ITPEC FE Morning October 2020 Question 11

A timer signalling that time has elapsed — the signal comes from hardware outside the running program, which makes it an external interrupt.

External interrupts are raised by sources independent of the instruction being executed: timers, I/O completion, power failure, a hardware fault. Internal interrupts (exceptions) are raised by the program itself while the CPU executes an instruction.

Answer (a)

Why not others:

• •(b) — a division by zero is detected by the CPU while executing that instruction: an internal interrupt
• •(c) — an undefined operation code is also detected while decoding the program's own instruction: internal
• •(d) — a page fault is raised when the executing instruction accesses a page not in main memory: internal

Key rule: Ask whether the interrupt would happen again if the same instruction ran with the same data: if so it is internal (caused by the program); if it depends on outside events such as timers or I/O, it is external.

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