Consider a disk with tracks numbered 0 to 199. The current head position is 50 (Head Movement is Right). The request queue is: 98, 183, 37, 122, 14, 124, 65, 67. How Many Head movements are there in servicing requests using FCFS?Tip: Enter numerical data ONLY. E.g: "5" and NOT "5 Head Movements".

Question

BlackTom AI · Accepted Answer

FCFS (First-Come, First-Served) moves the disk head to requests in the exact order given; the head movement between two consecutive requests equals the absolute difference of their track numbers.

1) Relevant knowledge points and concepts
- FCFS scheduling: service requests in arrival order, no reordering.
- Head movement cost: absolute difference between successive track positions.
- Total head movement: sum of all individual movements.

2) Step-by-step reasoning and calculations
Start position: 50. Requests order: 98, 183, 37, 122, 14, 124, 65, 67. Compute each move (absolute differences):
- Move 1: |98 − 50| = 48
- Move 2: |183 − 98| = 85
- Move 3: |37 − 183| = 146
- Move 4: |122 − 37| = 85
- Move 5: |14 − 122| = 108
- Move 6: |124 − 14| = 110
- Move 7: |65 − 124| = 59
- Move 8: |67 − 65| = 2

3) Intermediate sums (showing accumulation)
- After first two: 48 + 85 = 133
- Add third: 133 + 146 = 279
- Add fourth: 279 + 85 = 364
- Add fifth: 364 + 108 = 472
- Add sixth: 472 + 110 = 582
- Add seventh: 582 + 59 = 641
- Add eighth: 641 + 2 = 643

4) Why the correct answer is correct
Because FCFS mandates no reordering, each movement must be exactly the absolute difference between consecutive serviced track numbers. Summing those exact differences yields 643 total head movements.

5) Why other interpretations or methods would be incorrect
- Sorting or using SCAN/C-SCAN would reorder requests and give different totals; those are not FCFS.
- Forgetting absolute values or omitting any request movement produces incorrect totals.
- The head direction note (“Right”) does not change FCFS order here and is irrelevant to calculation.

6) Conclusion
The total head movements under FCFS is 643, which matches the provided answer.

类似问题

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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".

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