FCFS (First Come First Served) in Operating Systems - Step-by-Step Execution

Concept Flow - FCFS (First Come First Served)

Processes arrive in order

↓

Queue processes by arrival time

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Pick first process in queue

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Run process to completion

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Remove process from queue

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Repeat for next process

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All done

FCFS schedules processes in the order they arrive, running each to completion before starting the next.

Execution Sample

Operating Systems

Processes = ['P1', 'P2', 'P3']
Arrival = [0, 2, 4]
Burst = [5, 3, 1]
Schedule by arrival
Run each fully in order

This example shows three processes arriving at different times, scheduled and run one after another in arrival order.

Analysis Table

Step	Current Time	Process Selected	Process Start Time	Process End Time	Queue State
1	0	P1	0	5	[P1]
2	5	P2	5	8	[P2, P3]
3	8	P3	8	9	[P3]
4	9	None	N/A	N/A	[]

💡 All processes completed; queue is empty.

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Current Time	0	5	8	9	9
Queue	[P1]	[P2, P3]	[P3]	[]	[]
Process Selected	None	P1	P2	P3	None

Key Insights - 2 Insights

Why does the CPU wait if the next process hasn't arrived yet?

In FCFS, the CPU is idle if no process has arrived by the current time. For example, if the next process arrives after the current time, the CPU waits until that arrival.

Can a process be interrupted once it starts?

No, FCFS runs each process fully until it finishes before starting the next one, as shown by continuous start and end times in execution_table rows.

Visual Quiz - 3 Questions

Look at the execution_table at Step 2. What is the current time when process P2 starts?

A 2

B 5

C 8

D 0

💡 Hint

Check the 'Process Start Time' column for Step 2 in execution_table.

At which step does the queue become empty?

A Step 3

B Step 2

C Step 4

D Step 1

💡 Hint

Look at the 'Queue State' column in execution_table to see when it becomes [].

If process P3 arrived earlier at time 3, how would that affect the queue at Step 2?

A No change; queue stays the same

B P3 would be ahead of P2 in the queue

C P3 would run before P1

D P3 would be removed from the queue

💡 Hint

FCFS orders by arrival time; check variable_tracker for queue order changes.

Concept Snapshot

FCFS (First Come First Served):
- Processes run in order of arrival.
- Each process runs fully before next starts.
- Simple, non-preemptive scheduling.
- Can cause waiting if early processes are long.
- Easy to understand and implement.

Full Transcript

FCFS scheduling runs processes in the order they arrive. The CPU picks the first process in the queue and runs it fully before moving to the next. If no process has arrived, the CPU waits. This method is simple but can cause delays if early processes take long. The execution table shows how current time advances as each process runs, and the queue shrinks until empty.