--- title: "Disk scheduling, 1000 cylinders, head at 345" subject: "Operating System" chapter: 6 key: "semester_6/operating_system:disk-1000-cyl-fifo-sstf-scan" source: https://www.bctengineer.com/q/semester_6/operating_system/disk-1000-cyl-fifo-sstf-scan site: BCT Engineering --- # Disk scheduling, 1000 cylinders, head at 345 Operating System · Chapter 6: I/O Management and Disk Scheduling - Asked in: 2071 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-1000-cyl-fifo-sstf-scan - Whole chapter: https://www.bctengineer.com/md/semester_6/operating_system/chapter-6 ## Question A disk with 1000 cylinders, numbered 0 to 999, compute the number of tracks the disk arm must move to satisfy all the requests in the disk queue. Assume the last request serviced was at track 345 and the head is moving towards track 0 (zero). The queue in FIFO order contains request for the following tracks: 123, 874, 693, 475, 105, 376 Perform the computation for the following scheduling algorithms: a) FIFO b) SSTF c) SCAN ## Short answer Head at 345 moving toward 0; FIFO (FCFS) services in arrival order: 2013 tracks; SSTF selects the closest request: 1298 tracks; SCAN goes to cylinder 0 then reverses upward: 1219 tracks. ## Answer Disk: 1000 cylinders (0 to 999). Head at 345, moving towards 0. Queue (FIFO): 123, 874, 693, 475, 105, 376. #### a) FIFO **FIFO**: service order (head at 345): 123, 874, 693, 475, 105, 376 ```text 345 -> 123 = 222 123 -> 874 = 751 874 -> 693 = 181 693 -> 475 = 218 475 -> 105 = 370 105 -> 376 = 271 Total = 222 + 751 + 181 + 218 + 370 + 271 = 2013 ``` #### b) SSTF **SSTF**: service order (head at 345): 376, 475, 693, 874, 123, 105 ```text 345 -> 376 = 31 376 -> 475 = 99 475 -> 693 = 218 693 -> 874 = 181 874 -> 123 = 751 123 -> 105 = 18 Total = 31 + 99 + 218 + 181 + 751 + 18 = 1298 ``` #### c) SCAN (moving toward 0: go down to track 0, then reverse and go up) **SCAN**: service order (head at 345): 123, 105, 376, 475, 693, 874 ```text 345 -> 123 = 222 123 -> 105 = 18 105 -> 0 = 105 0 -> 376 = 376 376 -> 475 = 99 475 -> 693 = 218 693 -> 874 = 181 Total = 222 + 18 + 105 + 376 + 99 + 218 + 181 = 1219 ``` | Algorithm | Tracks moved | |---|---| | FIFO | 2013 | | SSTF | 1298 | | SCAN | 1219 | (If LOOK were used, the arm would turn back at 105 and the movement would be 1009.) **Answer: FIFO = 2013, SSTF = 1298, SCAN = 1219 tracks.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. FIFO total movement from 345 is: A. 1298 B. 1219 C. 2013 D. 1009 Answer: C. 222+751+181+218+370+271 = 2013. (Key point: FIFO total) 2. SSTF from 345 serves first: A. 123 B. 105 C. 475 D. 376 (distance 31) Answer: D. Nearest request to 345 is 376; total is 1298. (Key point: SSTF order) 3. The head moves toward 0, so SCAN first serves: A. 123 then 105, goes to track 0, reverses and serves 376 onward B. 376 first, then 475 C. 874 first D. 105 then 123 Answer: A. The arm goes down to 0 before reversing; total 1219. (Key point: SCAN direction) 4. If LOOK were used, the arm would turn back at: A. 0 B. 105, the lowest request, saving the trip to 0 and back C. 123 D. 999 Answer: B. LOOK reverses at the last request, giving 1009. (Key point: LOOK difference) ## 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 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: 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: 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