Discussion
Consider the following disk request sequence for a disk with 100 tracks:
60, 45, 21, 67
Head pointer starting at 60 and moving in the left direction. Find the number of head movements in cylinders using FCFS scheduling.
Answer: A
The head makes 85 movements under FCFS. FCFS serves the requests in the order given, so the head moves 60 to 60, then to 45, 21 and 67. \[ |60-60| + |45-60| + |21-45| + |67-21| \] \[ = 0 + 15 + 24 + 46 = 85 \] The starting direction of the head does not matter for FCFS, because it simply goes to each request in order. The figures 80, 90 and 193 do not come out of this sum. Total head movement is the sum of the distances between successive positions, so \[ 15 + 24 + 46 = 85 \] which agrees with the option.
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