LOOK and C-LOOK variants in Operating Systems - Step-by-Step Execution

Concept Flow - LOOK and C-LOOK variants

Start at initial head position

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Move head in one direction

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Check for requests in that direction

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Serve requests

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Continue moving

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For C-LOOK: jump to furthest request

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Repeat until all requests served

The disk head moves in one direction serving requests until none remain, then either reverses (LOOK) or jumps back (C-LOOK) to continue.

Execution Sample

Operating Systems

Requests = [10, 22, 20, 2, 40, 6]
Head = 20
Direction = up

Serve requests in direction until none left
Then reverse (LOOK) or jump (C-LOOK)

Simulates disk head moving to serve requests in one direction, then changing direction or jumping.

Analysis Table

Step	Head Position	Direction	Requests Served	Action
1	20	up	20	Serve request at 20
2	22	up	22	Serve request at 22
3	40	up	40	Serve request at 40
4	40	up	-	No more requests up, LOOK reverses direction to down
5	10	down	10	Serve request at 10
6	6	down	6	Serve request at 6
7	2	down	2	Serve request at 2
8	2	down	-	No more requests down, all served, STOP
1	20	up	20	Serve request at 20 (C-LOOK start)
2	22	up	22	Serve request at 22
3	40	up	40	Serve request at 40
4	2	up	2	Jump to lowest request 2, serve 2
5	6	up	6	Serve request at 6
6	10	up	10	Serve request at 10
7	10	up	-	All requests served, STOP

💡 All requests served; LOOK reverses direction when no requests remain in current direction; C-LOOK jumps to lowest request after reaching highest.

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	Final
Head Position	20	20	22	40	40	10	6	2	2
Direction (LOOK)	up	up	up	up	down	down	down	down	stop
Requests Served (LOOK)	[]	[20]	[20,22]	[20,22,40]	[20,22,40]	[20,22,40,10]	[20,22,40,10,6]	[20,22,40,10,6,2]	[20,22,40,10,6,2]
Direction (C-LOOK)	up	up	up	up	up	up	up	up	stop
Requests Served (C-LOOK)	[]	[20]	[20,22]	[20,22,40]	[20,22,40,2]	[20,22,40,2,6]	[20,22,40,2,6,10]	[20,22,40,2,6,10]	[20,22,40,2,6,10]

Key Insights - 3 Insights

Why does LOOK reverse direction instead of jumping back?

LOOK moves the head back along the track after reaching the last request in one direction, as shown in execution_table steps 4 and 5, to serve remaining requests.

How does C-LOOK differ in handling the head movement after reaching the last request?

C-LOOK jumps directly to the lowest request without reversing, as seen in execution_table step 4 for C-LOOK, skipping empty tracks.

Why are some requests served out of numerical order in C-LOOK?

Because C-LOOK only moves in one direction and jumps back to the lowest request, it serves requests in ascending order from the jump point, not reversing direction.

Visual Quiz - 3 Questions

Looking at the execution_table, what is the head position at step 3 for LOOK?

A 22

B 40

C 10

D 6

💡 Hint

Check the 'Head Position' column at step 3 in the LOOK part of the table.

At which step does LOOK reverse direction?

A Step 3

B Step 5

C Step 4

D Step 7

💡 Hint

Look at the 'Action' column where it mentions reversing direction.

In C-LOOK, after serving the highest request, what happens next?

A Head jumps to lowest request

B Head reverses direction

C Head stops

D Head moves to middle request

💡 Hint

See the action at step 4 in the C-LOOK section of the execution_table.

Concept Snapshot

LOOK and C-LOOK are disk scheduling algorithms.
LOOK moves the head in one direction serving requests until none remain, then reverses.
C-LOOK moves in one direction and jumps back to the lowest request without reversing.
Both reduce unnecessary head movement compared to SCAN.
Useful for optimizing disk access time.

Full Transcript

This visual execution shows how LOOK and C-LOOK disk scheduling algorithms work. The disk head starts at a position and moves in one direction serving requests. LOOK continues until no requests remain in that direction, then reverses to serve remaining requests. C-LOOK, instead of reversing, jumps back to the lowest request and continues moving in the same direction. The execution table traces each step, showing head position, direction, requests served, and actions taken. Variable tracking shows how head position and direction change over time. Key moments clarify why LOOK reverses and C-LOOK jumps. The quiz tests understanding of head positions and direction changes. This helps beginners see how these algorithms reduce disk head movement and improve efficiency.