4.10478 · Section 4: Software

Types of software and interrupts

Key learnings

System vs application software
The mark scheme rewards a two-element difference, not an example. System software provides the services the computer needs: it manages the hardware and gives a platform — the OS plus utility software (anti-virus, defragmenter, compression, backup). Application software serves the user's task, e.g. word processor, spreadsheet, web browser. The difference is whose needs it serves — computer vs user. Use generic names, never brand names.
OS: manages hardware, provides a platform
The operating system (OS) is system software that manages the computer's hardware and resources and gives a platform on which applications run. Functions to know: managing memory (RAM allocation so programs do not overwrite each other), peripherals/drivers, files, a user interface (GUI), multitasking (scheduling the CPU), system security, user accounts, and handling interrupts. Tip: examiners say candidates list functions with little detail — give what each does AND why.
Interrupts and the ISR
An interrupt is a signal sent to the CPU from a device or program that needs immediate attention, so the CPU pauses to deal with it. Handling chain: the CPU finishes the instruction, saves its state/registers, runs the interrupt service routine (ISR) that does the action, then restores the state and resumes where it left off. Types: hardware (key press, mouse move, printer out of paper) vs software (divide by zero, two processes accessing the same memory location).

Common mistakes

Drawn from real examiner reports.

Interrupt vs interrupt service routine

Keep the signal and the program separate. The interrupt is the signal that requests the processor's attention; the interrupt service routine (ISR) is the program the CPU runs to deal with it. Saying "the interrupt fixes it" blurs the two — the interrupt only requests attention, the ISR carries out the action, then the CPU restores its state and resumes the task.

Software types: state the difference

"Describe the difference between system and application software" needs a comparison of purpose, not a list, and no brand names. Form: system software serves the computer and controls the hardware (e.g. the OS); application software serves the user's task (e.g. a word processor). Two elements: whose needs it serves — computer vs user — and a generic example of each.

June 2023 Paper 1 examiner report (0478_s23_er): "Many candidates were able to give a suitable example of each type of software, but few were able to describe the difference between the type of software. Candidates are reminded that they should not use brand names when referring to examples of software."

Vague interrupt: name the mechanism

Candidates name a trigger ("a key is pressed") but not the mechanism, and wrongly add analogue-to-digital conversion (an interrupt does not do that). Form: an interrupt is a signal to the CPU that a device/program needs attention; the CPU finishes the instruction, saves state, runs the ISR, then restores state and resumes. Use: signal, CPU, save/restore state, ISR.

November 2023 Paper 1 examiner report (0478_w23_er): "Very few candidates could give an accurate technical description about how interrupts are used. Most candidates gave a vague statement about a key being pressed and analogue data being converted to digital data."

The hardware, firmware, OS, app stack

Candidates muddle the layers that let applications run. Correct order: the bootloader (firmware) runs on the hardware; the OS runs on the firmware; application software runs on the OS. In Nov 2023 the commonest missing answer here was firmware — candidates gave hardware and OS but not firmware. Never put applications directly on the hardware or omit the firmware/bootloader.

November 2023 Paper 1 examiner report (0478_w23_er): "Few candidates were able to give the answer for application software and firmware. The most common incorrect answer was firmware."

OS functions need a purpose, not a name

Asked for the purpose of an OS, candidates write "manages memory, manages files, manages peripherals" and stop. The mark scheme wants the purpose of each — e.g. "manages memory so each program gets space in RAM and does not overwrite others." Nov 2023 examiners: candidates "gave functions but gave little detail beyond this." Pair every function with what it achieves.

November 2023 Paper 1 examiner report (0478_w23_er): "Most candidates gave functions of an operating system but gave little detail beyond this."

Hardware vs software interrupt: the source

What distinguishes the two types is the source of the signal, not its effect. A hardware interrupt is from a device — key press, mouse move, printer out of paper. A software interrupt is from a program/error — divide by zero, or two processes accessing one memory location. Both use the same save/ISR/restore routine, so classify by the signal's source.

Exam tips

Interrupt chain: one mark per link
"Describe how an interrupt is handled" — one mark per link: Generate → Finish → Save → Service → Restore → Resume. A device sends the signal; the CPU finishes the instruction, saves its state (registers/PC), runs the ISR, restores state and resumes where it stopped.
Keep interrupt examples separate by type
For "give an example" parts, keep hardware examples (key press, mouse move, printer out of paper) separate from software examples (divide by zero, two processes accessing one memory location) — a mark is given for each type. Do not give two of the same type.
OS answers: function and purpose
When asked the purpose of the OS, name each function AND what it achieves — a bare list scores less; examiners note candidates list functions with little detail. For software examples always use generic terms (web browser, word processor), never brand names.

Revision notes