Discussion
On a disk with 1000 cylinders (0 to 999) find the number of tracks, the disk arm must move to satisfy all the requests in the disk queue. Assume the last request service was at track 345 and the head is moving toward track 0. The queue in FIFO order contains requests for the following tracks : 123, 874, 692, 475, 105, 376 (Assume SCAN algorithm)
Answer: B
With SCAN the arm keeps moving in its present direction, toward track 0, serving the requests in that direction as it goes, and it goes on to the end of the disk at track 0 before reversing. It starts at 345 and serves 123 and 105 on the way down, so it travels 345 tracks to reach 0. Then it reverses and moves up to the highest request, 874, serving 376, 475 and 692 on the way, which is 874 tracks.
\[ 345 + 874 = 1219 \]
So the arm moves 1219 tracks, option (B). A figure of 1507 would be the value if the arm stopped at 105 and did not go to the end.
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