📝 Dispatcher in CPU Scheduling (40 MCQs)

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

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What is Dispatcher in CPU Scheduling?

Definition:
The dispatcher is the module that gives control of the CPU to the process selected by the short-term scheduler, performing context switching with latency $L_d$ and mode switching.

Example:
After the scheduler picks Process B, the dispatcher saves Process A's registers, loads Process B's registers, updates the PCB, and jumps to Process B's instruction pointer.

Reason:
The dispatcher's speed is crucial because dispatch latency adds overhead to every context switch; minimizing $L_d$ ensures that more CPU time is spent on useful computation rather than administrative tasks.

Easy

Medium

Hard

📝 All Dispatcher in CPU Scheduling MCQs

Q1. What is the primary function of the dispatcher?

A.To select a process from the ready queue

B.To give control of the CPU to the selected process ✅

C.To manage memory allocation

D.To handle I/O operations

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The dispatcher is the module that gives control of the CPU to the process selected by the short-term scheduler. Its primary responsibility is to actually perform the context switch and transfer CPU control to the chosen process.

Q2. Which component works directly with the dispatcher in the CPU scheduling function?

A.Long-term scheduler

B.Short-term scheduler ✅

C.Medium-term scheduler

D.I/O scheduler

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The dispatcher works with the short-term scheduler, also called the CPU scheduler. The scheduler selects the process, and the dispatcher gives control of the CPU to that selected process, completing the scheduling operation.

Q3. What are the three main functions of the dispatcher?

A.Context switching, switching to user mode, jumping to proper location ✅

B.Process selection, context switching, terminating processes

C.Memory management, process creation, I/O handling

D.Priority calculation, context switching, process termination

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: The dispatcher performs context switching, switches from kernel to user mode, and jumps to the proper location in the user program to restart execution. These three operations transfer CPU control from the kernel to the selected process.

Q4. What is dispatch latency?

A.The time to select a process from the ready queue

B.The time for the dispatcher to stop one process and start another ✅

C.The time for a process to complete execution

D.The time for the scheduler to make a decision

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: Dispatch latency is the time it takes for the dispatcher to stop one process and start another running. This includes context switch time, mode switching, and program restart operations, all of which must be completed quickly.

Q5. Why must the dispatcher be as fast as possible?

A.To reduce memory usage

B.Because it is invoked during every process switch ✅

C.To simplify the scheduler design

D.To increase process priority

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The dispatcher must be fast because it is invoked during every process switch. Any delay in the dispatcher directly affects system performance and responsiveness, as context switches occur frequently in multitasking systems.

Q6. What happens during the context switch performed by the dispatcher?

A.The process is terminated

B.The state of the current process is saved and the new process's state is restored ✅

C.The process is moved to the waiting queue

D.The process's priority is increased

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: During context switching, the dispatcher saves the state of the current process (registers, program counter, etc.) and restores the state of the selected process. This allows the CPU to resume execution of the new process from where it was previously paused.

Q7. After the dispatcher switches to user mode, what is the next step?

A.Load the process control block

B.Jump to the proper location in the user program ✅

C.Execute the scheduler

D.Initialize the memory management unit

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: After switching to user mode, the dispatcher jumps to the proper location in the user program to restart that program. This final step transfers control to the process's execution code, allowing the process to continue or begin execution.

Q8. What determines the proper location for the jump in the user program during dispatching?

A.The program counter value in the PCB ✅

B.The process ID

C.The memory address of the process

D.The priority of the process

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: The dispatcher uses the program counter value stored in the process control block (PCB) to determine where to jump in the user program. This ensures that execution resumes at the correct instruction position for the selected process.

Q9. Which of the following is NOT a function of the dispatcher?

A.Switching context

B.Switching to user mode

C.Jumping to proper program location

D.Selecting the next process to run ✅

💡 Difficulty: medium | ✅ Correct: D

📖 Explanation: Selecting the next process to run is the function of the short-term scheduler, not the dispatcher. The dispatcher's role is to perform the actual context switch and transfer CPU control after the scheduler has made the selection decision.

Q10. What is the relationship between the scheduler and the dispatcher?

A.The dispatcher selects processes and the scheduler executes them

B.The scheduler selects processes and the dispatcher executes the selection ✅

C.Both perform identical functions

D.The dispatcher selects processes for the scheduler

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The scheduler selects which process should run next, and the dispatcher implements that decision by performing the context switch and giving CPU control to the selected process. They work sequentially in the scheduling process.

Q11. When does the dispatcher get invoked?

A.Only when a process terminates

B.During every process switch ✅

C.Only when I/O occurs

D.Only in nonpreemptive systems

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The dispatcher is invoked during every process switch, regardless of the reason for the switch. This includes process completion, I/O requests, timer interrupts, and any other event that causes a context switch between processes.

Q12. What happens to the dispatcher during a context switch?

A.The dispatcher executes the context switch ✅

B.The dispatcher is preempted

C.The dispatcher terminates

D.The dispatcher moves to user mode

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: The dispatcher itself executes the context switch operation. It saves the current process's state and restores the selected process's state. The dispatcher runs in kernel mode during this operation to access privileged CPU registers and memory structures.

Q13. What is the significance of dispatch latency in real-time systems?

A.It ensures process execution

B.It can affect meeting timing deadlines ✅

C.It increases memory usage

D.It simplifies scheduling

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Dispatch latency is crucial in real-time systems because it adds to the time required to start executing a real-time task. If dispatch latency is too high, tasks may miss their deadlines, making the system unable to meet its timing requirements.

Q14. How does context switching by the dispatcher affect system performance?

A.It increases CPU utilization

B.It introduces overhead ✅

C.It reduces memory fragmentation

D.It improves I/O throughput

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Context switching introduces overhead because the dispatcher must save and restore process states, switch modes, and perform other operations. While necessary for multitasking, this overhead reduces the time available for actual process execution.

Q15. What happens to the process state during the dispatcher's context switch?

A.It is stored in the process control block ✅

B.It is deleted

C.It is stored in memory only

D.It is cached

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: During context switching, the dispatcher saves the current process's state (registers, program counter, etc.) in its process control block (PCB). This allows the process to resume execution correctly when it is later selected to run again by the scheduler.

Q16. Why does the dispatcher need to switch from kernel mode to user mode?

A.User processes must execute in user mode ✅

B.Kernel mode is slower

C.User mode provides more privileges

D.To terminate the process

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: The dispatcher must switch from kernel mode to user mode because user processes execute with limited privileges in user mode. After the kernel performs scheduling and context switching operations, the CPU must be switched to user mode before executing the user process's instructions.

Q17. What is the relationship between dispatch latency and context switch time?

A.Dispatch latency includes context switch time ✅

B.Dispatch latency is unrelated to context switch time

C.Dispatch latency is shorter than context switch time

D.Dispatch latency refers only to scheduler time

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Dispatch latency includes context switch time along with other operations like mode switching and jumping to the program location. Context switch time is a component of the total dispatch latency, which measures the complete time to stop one process and start another.

Q18. How can operating systems reduce dispatch latency?

A.By increasing context switch frequency

B.By reducing the time spent in the dispatcher ✅

C.By using larger process control blocks

D.By selecting processes more quickly

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Operating systems reduce dispatch latency by optimizing the dispatcher code and minimizing the operations performed during context switching. Faster hardware, optimized context-switch routines, and efficient mode switching all contribute to lower dispatch latency.

Q19. What happens to the dispatcher when a process returns from kernel mode to user mode?

A.The dispatcher transfers control to the process ✅

B.The dispatcher terminates

C.The dispatcher is removed from memory

D.The dispatcher moves to user mode

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: When a process returns from kernel mode to user mode, the dispatcher is responsible for transferring control from the kernel to the user process. This is the final step in the scheduling process after the kernel has completed any necessary operations.

Q20. In a multiprocessor system, what additional complexity does the dispatcher face?

A.Memory allocation

B.Cache coherence and TLB flushing ✅

C.Process selection

D.Priority calculation

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: In multiprocessor systems, the dispatcher must handle cache coherence issues and TLB (Translation Lookaside Buffer) flushing. When moving processes between processors, the dispatcher must ensure that processor caches and TLBs are properly synchronized to maintain data consistency.

Q21. What is the primary constraint on dispatcher code complexity?

A.Memory limitations

B.Speed requirements ✅

C.Processor architecture

D.Scheduler complexity

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: The dispatcher code must be kept simple and efficient because it is invoked during every context switch. Any unnecessary complexity in the dispatcher directly increases dispatch latency and reduces overall system performance.

Q22. How does the dispatcher ensure proper program execution after a context switch?

A.By restoring the program counter ✅

B.By resetting all registers to zero

C.By clearing the CPU cache

D.By disabling interrupts

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: The dispatcher restores the program counter (PC) from the process control block, ensuring that execution resumes at the correct instruction when the process is scheduled again. This is essential for maintaining process execution continuity across multiple CPU assignments.

Q23. What would be the impact of a slow dispatcher on a time-sharing system?

A.Better user response time

B.Worse user response time ✅

C.No impact on response time

D.Improved system throughput

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: A slow dispatcher increases the time required to switch between processes, reducing the CPU time available for process execution. This leads to worse user response times in time-sharing systems because processes take longer to get CPU attention.

Q24. Why does the dispatcher switch to user mode before jumping to the user program?

A.To maintain security and prevent privilege escalation ✅

B.To improve execution speed

C.To simplify the jump operation

D.To reduce memory usage

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Switching to user mode before jumping to the user program is essential for security. Processes in user mode cannot execute privileged instructions, preventing them from interfering with the operating system or other processes. This protection mechanism is fundamental to operating system design.

Q25. What is the role of the dispatcher in virtual memory management?

A.Managing page tables

B.The dispatcher has no direct role in virtual memory ✅

C.Allocating memory to processes

D.Handling page faults

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: The dispatcher's primary role is context switching and CPU control transfer, not virtual memory management. While the dispatcher may save and restore memory management information during context switches, it is not directly involved in virtual memory operations like page table management.

Q26. How does the dispatcher handle the CPU register state during context switches?

A.It saves the current registers and loads the next process's registers ✅

B.It clears all registers

C.It leaves registers unchanged

D.It only saves the program counter

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: During context switching, the dispatcher saves the values of all CPU registers for the current process and loads the saved register values for the next process. This ensures each process continues execution with its correct register state, maintaining program correctness.

Q27. What is the relationship between the short-term scheduler and the dispatcher in terms of execution frequency?

A.The dispatcher runs more frequently than the scheduler ✅

B.The scheduler runs more frequently than the dispatcher

C.Both run with equal frequency

D.The dispatcher only runs when processes terminate

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: The dispatcher runs more frequently than the scheduler because it is invoked during every context switch, while the scheduler only makes decisions when scheduling events occur. Some context switches may be forced by interrupts without the scheduler making new scheduling decisions.

Q28. Why can't the dispatcher be implemented in user mode?

A.User mode cannot access privileged CPU operations ✅

B.User mode is too slow

C.User mode lacks memory management

D.User mode causes security issues

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: The dispatcher must run in kernel mode because context switching requires privileged CPU operations such as modifying CPU registers, manipulating the process control block, and switching between modes. These operations cannot be performed from user mode due to security restrictions.

Q29. What happens to the dispatcher when a process switch occurs due to a hardware interrupt?

A.The dispatcher handles the interrupt and performs context switch ✅

B.The dispatcher is bypassed

C.The dispatcher terminates the current process

D.The dispatcher moves the process to the waiting queue

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: When a hardware interrupt occurs that triggers a process switch, the dispatcher handles the context switch operation. The interrupt handler may call the dispatcher to perform the actual CPU control transfer after the scheduler determines which process should run next.

Q30. What information does the dispatcher need to perform a context switch?

A.The process control blocks of both processes ✅

B.Only the current process's PCB

C.Only the next process's PCB

D.The memory management unit

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: The dispatcher needs access to both the current and next process's PCBs to perform a context switch. It saves the current process's state to its PCB and loads the next process's state from its PCB, requiring access to both control blocks.

Q31. How does the dispatcher handle multiple processes in a round-robin scheduling system?

A.By performing context switches at each time quantum ✅

B.By selecting processes randomly

C.By executing processes to completion

D.By prioritizing interactive processes

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: In round-robin scheduling, the dispatcher performs context switches at each time quantum, saving the current process's state and loading the next process's state. This ensures fair allocation of CPU time among all ready processes by cycling through them regularly.

Q32. What is the impact of dispatching overhead on system throughput?

A.Reduces throughput ✅

B.Increases throughput

C.No effect on throughput

D.Improves throughput slightly

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Dispatching overhead reduces system throughput because the time spent in context switching is not available for executing user processes. Higher dispatch latency means more overhead per context switch, reducing the overall number of processes that can be completed.

Q33. Why must the dispatcher maintain process isolation?

A.To prevent processes from accessing each other's memory ✅

B.To improve CPU utilization

C.To simplify scheduling

D.To reduce context switch time

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: The dispatcher maintains process isolation by ensuring that context switches properly preserve each process's state and that processes cannot access each other's memory or registers. This isolation is essential for system security and stability, preventing one process from interfering with another.

Q34. What is the role of the dispatcher in process termination?

A.It performs the final context switch after termination ✅

B.It selects the next process

C.It frees the process's memory

D.It updates the process table

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: When a process terminates, the dispatcher performs the final context switch away from the terminated process. After the kernel handles process cleanup, the dispatcher transfers CPU control to the next scheduled process, completing the termination sequence.

Q35. How does the dispatcher contribute to ensuring fair CPU allocation?

A.It enforces scheduling policies through context switching ✅

B.It selects processes to run

C.It determines process priorities

D.It allocates memory to processes

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: The dispatcher enforces scheduling policies by performing context switches according to the scheduler's decisions. By switching processes at appropriate times, the dispatcher helps implement policies like round-robin or priority scheduling, ensuring fair allocation of CPU resources.

Q36. What hardware features does the dispatcher rely on?

A.Timer, MMU, and interrupt system ✅

B.Only the CPU

C.Only the memory management unit

D.Only the interrupt controller

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: The dispatcher relies on hardware features including the timer (for preemptive scheduling), memory management unit (for context switching), and interrupt system (for handling events that trigger context switches). These hardware components enable the dispatcher to perform its functions efficiently.

Q37. Why is dispatch latency critical for interactive systems?

A.It affects user-perceived responsiveness ✅

B.It determines CPU speed

C.It affects memory usage

D.It determines process priority

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Dispatch latency is critical for interactive systems because it directly affects user-perceived responsiveness. High dispatch latency means longer delays when switching between applications, resulting in a sluggish user experience and decreased system usability.

Q38. What is the relationship between dispatch latency and system overhead?

A.Higher dispatch latency increases system overhead ✅

B.Higher dispatch latency decreases system overhead

C.Dispatch latency has no effect on overhead

D.Overhead is only affected by scheduler decisions

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Higher dispatch latency increases system overhead because more CPU time is consumed in switching processes rather than executing them. This overhead reduces the efficiency of the system, as a larger portion of CPU time is spent on administrative tasks.

Q39. How does the dispatcher handle the transition from user mode to kernel mode?

A.The dispatcher does not handle this transition ✅

B.The dispatcher performs the transition

C.The hardware handles the transition

D.The scheduler handles the transition

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: The dispatcher does not handle the transition from user mode to kernel mode. This transition occurs through hardware mechanisms such as system calls, interrupts, or exceptions. The dispatcher is involved in the reverse transition (kernel to user mode) during context switching.

Q40. What optimization would most effectively reduce dispatch latency?

A.Minimizing the context switch code ✅

B.Increasing the context switch frequency

C.Using larger process control blocks

D.Reducing the number of processes

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Minimizing and optimizing the context switch code is the most effective way to reduce dispatch latency. By reducing the operations performed during context switching and optimizing memory accesses, the dispatcher can complete process switches more quickly, improving overall system performance.

🔗 Related Topics (MCQs)

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

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