'out of paper' message.
The saved register contents are restored and the video carries on.

Answer: The interrupt is handled by its ISR, then the video task continues from where it stopped.

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