C-SCAN (circular SCAN) in Operating Systems - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to describe the direction of the disk arm in C-SCAN.

Operating Systems

The disk arm moves in one direction until it reaches the end, then it [1] to the beginning.

Drag options to blanks, or click blank then click option'

Ajumps

Breverses

Cstops

Dslows

Attempts:

3 left

💡 Hint

Common Mistakes

Thinking the arm reverses direction like in SCAN.

Assuming the arm stops at the end.

In C-SCAN, the disk arm moves in one direction and when it reaches the end, it jumps back to the beginning without servicing requests on the return.

2fill in blank

medium

Complete the code to explain the main advantage of C-SCAN over SCAN.

Operating Systems

C-SCAN provides a more uniform wait time by servicing requests in a [1] direction only.

Drag options to blanks, or click blank then click option'

Asingle

Brandom

Cboth

Dreverse

Attempts:

3 left

💡 Hint

Common Mistakes

Thinking C-SCAN services requests in both directions.

Confusing C-SCAN with SCAN.

C-SCAN services requests in a single direction only, which helps provide a more uniform wait time compared to SCAN that moves back and forth.

3fill in blank

hard

Fix the error in the description of C-SCAN's movement.

Operating Systems

In C-SCAN, the disk arm moves from the start to the end and then [1] back servicing requests on the return.

Drag options to blanks, or click blank then click option'

Areverses

Bstops

Cjumps

Dpauses

Attempts:

3 left

💡 Hint

Common Mistakes

Saying the arm reverses and services requests on the return.

Assuming the arm stops at the end.

C-SCAN moves the arm from start to end servicing requests, then jumps back to the start without servicing requests on the return.

4fill in blank

hard

Fill both blanks to complete the C-SCAN algorithm description.

Operating Systems

C-SCAN moves the disk arm from [1] to [2], servicing requests only in this direction.

Drag options to blanks, or click blank then click option'

Astart

Bmiddle

Cend

Drandom

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing middle or random as start or end points.

Confusing the direction of movement.

C-SCAN moves the disk arm from the start to the end of the disk, servicing requests only in this direction before jumping back.

5fill in blank

hard

Fill all three blanks to complete the C-SCAN scheduling code snippet.

Operating Systems

requests = sorted(requests)
current_pos = [1]
for req in requests:
    if req >= current_pos:
        service(req)
current_pos = [2]
for req in requests:
    if req < current_pos:
        service(req)
current_pos = [3]

Drag options to blanks, or click blank then click option'

A0

Bmax_track

Dmiddle

Attempts:

3 left

💡 Hint

Common Mistakes

Using middle or random values instead of 0 or max_track.

Not resetting current_pos correctly.

The C-SCAN algorithm starts servicing requests from the current position (0), moves to the max track, then jumps back to 0 to service remaining requests.

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