C-SCAN (circular SCAN) in Operating Systems - Step-by-Step Execution

Concept Flow - C-SCAN (circular SCAN)

Start at current head position

↓

Move head in one direction

↓

Serve requests in order

↓

Reach last request in direction

↓

Jump to opposite end without serving

↓

Continue moving in same direction

↓

Repeat until all requests served

The disk head moves in one direction serving requests, then jumps back to the start without serving on return, continuing this cycle.

Execution Sample

Operating Systems

Requests = [40, 10, 22, 70, 90]
Head = 50
Direction = 'up'
Order = 'C-SCAN'

# Serve requests in ascending order from head, jump back to start after last request.

This simulates C-SCAN disk scheduling serving requests above head, then jumping to lowest request.

Analysis Table

Step	Head Position	Action	Served Request	Next Head Position
1	50	Move up to next request	70	70
2	70	Move up to next request	90	90
3	90	Reached last request, jump to start	None	10
4	10	Move up to next request	10	10
5	10	Move up to next request	22	22
6	22	Move up to next request	40	40
7	40	All requests served, stop	None	40

💡 All requests served after completing one full C-SCAN cycle.

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
Head Position	50	70	90	10	10	22	40	40
Served Requests	[]	[70]	[70,90]	[70,90]	[70,90,10]	[70,90,10,22]	[70,90,10,22,40]	[70,90,10,22,40]

Key Insights - 2 Insights

Why does the head jump from the last request back to the start without serving requests on the way back?

In C-SCAN, the head only serves requests moving in one direction to provide uniform wait times. The jump back is a fast move without serving, as shown in step 3 of the execution_table.

How does C-SCAN differ from SCAN in handling requests on the return trip?

Unlike SCAN, which serves requests on the return trip, C-SCAN skips serving requests when moving back, jumping directly to the start, as seen in the jump at step 3.

Visual Quiz - 3 Questions

According to the execution_table, what is the head position after step 3?

A 40

B 10

C 90

D 50

💡 Hint

Check the 'Next Head Position' column for step 3 in the execution_table.

At which step does the head serve the request at position 22?

A Step 4

B Step 6

C Step 5

D Step 7

💡 Hint

Look at the 'Served Request' column in the execution_table to find when 22 is served.

If the head started at 30 instead of 50, how would the first served request change?

A It would be 40

B It would be 10

C It would be 70

D It would be 22

💡 Hint

Refer to the variable_tracker and execution_table logic: head serves requests greater than current position.

Concept Snapshot

C-SCAN moves disk head in one direction only.
Serves requests in order while moving up.
After last request, jumps to start without serving.
Provides uniform wait times.
Useful for fair disk scheduling.

Full Transcript

C-SCAN (Circular SCAN) is a disk scheduling method where the disk head moves in one direction, serving requests as it goes. When it reaches the last request in that direction, it jumps back to the beginning without serving any requests on the way back. This cycle repeats until all requests are served. This method helps provide more uniform wait times compared to SCAN, which serves requests in both directions. The execution table shows the head starting at position 50, serving requests at 70 and 90, then jumping back to 10 and serving remaining requests in ascending order. Variables track the head position and served requests step-by-step. Key moments clarify why the head jumps without serving and how C-SCAN differs from SCAN. The quiz tests understanding of head positions and serving order based on the execution visuals.

Practice

1. What is the main characteristic of the C-SCAN disk scheduling algorithm?

easy

A. The disk arm moves back and forth serving requests in both directions.

B. The disk arm moves in one direction and jumps back to the start without serving requests on return.

C. The disk arm serves requests randomly without any specific order.

D. The disk arm serves only the nearest request to its current position.

Solution

1. Step 1: Understand C-SCAN movement

   C-SCAN moves the disk arm in one direction only, serving requests along the way.

2. Step 2: Identify behavior at the end of the disk

   After reaching the last track, it jumps back to the first track without serving requests during the return.

3. Final Answer:

   The disk arm moves in one direction and jumps back to the start without serving requests on return. -> Option B

4. Quick Check:

   C-SCAN moves one way and jumps back = A [OK]

Hint: Remember: C-SCAN moves one way, then jumps back [OK]

Common Mistakes:

• Thinking C-SCAN moves back and forth like SCAN
• Assuming requests are served during the return jump
• Confusing C-SCAN with random or nearest request scheduling

2. Which of the following is the correct way to describe the jump in C-SCAN?

easy

A. The disk arm jumps back to the first track without serving any requests during the jump.

B. The disk arm stops and waits for new requests before moving.

C. The disk arm reverses direction and serves requests immediately.

D. The disk arm jumps back to the first track and serves requests on the way.

Solution

1. Step 1: Analyze the jump behavior in C-SCAN

   C-SCAN jumps back to the first track after reaching the last track.

2. Step 2: Check if requests are served during the jump

   Requests are not served during this jump; the arm moves quickly without processing.

3. Final Answer:

   The disk arm jumps back to the first track without serving any requests during the jump. -> Option A

4. Quick Check:

   Jump without serving requests = D [OK]

Hint: Jump skips requests, no service during return [OK]

Common Mistakes:

• Assuming requests are served during the jump
• Confusing jump with reversing direction
• Thinking the arm waits before jumping

3. Consider a disk with tracks numbered 0 to 199. The disk arm is currently at track 50 and requests are at tracks 95, 180, 34, and 119. Using C-SCAN, which track will the arm serve immediately after 50?

medium

A. 95

B. 34

C. 119

D. 180

Solution

1. Step 1: Identify requests ahead of current position

   Requests are at 34, 95, 119, 180. Current position is 50. C-SCAN moves forward only, so it serves requests with track numbers greater than 50 first: 95, 119, 180.

2. Step 2: Find the nearest request ahead

   The nearest request ahead is at track 95, so it will be served immediately after 50.

3. Final Answer:

   95 -> Option A

4. Quick Check:

   Next request after 50 going forward = 95 [OK]

Hint: Serve next higher track number after current position [OK]

Common Mistakes:

• Serving requests behind current position before forward ones
• Choosing the closest track number regardless of direction
• Confusing C-SCAN with SCAN which moves back and forth

4. A student wrote that in C-SCAN, the disk arm serves requests while moving back to the first track after reaching the last track. What is wrong with this statement?

medium

A. C-SCAN does not move back to the first track; it reverses direction instead.

B. C-SCAN serves requests randomly, so direction does not matter.

C. C-SCAN serves requests only while moving forward, not during the jump back.

D. C-SCAN serves requests only at the last track.

Solution

1. Step 1: Review C-SCAN jump behavior

   C-SCAN moves forward serving requests, then jumps back to the first track without serving requests during the jump.

2. Step 2: Identify the error in the student's statement

   The student incorrectly said requests are served during the jump back, which is false.

3. Final Answer:

   C-SCAN serves requests only while moving forward, not during the jump back. -> Option C

4. Quick Check:

   Requests served only forward, not on jump = C [OK]

Hint: Requests served only forward, jump is skip [OK]

Common Mistakes:

• Thinking C-SCAN reverses direction instead of jumping
• Assuming requests are served during the jump
• Confusing C-SCAN with SCAN algorithm

5. Given a disk with tracks 0 to 199, the disk arm is at track 150. Requests are at tracks 10, 170, 15, 190, and 30. Using C-SCAN, what is the order in which the disk arm will serve these requests?

hard

A. 30, 15, 10, 190, 170

B. 10, 15, 30, 170, 190

C. 190, 170, 30, 15, 10

D. 170, 190, 10, 15, 30

Solution

1. Step 1: Identify requests ahead of current position 150

   Requests ahead are 170 and 190. C-SCAN serves these first moving forward.

2. Step 2: After reaching last track, jump to start and serve remaining requests

   After 190, the arm jumps to track 0 and serves requests at 10, 15, and 30 in ascending order.

3. Final Answer:

   170, 190, 10, 15, 30 -> Option D

4. Quick Check:

   Serve forward requests, jump, then serve from start = A [OK]

Hint: Serve forward requests first, then jump and serve from start [OK]

Common Mistakes:

• Serving requests behind current position before forward ones
• Serving requests in descending order
• Ignoring the jump and serving requests in wrong order