FCFS (First Come First Served) in Operating Systems - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the scheduling algorithm that executes processes in the order they arrive.

Operating Systems

algorithm = "[1]"

Drag options to blanks, or click blank then click option'

ARound Robin

BShortest Job First

CFCFS

DPriority Scheduling

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing FCFS with Round Robin or Priority Scheduling.

Choosing an algorithm that does not follow arrival order.

FCFS stands for First Come First Served, which means processes are executed in the order they arrive.

2fill in blank

medium

Complete the code to calculate the waiting time for the first process in FCFS scheduling.

Operating Systems

waiting_time[0] = [1]

Drag options to blanks, or click blank then click option'

Aarrival_time[0]

Bburst_time[0]

Ccompletion_time[0]

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Setting waiting time equal to burst time or arrival time.

Confusing waiting time with completion time.

The first process in FCFS does not wait, so its waiting time is zero.

3fill in blank

hard

Fix the error in the code to compute the waiting time for process i in FCFS scheduling.

Operating Systems

waiting_time[i] = completion_time[i-1] [1] arrival_time[i]

Drag options to blanks, or click blank then click option'

A-

B/

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of subtraction.

Using multiplication or division which are incorrect here.

Waiting time is calculated by subtracting the arrival time of the current process from the completion time of the previous process.

4fill in blank

hard

Fill both blanks to compute the completion time for process i in FCFS scheduling.

Operating Systems

completion_time[i] = completion_time[i-1] [1] burst_time[i] [2] 0

Drag options to blanks, or click blank then click option'

A+

B-

Cif

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction instead of addition.

Not using a condition to handle the first process.

The completion time is the sum of the previous completion time and the current burst time if the previous completion time is greater than zero.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps each process to its waiting time if waiting time is positive.

Operating Systems

waiting_times = [1]: [2] for [3] in processes if waiting_time[process] > 0

Drag options to blanks, or click blank then click option'

Aprocess

Bwaiting_time[process]

Cprocesses

Dwaiting_time

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect variable names.

Not filtering processes with positive waiting time.

This dictionary comprehension creates a mapping from each process to its waiting time, but only includes processes with positive waiting times.

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