FCFS disk scheduling in Operating Systems - Step-by-Step Execution

Concept Flow - FCFS disk scheduling

Start at initial head position

↓

Take next request in arrival order

↓

Move disk head to requested track

↓

Serve the request

↓

Check if more requests

↓

Repeat for next

FCFS disk scheduling serves disk requests in the exact order they arrive, moving the disk head sequentially to each requested track.

Execution Sample

Operating Systems

Initial head = 50
Requests = [82, 170, 43, 140, 24]
Serve requests in order
Calculate head movement each step

This example shows the disk head starting at track 50 and moving to each requested track in the order they arrive.

Analysis Table

Step	Current Head Position	Next Request	Head Movement	New Head Position
1	50	82	32 (82-50)	82
2	82	170	88 (170-82)	170
3	170	43	127 (170-43)	43
4	43	140	97 (140-43)	140
5	140	24	116 (140-24)	24
6	24	No more requests	0	24

💡 All requests served in arrival order; no more requests left.

State Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
Head Position	50	82	170	43	140	24	24
Request Served	None	82	170	43	140	24	All

Key Insights - 3 Insights

Why does the head sometimes move a large distance between requests?

Because FCFS serves requests strictly in arrival order without optimizing path, the head may jump far between distant tracks as shown in steps 2 and 3 in the execution_table.

Does FCFS reorder requests to reduce movement?

No, FCFS does not reorder requests; it always serves them in the order they arrive, which can cause inefficient head movement as seen in the execution_table.

What happens when there are no more requests?

The disk head stops moving, as shown in step 6 of the execution_table where no more requests remain.

Visual Quiz - 3 Questions

Look at the execution_table, what is the head position after serving the third request?

A 170

B 140

C 43

D 82

💡 Hint

Check the 'New Head Position' column at step 3 in the execution_table.

At which step does the head move the greatest distance?

A Step 1

B Step 3

C Step 5

D Step 2

💡 Hint

Look at the 'Head Movement' values in the execution_table to find the largest number.

If the initial head position was 100 instead of 50, how would the head movement in step 1 change?

A It would decrease

B It would increase

C It would stay the same

D It would be zero

💡 Hint

Compare the initial head position and next request in step 1 from the variable_tracker and execution_table.

Concept Snapshot

FCFS Disk Scheduling:
- Serve disk requests in order of arrival
- Move head directly to each requested track
- Simple but can cause long head movements
- No reordering or optimization
- Easy to implement but may be inefficient

Full Transcript

FCFS disk scheduling works by serving disk requests in the exact order they arrive. Starting from an initial head position, the disk head moves to each requested track one by one. The execution table shows each step: the current head position, the next request, the distance the head moves, and the new head position. The head can move large distances if requests are far apart because FCFS does not reorder requests to minimize movement. When no requests remain, the disk head stops. This method is simple but can be inefficient due to potentially long head movements.