📝 CPU Scheduler in Operating System (40 MCQs)

📖 From Operating System • 6. CPU Scheduling • 40 questions available

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What is CPU Scheduler in Operating System?

Definition:
The CPU scheduler, or short-term scheduler, selects from among the processes in memory that are ready to execute and allocates the CPU to one of them, making decisions at frequency $f_s$ whenever the CPU becomes idle.

Example:
Upon a timer interrupt indicating the end of a time slice, the scheduler evaluates the ready queue and assigns the CPU to the highest-priority runnable process.

Reason:
This component is critical for multiprogramming as it determines the order of execution, directly impacting response time, turnaround time, and fairness among competing processes.

Easy

Medium

Hard

📝 All CPU Scheduler in Operating System MCQs

Q1. What is the primary responsibility of the CPU scheduler in an operating system?

A.Managing virtual memory

B.Selecting a ready process and allocating the CPU ✅

C.Creating new processes

D.Handling file system requests

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The CPU scheduler selects one of the ready processes waiting in memory and allocates the CPU for execution. Memory management, process creation, and file system operations are handled by other operating system components, making them incorrect choices.

Q2. Under which condition does the operating system invoke the CPU scheduler described in this topic?

A.When a process creates a file

B.Whenever the CPU becomes idle ✅

C.Whenever a user logs in

D.Whenever secondary storage is formatted

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The scheduler becomes active whenever the CPU has no process to execute. At that point, it selects an eligible process from the ready queue. File operations, user authentication, and storage formatting are unrelated to this scheduling decision.

Q3. Which scheduler is directly responsible for selecting the next process from the ready queue?

A.Long-term scheduler

B.Medium-term scheduler

C.Short-term scheduler ✅

D.Memory scheduler

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: The short-term scheduler, also called the CPU scheduler, chooses the next ready process for execution. The long-term scheduler controls admission of new processes, while the medium-term scheduler manages process suspension and resumption.

Q4. A process has completed its waiting period and is now eligible to execute. Which component decides whether it receives the CPU next?

A.CPU scheduler ✅

B.Device controller

C.Memory management unit

D.File allocator

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: Once a process is in the ready queue, the CPU scheduler determines whether it will receive the processor. Device controllers, memory management hardware, and file allocators perform different operating system functions and do not select executable processes.

Q5. From where does the CPU scheduler choose the next process to execute?

A.Waiting queue

B.Job queue

C.Ready queue ✅

D.I/O queue

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: The scheduler selects only from the ready queue because these processes are prepared for execution. Waiting queues contain blocked processes, while job queues and I/O queues serve different purposes within process management.

Q6. Which statement correctly describes the relationship between the short-term scheduler and the CPU scheduler?

A.They perform unrelated operating system functions.

B.The CPU scheduler replaces the long-term scheduler.

C.They refer to the same scheduling component. ✅

D.The short-term scheduler only manages memory allocation.

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: The terms short-term scheduler and CPU scheduler describe the same operating system component. Its role is to choose a ready process and allocate the processor. It neither replaces the long-term scheduler nor performs memory management.

Q7. An operating system designer wants maximum flexibility in organizing ready processes. Which statement is correct?

A.The ready queue must always use FIFO ordering.

B.The ready queue can only be implemented as a tree.

C.The ready queue implementation is independent of the scheduler.

D.The ready queue may be implemented using different data structures. ✅

💡 Difficulty: medium | ✅ Correct: D

📖 Explanation: The ready queue is a logical concept rather than a fixed implementation. Operating systems may implement it using FIFO queues, priority queues, trees, or linked lists depending on scheduling requirements and desired performance characteristics.

Q8. Which data structure is explicitly identified as one possible implementation of the ready queue?

A.Hash table

B.Priority queue ✅

C.Stack

D.Graph

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: A priority queue is specifically mentioned as a valid implementation for the ready queue. Hash tables, stacks, and graphs are useful in computing but are not identified here as standard ready queue implementations.

Q9. A developer assumes that every ready queue must behave as a first-in, first-out queue. Why is this assumption incorrect?

A.Ready queues always remove the newest process first.

B.Ready queues can be implemented using several scheduling structures. ✅

C.Ready queues are limited to circular lists.

D.Ready queues contain only terminated processes.

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The ready queue is not inherently FIFO. Depending on the scheduling algorithm, it may use FIFO ordering, priorities, trees, or linked lists. Therefore, assuming FIFO behavior for every operating system would be technically incorrect.

Q10. Why does the operating system maintain a ready queue?

A.To store completed processes permanently

B.To hold processes waiting for CPU execution ✅

C.To save backup copies of process code

D.To manage available disk partitions

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The ready queue contains processes that are prepared to execute and are waiting for CPU allocation. Completed processes are removed from execution, while backups and disk partition management belong to separate operating system services.

Q11. Which characteristic best describes the ready queue in modern operating systems?

A.It always stores processes in FIFO order.

B.It is conceptually a collection of processes waiting for CPU execution regardless of its internal implementation. ✅

C.It permanently stores completed process information.

D.It contains only processes performing I/O operations.

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The ready queue is a conceptual collection of processes waiting for CPU execution. Its internal implementation may vary depending on the scheduling algorithm, allowing structures such as FIFO queues, priority queues, trees, or linked lists instead of enforcing a single organization.

Q12. A scheduling algorithm requires the highest-priority ready process to execute next. Which ready queue implementation most naturally supports this requirement?

A.Priority queue ✅

B.FIFO queue

C.Circular buffer

D.Sequential array sorted by arrival time

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: A priority queue efficiently retrieves the process with the highest scheduling priority. Although other structures can be adapted, a priority queue directly supports priority-based scheduling without relying solely on arrival order.

Q13. A system designer chooses a tree structure for the ready queue. What is the primary reason for selecting such an implementation?

A.To guarantee FIFO execution

B.To provide efficient organization and retrieval according to scheduling criteria ✅

C.To eliminate the need for scheduling algorithms

D.To store only terminated processes

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Tree-based implementations can efficiently organize ready processes according to scheduling keys such as priority or execution characteristics. They do not guarantee FIFO behavior, replace scheduling algorithms, or store terminated processes.

Q14. What does the CPU scheduler allocate to the selected process?

A.A memory page

B.The processor for execution ✅

C.A disk partition

D.An input device

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The scheduler's immediate responsibility is to allocate the CPU to one of the ready processes. Memory pages, storage resources, and I/O devices are managed through separate operating system mechanisms.

Q15. Which statement about processes in the ready queue is correct?

A.They are waiting for CPU execution. ✅

B.They have permanently terminated.

C.They are blocked waiting for I/O completion.

D.They have not yet been admitted into memory.

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: Processes in the ready queue are ready to execute as soon as they receive CPU time. Blocked processes belong to waiting queues, terminated processes have finished execution, and processes not admitted into memory are outside the ready state.

Q16. Which operating system structure typically represents the entries stored in the ready queue?

A.Page tables

B.Process Control Blocks (PCBs) ✅

C.File descriptors

D.Disk allocation tables

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Ready queues generally store references to Process Control Blocks rather than entire process images. PCBs contain the management information required by the operating system to schedule and control process execution efficiently.

Q17. A student claims that the ready queue stores complete executable programs instead of management records. Which response is most accurate?

A.The queue stores Process Control Blocks that represent the processes. ✅

B.The queue stores only processor registers.

C.The queue stores only executable files from disk.

D.The queue stores only memory pages.

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Operating systems normally maintain Process Control Blocks within scheduling queues. These structures contain process management information needed for scheduling decisions, rather than storing complete executable files or isolated hardware state.

Q18. Which statement best explains why different scheduling algorithms may require different ready queue implementations?

A.Every algorithm executes processes randomly.

B.Different algorithms organize ready processes according to different selection criteria. ✅

C.Scheduling algorithms never interact with queue structures.

D.Queue implementation has no influence on process selection.

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Scheduling algorithms often rely on different selection policies such as arrival order or priority. Consequently, the underlying data structure may differ to support efficient retrieval while maintaining the algorithm's intended behavior.

Q19. An operating system keeps all ready processes in an unordered linked list. What does this imply?

A.The implementation still satisfies the conceptual definition of a ready queue. ✅

B.The scheduler can no longer schedule any process.

C.Only FIFO scheduling becomes possible.

D.Every process automatically receives equal CPU time.

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: The ready queue is defined conceptually rather than by a fixed implementation. An unordered linked list still qualifies because it contains ready processes awaiting CPU allocation, although selection efficiency depends on the scheduling algorithm.

Q20. A process enters the ready queue after becoming eligible to execute. What is its immediate status before being selected by the CPU scheduler?

A.Running on the processor

B.Waiting in the ready queue for CPU allocation ✅

C.Performing an I/O operation

D.Terminated after completing execution

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Once a process is ready, it waits in the ready queue until the CPU scheduler selects it for execution. It is not yet running, blocked for I/O, or terminated, making CPU allocation the next expected scheduling event.

Q21. Which event directly causes the CPU scheduler to make a new scheduling decision in the context of this topic?

A.A printer finishes printing a document

B.The CPU becomes idle and requires a new process to execute ✅

C.A new file is created on secondary storage

D.A user changes account settings

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The scheduler is activated when the CPU has no process to execute. It then selects a process from the ready queue. Events such as printing, file creation, or account management do not directly require CPU scheduling decisions.

Q22. Why is the ready queue described as a conceptual waiting line rather than a fixed data structure?

A.Because it always behaves like a stack

B.Because its internal organization depends on the scheduling algorithm used ✅

C.Because it exists only in secondary storage

D.Because it contains only suspended processes

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The ready queue represents the set of processes waiting for CPU service. Its physical implementation can vary according to the scheduling algorithm, allowing FIFO queues, priority queues, trees, or linked lists while preserving the same conceptual purpose.

Q23. Which of the following is NOT identified as a possible implementation of the ready queue?

A.FIFO queue

B.Priority queue

C.Binary tree

D.Hash table ✅

💡 Difficulty: medium | ✅ Correct: D

📖 Explanation: FIFO queues, priority queues, and tree-based structures are recognized implementations of the ready queue. A hash table is not identified for this purpose because scheduling focuses on selecting the next executable process rather than key-based lookups.

Q24. A scheduler examines several ready processes and grants the CPU to one of them. Which operating system function is being performed?

A.Memory allocation

B.CPU scheduling ✅

C.Deadlock detection

D.Virtual memory translation

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: Choosing one ready process and assigning it the processor is the fundamental responsibility of CPU scheduling. Memory allocation, deadlock detection, and address translation are separate operating system functions.

Q25. Which statement best distinguishes a ready queue from a waiting queue?

A.A ready queue contains executable processes awaiting CPU time, whereas a waiting queue contains blocked processes awaiting events. ✅

B.A ready queue stores completed processes, whereas a waiting queue stores active processes.

C.Both queues always contain identical processes.

D.A waiting queue always has higher scheduling priority than a ready queue.

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Ready processes can execute as soon as the CPU becomes available, while waiting processes are blocked until an event such as I/O completion occurs. This distinction is fundamental to process state management and scheduling.

Q26. Which operating system structure contains the information needed to manage an individual process and is commonly stored in scheduling queues?

A.Interrupt vector

B.Process Control Block (PCB) ✅

C.Page directory

D.Cache line descriptor

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: A Process Control Block stores process-related management information such as state and scheduling data. Scheduling queues typically maintain PCBs or references to them, enabling the operating system to manage processes efficiently.

Q27. A developer replaces a FIFO ready queue with a priority queue while keeping the same set of ready processes. What changes immediately?

A.The physical memory occupied by each process

B.The method used to select the next process for execution ✅

C.The CPU instruction set

D.The definition of the ready state

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Changing the queue implementation affects how the scheduler selects the next process. It does not alter processor architecture, process memory size, or the meaning of the ready state itself.

Q28. Why are Process Control Blocks stored in scheduling queues instead of complete process images?

A.PCBs contain the management information required for scheduling decisions ✅

B.PCBs permanently replace executable programs

C.Complete process images cannot reside in memory

D.PCBs eliminate the need for context switching

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: A PCB provides the scheduling and management information needed to identify and control a process efficiently. The executable code and memory image remain elsewhere, making PCBs far more suitable for queue management.

Q29. An operating system implements the ready queue as an unordered linked list. Which statement remains true regardless of this implementation?

A.The scheduler still selects from the collection of ready processes ✅

B.Processes are always executed in arrival order

C.Priority scheduling becomes impossible

D.The ready queue can no longer contain PCBs

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: The implementation changes only the internal organization of ready processes. Regardless of the data structure, the scheduler still chooses a process from the ready queue, which generally contains PCBs representing executable processes.

Q30. Which statement accurately summarizes the relationship among the CPU scheduler, ready queue, and CPU?

A.The ready queue allocates the CPU directly without scheduler involvement.

B.The CPU scheduler selects a ready process from the ready queue and allocates the CPU to it. ✅

C.The CPU allocates itself to the highest-priority process automatically.

D.The ready queue executes processes independently of the processor.

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: The scheduling sequence consists of selecting a process from the ready queue and allocating the CPU through the CPU scheduler. The queue itself does not execute processes, and the processor does not independently determine scheduling decisions.

Q31. Which operating system component makes the final decision about which ready process executes next?

A.Memory manager

B.CPU scheduler ✅

C.Device driver

D.File system manager

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The CPU scheduler is responsible for selecting one process from the ready queue whenever the processor becomes available. Memory management, device drivers, and file systems provide different operating system services and do not decide which ready process receives CPU time.

Q32. What is the primary purpose of maintaining a ready queue in memory?

A.To organize processes that are prepared for execution ✅

B.To archive completed processes

C.To permanently store executable files

D.To manage physical storage devices

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: The ready queue keeps track of processes that are ready to execute but are waiting for CPU allocation. Completed processes, executable files, and storage devices are managed by other operating system components.

Q33. A scheduling policy requires selecting processes according to dynamically changing priorities. Which ready queue implementation best supports this policy?

A.Priority queue ✅

B.FIFO queue

C.Simple sequential array based only on arrival order

D.Static stack

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: A priority queue naturally supports scheduling decisions based on changing process priorities. FIFO ordering emphasizes arrival sequence, while stacks and fixed arrays do not efficiently support dynamic priority-based selection.

Q34. Which statement correctly describes the ready queue regardless of its implementation?

A.It always executes processes in arrival order.

B.It contains processes waiting for CPU allocation. ✅

C.It stores only blocked processes.

D.It permanently contains every process created by the operating system.

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The defining characteristic of the ready queue is that it contains processes ready to execute and waiting for CPU time. Its internal organization and scheduling order may differ depending on the scheduling algorithm employed.

Q35. An operating system designer replaces a linked-list ready queue with a tree-based implementation. Which aspect remains unchanged?

A.The conceptual purpose of the ready queue ✅

B.The scheduling algorithm automatically becomes FIFO

C.Every process receives identical execution time

D.The CPU scheduler is no longer required

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Changing the underlying data structure affects implementation efficiency rather than the conceptual role of the ready queue. The scheduler still selects ready processes, and neither FIFO behavior nor equal execution time is guaranteed.

Q36. Why are Process Control Blocks suitable entries for the ready queue?

A.They contain process management information needed for scheduling. ✅

B.They replace the executable code of every process.

C.They permanently store user application data.

D.They eliminate context switching overhead.

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: PCBs store scheduling-related information such as process state and other control data. This allows the scheduler to manage processes efficiently without storing complete program images or user data in the scheduling queue.

Q37. Which of the following best explains why the ready queue is not inherently a FIFO queue?

A.Scheduling algorithms may require different selection mechanisms. ✅

B.FIFO queues cannot be implemented in operating systems.

C.Ready processes never arrive in sequence.

D.The CPU executes all ready processes simultaneously.

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: The ready queue is a logical collection of executable processes. Different scheduling algorithms require different methods for selecting the next process, making FIFO only one possible implementation rather than a mandatory characteristic.

Q38. A process has just been selected from the ready queue by the CPU scheduler. What is the immediate outcome?

A.The process is allocated the CPU for execution. ✅

B.The process is moved directly to secondary storage.

C.The process becomes permanently blocked.

D.The process is automatically terminated.

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Once selected by the CPU scheduler, the process receives the processor and begins or resumes execution. It is not blocked, terminated, or transferred to secondary storage as part of this scheduling decision.

Q39. Which statement accurately reflects the flexibility of ready queue implementation?

A.Only linked lists can represent ready queues.

B.Only priority queues are valid implementations.

C.Different data structures may be used while preserving the same scheduling concept. ✅

D.Every implementation guarantees identical scheduling performance.

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: The ready queue is a conceptual structure whose implementation may vary. FIFO queues, priority queues, trees, and linked lists all satisfy the same conceptual role, although they differ in efficiency and scheduling behavior.

Q40. Which sequence correctly describes the scheduling activity discussed in this topic?

A.CPU becomes idle → CPU scheduler selects a ready process → CPU is allocated to that process ✅

B.Ready queue becomes empty → Process terminates → CPU scheduler starts

C.Process completes execution → CPU scheduler formats the ready queue → CPU shuts down

D.Memory manager selects a PCB → Device controller allocates CPU → Process executes

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: When the processor becomes idle, the CPU scheduler examines the ready queue, selects an eligible process, and allocates the CPU to it. This sequence captures the essential scheduling workflow described in the topic.

🔗 Related Topics (MCQs)

📝 CPU Scheduling Basic Concepts MCQs 📝 CPU I/O Burst Cycle MCQs 📝 Preemptive Scheduling in Operating System MCQs 📝 Dispatcher in CPU Scheduling MCQs 📝 Scheduling Criteria in Operating System MCQs

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