--- title: "Disk scheduling with 100 cylinders" subject: "Operating System" chapter: 6 key: "semester_6/operating_system:disk-100-cyl-fcfs-sstf-look-cscan-scan" source: https://www.bctengineer.com/q/semester_6/operating_system/disk-100-cyl-fcfs-sstf-look-cscan-scan site: BCT Engineering --- # Disk scheduling with 100 cylinders Operating System · Chapter 6: I/O Management and Disk Scheduling - Asked in: 2070 Magh, 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-100-cyl-fcfs-sstf-look-cscan-scan - Whole chapter: https://www.bctengineer.com/md/semester_6/operating_system/chapter-6 ## Question Suppose that a disk drive has 100 cylinders, numbered 0 to 99. The drive is currently serving a request at cylinder 43, and previous request was at cylinder 25. The queue of pending request, in FIFO order is: 86, 70, 13, 74, 48, 9, 22, 50, 30 Starting from the current head position, what is the total distance (in cylinders) that the disk arm moves to satisfy all pending requests for each of the following disk scheduling algorithms? a) First-come, First served b) Shortest Seek Time First c) Look d) C-Scan e) Scan ## Short answer Head at 43 moving upward; FCFS services in queue order: 303 cylinders; SSTF picks nearest: 120; LOOK goes up to 86 then reverses: 120; C-SCAN goes to end 99, jumps to 0, continues upward: 185; SCAN goes to end then reverses: 146 cylinders. ## Answer Disk: 100 cylinders (0 to 99), head at 43, previous request at 25, so the head is moving **upward** (towards 99). Queue (FIFO): 86, 70, 13, 74, 48, 9, 22, 50, 30. #### a) FCFS **FCFS**: service order (head at 43): 86, 70, 13, 74, 48, 9, 22, 50, 30 ```text 43 -> 86 = 43 86 -> 70 = 16 70 -> 13 = 57 13 -> 74 = 61 74 -> 48 = 26 48 -> 9 = 39 9 -> 22 = 13 22 -> 50 = 28 50 -> 30 = 20 Total = 43 + 16 + 57 + 61 + 26 + 39 + 13 + 28 + 20 = 303 ``` #### b) SSTF After 50, the cylinders 30 and 70 are equally near (20); the one in the direction of travel (70) is chosen. **SSTF**: service order (head at 43): 48, 50, 70, 74, 86, 30, 22, 13, 9 ```text 43 -> 48 = 5 48 -> 50 = 2 50 -> 70 = 20 70 -> 74 = 4 74 -> 86 = 12 86 -> 30 = 56 30 -> 22 = 8 22 -> 13 = 9 13 -> 9 = 4 Total = 5 + 2 + 20 + 4 + 12 + 56 + 8 + 9 + 4 = 120 ``` #### c) LOOK (go up as far as the last request, then reverse) **LOOK**: service order (head at 43): 48, 50, 70, 74, 86, 30, 22, 13, 9 ```text 43 -> 48 = 5 48 -> 50 = 2 50 -> 70 = 20 70 -> 74 = 4 74 -> 86 = 12 86 -> 30 = 56 30 -> 22 = 8 22 -> 13 = 9 13 -> 9 = 4 Total = 5 + 2 + 20 + 4 + 12 + 56 + 8 + 9 + 4 = 120 ``` #### d) C-SCAN (go to the end 99, jump to 0, continue upward) **C-SCAN**: service order (head at 43): 48, 50, 70, 74, 86, 9, 13, 22, 30 ```text 43 -> 48 = 5 48 -> 50 = 2 50 -> 70 = 20 70 -> 74 = 4 74 -> 86 = 12 86 -> 99 = 13 99 -> 0 = 99 (return jump) 0 -> 9 = 9 9 -> 13 = 4 13 -> 22 = 9 22 -> 30 = 8 Total = 5 + 2 + 20 + 4 + 12 + 13 + 99 + 9 + 4 + 9 + 8 = 185 ``` #### e) SCAN (go to the end 99, reverse) **SCAN**: service order (head at 43): 48, 50, 70, 74, 86, 30, 22, 13, 9 ```text 43 -> 48 = 5 48 -> 50 = 2 50 -> 70 = 20 70 -> 74 = 4 74 -> 86 = 12 86 -> 99 = 13 99 -> 30 = 69 30 -> 22 = 8 22 -> 13 = 9 13 -> 9 = 4 Total = 5 + 2 + 20 + 4 + 12 + 13 + 69 + 8 + 9 + 4 = 146 ``` #### Summary | Algorithm | Total head movement (cylinders) | |---|---| | a) FCFS | 303 | | b) SSTF | 120 | | c) LOOK | 120 | | d) C-SCAN | 185 (86 if the return sweep is not counted) | | e) SCAN | 146 | If the tie at cylinder 50 is broken the other way (30 first), SSTF would be 125. **Answer: FCFS 303, SSTF 120, LOOK 120, C-SCAN 185, SCAN 146 cylinders.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Head at 43, previous request at 25: which direction is the head moving? A. Downward, towards 0 B. Upward, towards 99 C. Stationary D. Cannot be determined Answer: B. 43 > 25, so it is moving up. (Key point: Direction) 2. FCFS total head movement is: A. 120 B. 185 C. 303 D. 146 Answer: C. 43+16+57+61+26+39+13+28+20 = 303. (Key point: FCFS total) 3. At cylinder 50, with 30 and 70 both 20 away, SSTF here picks: A. 30, because it is lower B. Neither, it stops C. The one added to the queue first D. 70, the one in the direction of travel Answer: D. Total then is 120; picking 30 first would give 125. (Key point: SSTF tie) 4. Why is SCAN 146 while LOOK is 120? A. SCAN goes all the way to cylinder 99 before reversing; LOOK reverses at the last request (86) B. LOOK serves fewer requests C. SCAN starts downward D. SCAN skips cylinder 9 Answer: A. The extra 13 cylinders up and 13 back to 86 account for 26. (Key point: SCAN vs LOOK) 5. In C-SCAN the head after reaching 99: A. Reverses and serves downward B. Returns to 0 without serving and then sweeps up serving 9,13,22,30 C. Stops D. Serves requests on the return Answer: B. Total is 185 including the 99-cylinder return. (Key point: C-SCAN) ## Close variants set in other exams - [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× ## Related in Operating System - [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, 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 - [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 other subjects - [Conductor selection, 100 MW 70 km 132 kV](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aconductor-selection-100mw-70km-132kv) — Transmission and Distribution System Design, chapter 4