--- title: "Disk scheduling with 150 cylinders" subject: "Operating System" chapter: 6 key: "semester_6/operating_system:disk-150-cyl-sstf-scan-cscan-look-fcfs" source: https://www.bctengineer.com/q/semester_6/operating_system/disk-150-cyl-sstf-scan-cscan-look-fcfs site: BCT Engineering --- # Disk scheduling with 150 cylinders Operating System · Chapter 6: I/O Management and Disk Scheduling - Asked in: 2075 Baisakh, 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-150-cyl-sstf-scan-cscan-look-fcfs - Whole chapter: https://www.bctengineer.com/md/semester_6/operating_system/chapter-6 ## Question Suppose a disk drive with 150 cylinders numbered from 0-149. The drive is currently serving a request at 35 and previous request was at 120. The queue of pending request is 98, 103, 38, 122, 10, 128, 65, 75. Starting from the current head position, calculate the total head movement (in cylinder) that the disk arm moves to satisfy all the pending request for SSTF, SCAN, C-SCAN, LOOK and FCFS Disk Arm Scheduling Algorithm. ## Short answer SSTF: 211, SCAN: 163, C-SCAN: 295, LOOK: 143, FCFS: 520 cylinders. LOOK yields the lowest head movement. ## Answer Cylinders 0–149, head at 35, previous request at 120. Since the head came from 120 to 35, it is moving **towards lower cylinders**. Queue = 98, 103, 38, 122, 10, 128, 65, 75. #### 1. SSTF ```text 35 → 38 → 65 → 75 → 98 → 103 → 122 → 128 → 10 ``` 3 + 27 + 10 + 23 + 5 + 19 + 6 + 118 = 211 #### 2. SCAN Move down serving 10, go to 0, reverse, then serve the rest going up (the arm stops at 128, the last request, since nothing lies beyond). ```text 35 → 10 → 0 → 38 → 65 → 75 → 98 → 103 → 122 → 128 ``` 25 + 10 + 38 + 27 + 10 + 23 + 5 + 19 + 6 = 163 #### 3. C-SCAN Move down to 0, jump to the other end (149), then continue downwards. The return jump is counted. ```text 35 → 10 → 0 → 149 → 128 → 122 → 103 → 98 → 75 → 65 → 38 ``` 25 + 10 + 149 + 21 + 6 + 19 + 5 + 23 + 10 + 27 = 295 #### 4. LOOK Same as SCAN, but reverses at the last request (10), not at cylinder 0. ```text 35 → 10 → 38 → 65 → 75 → 98 → 103 → 122 → 128 ``` 25 + 28 + 27 + 10 + 23 + 5 + 19 + 6 = 143 #### 5. FCFS ```text 35 → 98 → 103 → 38 → 122 → 10 → 128 → 65 → 75 ``` 63 + 5 + 65 + 84 + 112 + 118 + 63 + 10 = 520 #### Summary | Algorithm | Head movement (cylinders) | |---|---| | SSTF | 211 | | SCAN | 163 | | C-SCAN | 295 | | LOOK | 143 | | FCFS | 520 | **Answer: SSTF = 211, SCAN = 163, C-SCAN = 295, LOOK = 143, FCFS = 520 cylinders.** LOOK is the best. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Head at 35, previous request at 120. Which direction? A. Towards higher cylinders B. Towards 149 first C. Towards lower cylinders D. Not determinable Answer: C. The head moved from 120 down to 35. (Key point: Direction from previous request) 2. What is the LOOK total moving down first? A. 143 B. 163 C. 211 D. 295 Answer: A. 35 to 10 (25), then 10 to 128 (118) = 143. (Key point: LOOK total) 3. SCAN is 163 and LOOK is 143. Why? A. SCAN serves extra requests B. SCAN goes to 0 before reversing: 10 extra down plus 10 extra back C. LOOK goes to 149 D. SCAN starts at 0 Answer: B. The extra 10 to cylinder 0 and 10 back adds 20. (Key point: SCAN vs LOOK) 4. In C-SCAN (295), what is the 149 term? A. Distance from 35 to 10 B. Serving request 128 C. The size of the queue D. The return jump from 0 to 149 Answer: D. After reaching 0, the head jumps to the other end, which is counted. (Key point: C-SCAN counting) ## Close variants set in other exams - [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× ## 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 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, 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, 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 - [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×