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

↓

Pick first process in queue

↓

Run process to completion

↓

Remove process from queue

↓

Repeat for next process

↓

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.

Practice

1. What does FCFS (First Come First Served) scheduling mean in operating systems?

easy

A. Processes with the shortest time are handled first.

B. Processes are handled in the order they arrive.

C. Processes are handled randomly.

D. Processes with the highest priority are handled first.

Solution

1. Step 1: Understand FCFS concept

   FCFS means tasks are processed in the order they come, like a queue.

2. Step 2: Compare options

   Only Processes are handled in the order they arrive. describes this order-based processing correctly.

3. Final Answer:

   Processes are handled in the order they arrive. -> Option B

4. Quick Check:

   FCFS = Order of arrival [OK]

Hint: Remember: FCFS is like waiting in line [OK]

Common Mistakes:

• Confusing FCFS with priority scheduling
• Thinking shortest tasks go first
• Assuming random order

2. Which of the following is the correct way to describe FCFS scheduling?

easy

A. Processes are scheduled in the order they arrive without preemption.

B. Processes are scheduled based on their priority levels.

C. Processes are scheduled by shortest remaining time first.

D. Processes are scheduled randomly to balance load.

Solution

1. Step 1: Identify FCFS scheduling traits

   FCFS schedules tasks in arrival order and does not interrupt running tasks.

2. Step 2: Match options to traits

   Processes are scheduled in the order they arrive without preemption. correctly states no preemption and order of arrival.

3. Final Answer:

   Processes are scheduled in the order they arrive without preemption. -> Option A

4. Quick Check:

   FCFS = Arrival order + no preemption [OK]

Hint: FCFS runs tasks fully in arrival order [OK]

Common Mistakes:

• Mixing FCFS with priority or shortest job scheduling
• Assuming tasks can be interrupted
• Thinking scheduling is random

3. Given three processes arriving at times 0, 2, and 4 with burst times 5, 3, and 1 respectively, what is the completion time of the second process using FCFS?

medium

A. 10

B. 5

C. 8

D. 9

Solution

1. Step 1: Calculate completion of first process

   Process 1 arrives at 0 and runs for 5 units, finishing at time 5.

2. Step 2: Calculate completion of second process

   Process 2 arrives at 2 but waits until process 1 finishes at 5, then runs for 3 units, finishing at 8.

3. Final Answer:

   8 -> Option C

4. Quick Check:

   Process 2 finishes at 5+3=8 [OK]

Hint: Add burst times in arrival order [OK]

Common Mistakes:

• Starting second process at its arrival time instead of after first finishes
• Adding arrival times incorrectly
• Ignoring waiting time

4. A student wrote this FCFS scheduling code but it gives wrong completion times. What is the likely error?

processes = [(0, 4), (1, 3), (2, 1)]  # (arrival, burst)
completion = []
current_time = 0
for arrival, burst in processes:
    if arrival > current_time:
        current_time = arrival
    completion.append(current_time)
    current_time += burst
print(completion)

medium

A. Not updating current_time after appending completion time.

B. Using a list instead of a queue for processes.

C. Not considering arrival time when updating current_time.

D. Appending completion time before updating current_time.

Solution

1. Step 1: Analyze code logic

   Completion time is appended before current_time is updated with burst time, causing wrong values.

2. Step 2: Identify correct order

   We must update current_time by adding burst before appending completion time to reflect actual finish time.

3. Final Answer:

   Appending completion time before updating current_time. -> Option D

4. Quick Check:

   Update time before recording completion [OK]

Hint: Update current time before saving completion [OK]

Common Mistakes:

• Appending completion time too early
• Ignoring arrival time adjustments
• Confusing process order

5. In an FCFS system, three processes arrive at times 0, 1, and 2 with burst times 4, 2, and 6. If the first process takes longer than expected and runs for 8 units instead of 4, how does this affect the waiting time of the third process?

hard

A. The third process waits longer because the first process delays the queue.

B. The third process waiting time remains the same.

C. The third process starts earlier due to the delay.

D. The third process is skipped and runs immediately.

Solution

1. Step 1: Understand FCFS impact of longer burst

   FCFS runs processes fully in arrival order, so a longer first process delays all others.

2. Step 2: Analyze waiting time effect on third process

   The third process must wait until the first and second finish, so longer first process increases its waiting time.

3. Final Answer:

   The third process waits longer because the first process delays the queue. -> Option A

4. Quick Check:

   Longer first task = longer wait for later tasks [OK]

Hint: Long first task delays all later tasks [OK]

Common Mistakes:

• Assuming later tasks start earlier
• Ignoring impact of burst time changes
• Thinking FCFS skips tasks