Question
Use the following list of cylinder requests. They are listed in the order in which they were received:
40, 12, 22, 66, 67, 33, 80
a. List the order in which these requests are handled if the FCFS algorithm is used. Assume that the disk is positioned at cylinder 50.
b. List the order in which these requests are handled if the SSTF algorithm is used. Assume that the disk is positioned at cylinder 50.
c. List the order in which these requests are handled if the SCAN algorithm is used. Assume that the disk is positioned at cylinder 50.
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40, 12, 22, 66, 67, 33, 80
a. List the order in which these requests are handled if the FCFS algorithm is used. Assume that the disk is positioned at cylinder 50.
b. List the order in which these requests are handled if the SSTF algorithm is used. Assume that the disk is positioned at cylinder 50.
c. List the order in which these requests are handled if the SCAN algorithm is used. Assume that the disk is positioned at cylinder 50.
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Answers
GPT-4o mini
Certainly! Let’s handle the cylinder requests using the specified disk scheduling algorithms.
### Given Requests:
- 40
- 12
- 22
- 66
- 67
- 33
- 80
### a. **First-Come, First-Served (FCFS) Algorithm**
In FCFS, the requests are handled in the order they are received. Since the disk is at cylinder 50 initially, we simply process the requests in their arrival order:
1. 40 (move from 50 to 40)
2. 12 (move from 40 to 12)
3. 22 (move from 12 to 22)
4. 66 (move from 22 to 66)
5. 67 (move from 66 to 67)
6. 33 (move from 67 to 33)
7. 80 (move from 33 to 80)
**Order of requests handled in FCFS:**
- 40, 12, 22, 66, 67, 33, 80
### b. **Shortest Seek Time First (SSTF) Algorithm**
In SSTF, we select the request that is closest to the current position of the disk.
1. Start at 50. Closest request is 40.
2. Next at 40. Closest is 33.
3. Next at 33. Closest is 22.
4. Next at 22. Closest is 12.
5. Next at 12. Closest is 67 (note it's the next closest).
6. Next at 67. Closest is 66.
7. Finally, 80.
**Order of requests handled in SSTF:**
- 40, 33, 22, 12, 67, 66, 80
### c. **SCAN Algorithm**
In SCAN, the disk moves in one direction until it reaches the end and then reverses. We assume a specific direction; let’s say the disk starts scanning towards lower cylinder numbers before reversing.
1. Start at 50. Move down to 40.
2. Move down to 33.
3. Move down to 22.
4. Move down to 12.
5. There are no lower requests (bottom of the range), so now move up.
6. Move up to 66.
7. Move up to 67.
8. Finally, move up to 80.
**Order of requests handled in SCAN:**
- 40, 33, 22, 12, 66, 67, 80
In summary:
- **FCFS Order:** 40, 12, 22, 66, 67, 33, 80
- **SSTF Order:** 40, 33, 22, 12, 67, 66, 80
- **SCAN Order:** 40, 33, 22, 12, 66, 67, 80
### Given Requests:
- 40
- 12
- 22
- 66
- 67
- 33
- 80
### a. **First-Come, First-Served (FCFS) Algorithm**
In FCFS, the requests are handled in the order they are received. Since the disk is at cylinder 50 initially, we simply process the requests in their arrival order:
1. 40 (move from 50 to 40)
2. 12 (move from 40 to 12)
3. 22 (move from 12 to 22)
4. 66 (move from 22 to 66)
5. 67 (move from 66 to 67)
6. 33 (move from 67 to 33)
7. 80 (move from 33 to 80)
**Order of requests handled in FCFS:**
- 40, 12, 22, 66, 67, 33, 80
### b. **Shortest Seek Time First (SSTF) Algorithm**
In SSTF, we select the request that is closest to the current position of the disk.
1. Start at 50. Closest request is 40.
2. Next at 40. Closest is 33.
3. Next at 33. Closest is 22.
4. Next at 22. Closest is 12.
5. Next at 12. Closest is 67 (note it's the next closest).
6. Next at 67. Closest is 66.
7. Finally, 80.
**Order of requests handled in SSTF:**
- 40, 33, 22, 12, 67, 66, 80
### c. **SCAN Algorithm**
In SCAN, the disk moves in one direction until it reaches the end and then reverses. We assume a specific direction; let’s say the disk starts scanning towards lower cylinder numbers before reversing.
1. Start at 50. Move down to 40.
2. Move down to 33.
3. Move down to 22.
4. Move down to 12.
5. There are no lower requests (bottom of the range), so now move up.
6. Move up to 66.
7. Move up to 67.
8. Finally, move up to 80.
**Order of requests handled in SCAN:**
- 40, 33, 22, 12, 66, 67, 80
In summary:
- **FCFS Order:** 40, 12, 22, 66, 67, 33, 80
- **SSTF Order:** 40, 33, 22, 12, 67, 66, 80
- **SCAN Order:** 40, 33, 22, 12, 66, 67, 80