Suppose that a disk drive has 5000 cylinders, numbered 0 to 4999. The drive is…
Suppose that a disk drive has 5000 cylinders, numbered 0 to 4999. The drive is currently serving a request at cylinder 2150, and the previous request was at cylinder 1805. The queue of pending requests, in FIFO order, is 2069, 1212, 2296, 2800, 544, 1618, 356, 1523, 4965, 3681. Starting from the current head position, what is the total distance (in cylinders) that the disk armmoves to satisfy all the pending requests, using C-SCAN disk scheduling?
Answer: C. 9917 — In C-SCAN (Circular SCAN), the disk arm moves from the current position to the end of the disk, servicing requests in increasing order. It then jumps to the…
- A.
9900
- B.
900
- C.
9917
- D.
9797
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Correct answer: C
In C-SCAN (Circular SCAN), the disk arm moves from the current position to the end of the disk, servicing requests in increasing order. It then jumps to the beginning (cylinder 0) and services remaining requests up to the original starting point.
Given: Current head = 2150, Previous request = 1805 (implies direction is increasing).
1. Move from 2150 to the end of the disk (4999):
|4999 - 2150| = 2849 cylinders
2. Jump from the end (4999) to the start (0):
|0 - 4999| = 4999 cylinders
3. Move from 0 to the last pending request (2069):
|2069 - 0| = 2069 cylinders
Total Distance = 2849 + 4999 + 2069 = 9917 cylinders.