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

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Jump to opposite end without serving

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Continue moving in same direction

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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.