--- title: "Disk scheduling: FCFS, SSF, SCAN at 53" subject: "Operating System" chapter: 6 key: "semester_6/operating_system:disk-200-cyl-fcfs-ssf-scan-53" source: https://www.bctengineer.com/q/semester_6/operating_system/disk-200-cyl-fcfs-ssf-scan-53 site: BCT Engineering --- # Disk scheduling: FCFS, SSF, SCAN at 53 Operating System · Chapter 6: I/O Management and Disk Scheduling - Asked in: 2073 Magh, 6 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-200-cyl-fcfs-ssf-scan-53 - Whole chapter: https://www.bctengineer.com/md/semester_6/operating_system/chapter-6 ## Question Suppose a disk drive with 200 cylinders numbered from 0 to 199. The drive is currently serving a request at 53. The queue of pending request is 98, 183, 37, 122, 14, 124, 65, 67. Starting from the current head position, calculate total head movement (in cylinder) that the disk arm moves to satisfy all the pending request for FCFS, SSF and SCAN disk scheduling algorithm. ## Short answer For head at 53 and queue 98,183,37,122,14,124,65,67, FCFS serves arrival order and moves 640 cylinders. SSTF order 53,65,67,37,14,98,122,124,183 moves 236; SCAN moving toward 0 first also moves 236 cylinders. ## Answer Disk has 200 cylinders (0–199), head at 53, queue = 98, 183, 37, 122, 14, 124, 65, 67. Total head movement = sum of absolute differences between successive positions. #### 1. FCFS Requests are served in arrival order. ```text 53 → 98 → 183 → 37 → 122 → 14 → 124 → 65 → 67 ``` | Move | Distance | |---|---| | 53 → 98 | 45 | | 98 → 183 | 85 | | 183 → 37 | 146 | | 37 → 122 | 85 | | 122 → 14 | 108 | | 14 → 124 | 110 | | 124 → 65 | 59 | | 65 → 67 | 2 | | **Total** | **640** | #### 2. SSF (Shortest Seek First / SSTF) Always go to the nearest pending request. ```text 53 → 65 → 67 → 37 → 14 → 98 → 122 → 124 → 183 ``` | Move | Distance | |---|---| | 53 → 65 | 12 | | 65 → 67 | 2 | | 67 → 37 | 30 | | 37 → 14 | 23 | | 14 → 98 | 84 | | 98 → 122 | 24 | | 122 → 124 | 2 | | 124 → 183 | 59 | | **Total** | **236** | #### 3. SCAN (elevator) Direction is not given, so assume the arm moves towards cylinder 0 first (as in Silberschatz), serves requests on the way, reverses at cylinder 0 and then moves up. ```text 53 → 37 → 14 → 0 → 65 → 67 → 98 → 122 → 124 → 183 ``` | Move | Distance | |---|---| | 53 → 37 | 16 | | 37 → 14 | 23 | | 14 → 0 | 14 | | 0 → 65 | 65 | | 65 → 67 | 2 | | 67 → 98 | 31 | | 98 → 122 | 24 | | 122 → 124 | 2 | | 124 → 183 | 59 | | **Total** | **236** | If the head instead moved towards 199 first, SCAN would give 53 → 65 → 67 → 98 → 122 → 124 → 183 → 199 → 37 → 14 = 331 cylinders. #### Summary | Algorithm | Total head movement | |---|---| | FCFS | 640 | | SSF (SSTF) | 236 | | SCAN (towards 0 first) | 236 | **Answer: FCFS = 640, SSF = 236, SCAN = 236 cylinders.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Head at 53, queue 98, 183, 37, 122, 14, 124, 65, 67. What is the FCFS total? A. 236 B. 640 C. 331 D. 583 Answer: B. 45+85+146+85+108+110+59+2 = 640. (Key point: FCFS total) 2. The question gives no direction. What does the answer assume for SCAN? A. The arm moves towards 199 first B. The arm does not reverse C. The arm serves requests in FIFO order D. The arm moves towards 0 first, reverses at 0, then moves up Answer: D. Following Silberschatz, it assumes downward first, giving 236. (Key point: SCAN direction assumption) 3. If SCAN moved towards 199 first, what total results? A. 331 B. 236 C. 640 D. 208 Answer: A. 53 to 199 is 146, then 199 to 14 is 185: 146+185 = 331. (Key point: SCAN other direction) 4. SSTF gives 236 here. Which is its order? A. 37, 14, 65, 67, 98, 122, 124, 183 B. 98, 183, 37, 122, 14, 124, 65, 67 C. 65, 67, 37, 14, 98, 122, 124, 183 D. 65, 67, 98, 122, 124, 183, 37, 14 Answer: C. SSTF picks the nearest each time, starting with 65 (distance 12). (Key point: Why SSTF and SCAN equal) ## Close variants set in other exams - [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× - [Device driver; FCFS, SSF, SCAN numerical](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adevice-driver-disk-fcfs-ssf-scan) — Operating System, chapter 6, asked 2× ## Related in Operating System - [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 - [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 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 50 cylinders](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adisk-50-cyl-fcfs-sstf-scan-look-cscan) — Operating System, chapter 6 - [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