Project ForgeConsider the following five disk access requests of the form (request id, cylinder number) that are present in the disk scheduler queue at a given time.
(P, 155), (Q, 85), (R, 110), (S, 30), (T, 115)
Assume the head is positioned at cylinder 100. The scheduler follows Shortest Seek Time First scheduling to service the requests. Which one of the following statements is FALSE?
- AT is serviced before P.
- Q is serviced after S, but before T.
- CThe head reverses its direction of movement between servicing of Q and P.
- DR is serviced before P.
Explanation
From head=100, seek distances: P=55, Q=15, R=10, S=70, T=15 (R=10 is smallest) → service R(110). From 110: P=45,Q=25,S=80,T=5 → service T(115). From 115: P=40,Q=30,S=85 → service Q(85). From 85: P=125,S=55 → service S(30). Finally P(155). Order: R, T, Q, S, P. (A) T (2nd) before P (5th) ✓ true. (B) Q is serviced 3rd and S is serviced 4th, so Q comes BEFORE S, not after — this statement is FALSE, making it the answer. (C) the head moves down through Q(85) and S(30), then reverses upward to reach P(155) — a genuine direction reversal occurs in that span ✓ true. (D) R (1st) before P (5th) ✓ true.
Source: GATE CS 2020 (2020)
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