Consider a disk with tracks numbered 0 to 199. The current head position is 157 (Head Movement is Right). The request queue is: 17, 93, 151, 4, 128, 67, 199, 35. How Many Head movements are there in servicing requests using C-SCAN?Tip: Enter numerical data ONLY. E.g: "5" and NOT "5 Head Movements".

Question

BlackTom AI · Accepted Answer

C-SCAN moves the head only in one direction (right in this case), serving requests encountered in increasing track order; when it reaches the highest track it jumps to track 0 and continues rightward. 1) Relevant concepts - C-SCAN services requests in one direction (right), visiting all requests with track >= current head position in ascending order, then moves to the end and performs a circular jump to track 0, and then services remaining requests in ascending order. - Total head movement includes the traveled distances while servicing requests and the jump from end to start (this problem counts that jump as head movement). 2) Step-by-step ordering and reasoning - Current head = 157. Request list: 17, 93, 151, 4, 128, 67, 199, 35. - Separate requests into those >=157 and those <157. - >=157: 199 - <157: 151, 128, 93, 67, 35, 17, 4 - Service order under C-SCAN (moving right): first 199, then jump to 0, then service remaining requests in ascending order: 4, 17, 35, 67, 93, 128, 151. 3) Calculations (all intermediate steps) - Move from 157 to 199: |199 - 157| = 42 - Jump from 199 to 0 (circular jump): |199 - 0| = 199 - Service remaining requests starting at 0 in ascending order: - 0 -> 4 = 4 - 4 -> 17 = 13 - 17 -> 35 = 18 - 35 -> 67 = 32 - 67 -> 93 = 26 - 93 -> 128 = 35 - 128 -> 151 = 23 - Sum of these = 4 + 13 + 18 + 32 + 26 + 35 + 23 = 151 Total head movement = 42 (initial right moves) + 199 (jump) + 151 (servicing after jump) = 392 4) Why this is correct - This correctly implements C-SCAN ordering and includes the circular jump distance, matching the usual exam convention used here. 5) Common incorrect approaches - Omitting the jump distance yields 42 + 151 = 193 (incorrect for this question). - Serving left-side requests before jumping (as in normal SCAN) would produce a different total; that is not C-SCAN behavior. 6) Conclusion - Final confirmed total head movements = 392, which matches the provided answer.

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