--- title: "Disk scheduling: FCFS, SSTF, SCAN, LOOK, C-SCAN" subject: "Operating System" chapter: 6 key: "semester_6/operating_system:disk-200-cyl-head-moving-60" source: https://www.bctengineer.com/q/semester_6/operating_system/disk-200-cyl-head-moving-60 site: BCT Engineering --- # Disk scheduling: FCFS, SSTF, SCAN, LOOK, C-SCAN Operating System · Chapter 6: I/O Management and Disk Scheduling - Asked in: 2071 Magh, 11 marks - Video: [7.8- All Disk Scheduling Algorithm In Operating System | FCFS | SSTF | SCAN | CSCAN | LOOK | CLOOK](https://www.youtube.com/watch?v=uEM7LfP_0mw) — TutorialsSpace- Er. Deepak Garg - 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-head-moving-60 - Whole chapter: https://www.bctengineer.com/md/semester_6/operating_system/chapter-6 ## Question Consider a disk drive having 200 cylinders, numbered from 0 to 199. The head is currently positioned at cylinder 53 and moving toward the cylinder 60. The queue of pending requests are 98, 183, 37, 122, 14, 124, 65, 67. Starting from the current head position, what is the total head movement (in cylinders) to service the pending requests for each of the following disk arm scheduling algorithms? (a) FCFS (b) SSTF (c) SCAN (d) LOOK (e) C-SCAN. ## Short answer Head moving upward; FCFS total head movement = 640 cylinders, SSTF = 236, SCAN = 331 (to end 199 then reverse), LOOK = 299 (reverse at 183), C-SCAN = 382 (to 199, jump to 0, continue upward). ## Answer Disk: 200 cylinders (0 to 199), head at 53 moving towards 60 (upward). Queue: 98, 183, 37, 122, 14, 124, 65, 67. #### a) FCFS **FCFS**: service order (head at 53): 98, 183, 37, 122, 14, 124, 65, 67 ```text 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 = 45 + 85 + 146 + 85 + 108 + 110 + 59 + 2 = 640 ``` #### b) SSTF **SSTF**: service order (head at 53): 65, 67, 37, 14, 98, 122, 124, 183 ```text 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 = 12 + 2 + 30 + 23 + 84 + 24 + 2 + 59 = 236 ``` #### c) SCAN (up to cylinder 199, then reverse) **SCAN**: service order (head at 53): 65, 67, 98, 122, 124, 183, 37, 14 ```text 53 -> 65 = 12 65 -> 67 = 2 67 -> 98 = 31 98 -> 122 = 24 122 -> 124 = 2 124 -> 183 = 59 183 -> 199 = 16 199 -> 37 = 162 37 -> 14 = 23 Total = 12 + 2 + 31 + 24 + 2 + 59 + 16 + 162 + 23 = 331 ``` #### d) LOOK (up to 183, the last request, then reverse) **LOOK**: service order (head at 53): 65, 67, 98, 122, 124, 183, 37, 14 ```text 53 -> 65 = 12 65 -> 67 = 2 67 -> 98 = 31 98 -> 122 = 24 122 -> 124 = 2 124 -> 183 = 59 183 -> 37 = 146 37 -> 14 = 23 Total = 12 + 2 + 31 + 24 + 2 + 59 + 146 + 23 = 299 ``` #### e) C-SCAN (up to 199, jump to 0, continue upward) **C-SCAN**: service order (head at 53): 65, 67, 98, 122, 124, 183, 14, 37 ```text 53 -> 65 = 12 65 -> 67 = 2 67 -> 98 = 31 98 -> 122 = 24 122 -> 124 = 2 124 -> 183 = 59 183 -> 199 = 16 199 -> 0 = 199 (return jump) 0 -> 14 = 14 14 -> 37 = 23 Total = 12 + 2 + 31 + 24 + 2 + 59 + 16 + 199 + 14 + 23 = 382 ``` #### Summary | Algorithm | Total head movement (cylinders) | |---|---| | FCFS | 640 | | SSTF | 236 | | SCAN | 331 | | LOOK | 299 | | C-SCAN | 382 (183 if the return sweep is not counted) | **Answer: FCFS 640, SSTF 236, SCAN 331, LOOK 299, C-SCAN 382 cylinders.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Head at 53 moving toward 60 means: A. It sweeps downward first B. It serves the nearest only C. It sweeps upward first D. It starts at cylinder 0 Answer: C. 60 > 53, so the direction is up. (Key point: Direction) 2. SSTF total movement is: A. 640 B. 331 C. 299 D. 236 Answer: D. 12+2+30+23+84+24+2+59 = 236. (Key point: SSTF total) 3. LOOK total is 299 vs SCAN 331 because: A. LOOK reverses at 183, the last request, not at 199 B. LOOK serves fewer requests C. LOOK moves downward first D. LOOK skips cylinder 14 Answer: A. The 16 extra cylinders up and back account for 32. (Key point: LOOK vs SCAN) 4. C-SCAN total including the return jump is: A. 331 B. 382 C. 299 D. 183 Answer: B. Includes 199 to 0 return; 183 if the return is not counted. (Key point: C-SCAN) ## Related in Operating System - [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 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 - [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× - [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×