Past exam of the year 1 of the computer science course of the University of Oxford / 2016 / Digital systems, linear algebra, introduction to formal proof (resit) / 3 / b / iii / Solution 2026-10-31
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The previous motion from 135 to 133 fixes the initial direction as downward. Under SCAN disk scheduling, the head continues to the physical endpoint 0 before reversing:
133 -> 112 -> 97 -> 91 -> 76 -> 23 -> 0 -> 135 -> 142 -> 157
Thus SCAN crosses tracks, stopping once the final pending request has been served.
If “elevator” means LOOK disk scheduling, reversal occurs at the lowest pending request, 23, rather than track 0. Its route omits 0, and the total is tracks. Stating the turnaround convention is necessary to make the numerical answer unambiguous.
Figure 1.
Disk scheduling service paths and the SCAN–LOOK turnaround distinction
. Original paths from the given starting position. The extra zero-track endpoint in SCAN is a movement step rather than a serviced request. Totals measure absolute track displacement. Details.
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SCAN disk scheduling 2026-10-31
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Sweep in one direction, servicing requests encountered in track order, continue to the physical endpoint, and then reverse. A calculation must state the initial direction and whether movement after the final requested service is included. LOOK disk scheduling instead turns around at the last pending request in that direction.
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