--- title: "Disk scheduling with 50 cylinders" subject: "Operating System" chapter: 6 key: "semester_6/operating_system:disk-50-cyl-fcfs-sstf-scan-look-cscan" source: https://www.bctengineer.com/q/semester_6/operating_system/disk-50-cyl-fcfs-sstf-scan-look-cscan site: BCT Engineering --- # Disk scheduling with 50 cylinders Operating System · Chapter 6: I/O Management and Disk Scheduling - Asked in: 2074 Bhadra, 10 marks - Video: [FCFS SSTF SCAN CSCAN LOOK CLOOK Solved Example in Hindi | Disk Scheduling Sums | OS Lectures](https://www.youtube.com/watch?v=dEt7mr9R_Z8) — Last moment tuitions - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/operating_system/disk-50-cyl-fcfs-sstf-scan-look-cscan - Whole chapter: https://www.bctengineer.com/md/semester_6/operating_system/chapter-6 ## Question A disk drive has 50 cylinders, numbered 0 to 49. The drive is currently serving a request at cylinder 15, and the queue of pending requests, in FIFO order is 4, 40, 11, 35, 7, 14. What is the total distance that the disk arm moves for the following algorithms: a) FCFS b) SSTF c) SCAN d) LOOK e) C-SCAN ## Short answer For head at 15 moving down first, FCFS total movement is 135 cylinders, SSTF is 47, SCAN to 0 then up is 55, LOOK reversing at the last request is 47, and C-SCAN with return jump counted is 78. ## Answer Cylinders 0–49, head at 15, queue (FIFO) = 4, 40, 11, 35, 7, 14. The direction of head movement is not given, so assume the head is moving towards the lower cylinders (towards 0) at first. The result for the other direction is also shown. #### a) FCFS ```text 15 → 4 → 40 → 11 → 35 → 7 → 14 ``` 11 + 36 + 29 + 24 + 28 + 7 = 135 #### b) SSTF ```text 15 → 14 → 11 → 7 → 4 → 35 → 40 ``` 1 + 3 + 4 + 3 + 31 + 5 = 47 #### c) SCAN (towards 0 first, goes to the end of the disk) ```text 15 → 14 → 11 → 7 → 4 → 0 → 35 → 40 ``` 1 + 3 + 4 + 3 + 4 + 35 + 5 = 55 #### d) LOOK (reverses at the last request, not at the disk end) ```text 15 → 14 → 11 → 7 → 4 → 35 → 40 ``` 1 + 3 + 4 + 3 + 31 + 5 = 47 #### e) C-SCAN (sweep down to 0, jump to 49, continue down) ```text 15 → 14 → 11 → 7 → 4 → 0 → 49 → 40 → 35 ``` 1 + 3 + 4 + 3 + 4 + 49 + 9 + 5 = 78 (the return jump 0 → 49 is counted) #### Result | Algorithm | Head moving down first | Head moving up first | |---|---|---| | FCFS | 135 | 135 | | SSTF | 47 | 47 | | SCAN | 55 | 79 | | LOOK | 47 | 61 | | C-SCAN | 78 | 97 | **Answer (head moving down first): FCFS = 135, SSTF = 47, SCAN = 55, LOOK = 47, C-SCAN = 78 cylinders.** SSTF and LOOK are the best here. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Head at 15, queue 4, 40, 11, 35, 7, 14. FCFS total? A. 47 B. 135 C. 55 D. 78 Answer: B. 11+36+29+24+28+7 = 135. (Key point: FCFS total) 2. Moving down first, SCAN = 55 and LOOK = 47. Why 8 more for SCAN? A. SCAN goes on to cylinder 0 (4 extra) and returns 4 extra, whereas LOOK turns at 4 B. SCAN serves two more requests C. LOOK starts from cylinder 0 D. SCAN moves up first Answer: A. SCAN reaches 0 (4+4 = 8 extra movement) before reversing. (Key point: SCAN vs LOOK difference) 3. In the C-SCAN answer 78, what is the 49 term? A. Serving request 40 B. The distance 15 to 0 C. The head start position D. The return jump from cylinder 0 to 49 Answer: D. C-SCAN jumps to the other end after reaching 0, and this jump is counted. (Key point: C-SCAN counting) 4. What is the SSTF order here? A. 4, 7, 11, 14, 35, 40 B. 14, 11, 7, 4, 35, 40 C. 14, 35, 40, 11, 7, 4 D. 4, 40, 11, 35, 7, 14 Answer: B. The nearest request is chosen each time, giving 1+3+4+3+31+5 = 47. (Key point: SSTF order) ## Related in Operating System - [Disk scheduling with 100 cylinders](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adisk-100-cyl-fcfs-sstf-look-cscan-scan) — Operating System, chapter 6, asked 2× - [Disk scheduling with 5000 cylinders](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adisk-5000-cyl-fcfs-scan-look-cscan-sstf) — Operating System, chapter 6, asked 2× - [Disk scheduling with 150 cylinders](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adisk-150-cyl-sstf-scan-cscan-look-fcfs) — Operating System, chapter 6, asked 2× - [Disk scheduling: FCFS, SSF, SCAN at 53](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adisk-200-cyl-fcfs-ssf-scan-53) — Operating System, chapter 6, asked 3× - [Disk scheduling, 1000 cylinders, head at 345](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adisk-1000-cyl-fifo-sstf-scan) — Operating System, chapter 6 - [Disk scheduling: FCFS, SSTF, SCAN, LOOK, C-SCAN](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adisk-200-cyl-head-moving-60) — Operating System, chapter 6