CAP366

Operating System Design and Principles

8 units · 32 topics · 347 questions · 32 topic notes

Unit 1 Introduction to operating systems

Tuesday, 13 October 2026, 12:30 pm · 90 minutes · 30 marks

Monday, 28 December 2026, 10:00 am · 3 hours · 70 marks

Question 1 · Tier 4, Analyse

Why must the instruction that disables timer interrupts be privileged?

A program could otherwise keep the CPU forever and the OS could never regain control
The timer hardware would otherwise lose its count and the system clock would run fast
Disk controllers would otherwise stop delivering their completion interrupts to drivers
The process would otherwise be unable to make system calls into the kernel at all

Show the answer

A. The timer lets the OS preempt a running program. If user code could disable it, one program could monopolise the CPU.

Why not B: Disabling interrupts does not corrupt the counter; the issue is loss of OS control.

Why not C: Timer interrupts are separate from device interrupts, and this is not the protection reason.

Why not D: System calls use a trap instruction and do not depend on the timer interrupt.