ITPEC FE Morning April 2020 Question 10

An interrupt due to division by zero — an internal interrupt is raised by the instruction the CPU is executing.

Interrupts are classified by where the cause lies:

• •Internal interrupts (also called exceptions or traps) come from executing an instruction: division by zero, overflow, an invalid instruction, a page fault, a supervisor call
• •External interrupts come from outside the running program: I/O completion, a timer, a power failure, a hardware fault such as a memory parity error (a machine check interrupt)

Answer (b)

Why not others:

• •(a) — a loss of power is detected by the hardware and raises a machine check interrupt, an external interrupt
• •(c) — an I/O device signalling completion raises an I/O interrupt, which is external
• •(d) — a memory parity error is a hardware fault reported through a machine check interrupt, also external

Key rule: Ask whether the cause is the instruction being executed: if rerunning the same program would raise it again, it is internal; if it comes from the hardware or a device, it is external.

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