ITPEC FE Subject A October 2025 Question 8

Topic: Computer Component

There exists a main process and the two (2) interrupt processes A and B, as shown in the table below. Multiplex interrupts are permitted. When interrupts A and B occur with the timing shown in the diagram below, how many milliseconds (ms) can the main process use the CPU during the time period between 0 ms and 5 ms? Here, the overhead from calling an interrupt process and resuming from it can be ignored.

InterruptProcessing time (ms)Interrupt priority
A0.5High
B1.5Low
Occurrence of interrupt AOccurrence of interrupt B012345Time (ms)Note:indicates the timing of an interrupt.

Explanation

2 ms — lay the interrupts on a timeline, let the high-priority A pre-empt B, and count the gaps left for the main process.

From the diagram, B occurs at 0 ms (and again at 5 ms, the end of the period) and A occurs at 1, 2.5 and 3.5 ms.

Time (ms)CPU runs
0–1B, 1 ms of its 1.5 ms
1–1.5A, pre-empting B
1.5–2B resumes and finishes its last 0.5 ms
2–2.5main process
2.5–3A
3–3.5main process
3.5–4A
4–5main process

Main process: 0.5 + 0.5 + 1 = 2 ms. As a check, the interrupts need 1.5 + 3 × 0.5 = 3 ms, all inside the 5 ms window, which leaves 2 ms.

Answer (a)

Why not others:

  • •(b) 2.5 — lets the A at 1 ms run inside B's 1.5 ms; A pre-empts B, and B still needs its full 1.5 ms afterwards
  • •(c) 3.5 — subtracts only 1.5 ms, the time of one interrupt type (A's three runs or B's one run), not both
  • •(d) 5 — ignores the interrupts entirely

Key rule: Pre-emption changes the order of the work, not its amount: when all interrupt processing fits in the window, the main process gets the window minus the total interrupt time.

Computer Component · AI-written, may contain errors

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