25-Comp-A5 Operating Systems, December 2019: worked solutions

17-Comp-A5, Operating Systems — National Examinations, December 2019. 3-hour closed-book paper, 7 questions of 20 marks each (candidates asked to answer any 5; all 7 answered here). Total 100 marks.

Reference texts: Silberschatz, Galvin & Gagne, Operating System Concepts (10th ed., Wiley) — CPU scheduling (Ch.5), process synchronization (Ch.6–7), deadlocks (Ch.8), main memory (Ch.9), virtual memory (Ch.10), mass-storage/disk scheduling (Ch.11), file-system implementation/free-space management (Ch.12–14).

Data used below. (1) Q1(a): Proc3's execution time is 21 s (the printed column reads 14, 7, 21, 2, 1 for Proc1–5). (2) Q3(c): the base (relocation) register is 1500. (3) Q5(a) states that the disk has 160 tracks numbered 0 to 159, yet its request queue includes tracks 160, 174 and 176. This inconsistency in the paper is resolved by adopting a 200-track disk (0–199) for the C-SCAN calculation, the only sub-part affected. (4) Q6(b) lists the free holes as 305K/245K/405K/470K/270K/291K/325K/350K, but the next sentence restates the first two as 302K and 243K, a proofing slip in the paper; the full eight-value list is used throughout.

25-Comp-A5 Operating Systems: December 2019