in Operating System edited by
53 votes

Which of the following does not interrupt a running process?

  1. A device
  2. Timer
  3. Scheduler process
  4. Power failure
in Operating System edited by


is answer C or D?
@sushmita-Yeah assume you are about to save a file, you pressed cntrl+s, and suddenly power goes before your content is saved!!

(C) Scheduler Process is the correct answer. 
The reason being, A scheduler process only schedules or we can say selects a process from ready queue to be run on CPU. So no interrupt is generated in that case. When there is power failure, surely an interrupt "power-off signal" will be generated.

@arjun sir, what is the difference between prempt and interupt ?
It is not easy but tagged easy :p

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4 Answers

52 votes
Best answer

Answer is (C).

Timer and disk both makes interrupt and power failure will also interrupt the system. Only a scheduler process will not interrupt the running process as schduler process gets called only when no other process is running (preemption if any would have happened before scheduler starts execution).

Quote from wikipedia

In the Linux kernel, the scheduler is called after each timer interrupt (that is, quite a few times per second). It determines what process to run next based on a variety of factors, including priority, time already run, etc. The implementation of preemption in other kernels is likely to be similar.

edited by


but what is it is say SRTF.....a process can be interrupted here even though scheduled?
So ans is NONE or C ?
When a process A running, and to run another process B. I need scheduler to schedule.
But scheduler itself is a process, so to schedule the scheduler i need one more scheduler? and this goes on ?.

No, it doesn't happen that way, otherwise it will be vicious cycle.
We take support from hardware here. A Hardware timer runs along with process, when this timer expires, it generates a hardware interrupt. The interrupt handle routine pauses execution of current process and schedules the scheduler. Now scheduler decides which process to schedule next.
@Arjun sir but in case of priority scheduling algorithm it will be interrupt the current running process .So how can we say that scheduled process will not interrupt the running process ?
Scheduler is not a process and invokes by timer interrupt.It does not generate interrupt by itself. The timer interrupt handler runs the OS scheduler. If the process that was just interrupted has used up its time quantum, and there is some other runnable process, then the scheduler "returns" to that other process. (Later, when the timer interrupts some other process, the scheduler will "return" to this process).
In case of priority scheduling who will pre-empt running process if the process with high priority arrives in ready queue?
SCHEDULER is an operating service routine whose function is to determine  which user process should run in the next time slice .

The scheduler is called after each timer interrupt (that is, quite a few times per second). It determines what process to run next based on a variety of factors, including priority, time already run, etc

So basically scheduler process never interrupt a running process Scheduler just select process which is in runnable state not blocked state.
Yes. But who will interrupt the running process?
except scheduler process all 3 option interrupt running process
Scheduler is like Boss who only decide which process will run next and assign the work of preemption to dispatcher.Dispatcher is like worker who will stop the current process do Context switch and all other work like saving the value of registers and all.Thus Scheduler itself never interrupt the running process.
My question is in case of high priority process arrives in the ready queue ,then who will interrupt the running process?I dont think from any of the three options interrupting the process,no one will interrupt in this particular case,so who will interrupt in case a high priority process arrives?
Irrespective of any case, schedular choose the next process and dispatcher will interrupt the current process and do preemption.
Ok got it.Thanks.:)
edited by
@reena everything is fine in your answer except the fact that how can you say that long term scheduler is scheduler only i think dispatcher is also scheduler(short term scheduler ) because scheduler have three types and we all know them .(short,medium,long)

Correct me if i am wrong?

according to key answer is scheduler processs

edited by
How will device interrupt a running process?
@rahul, Device can trigger interrupt at any time by sending a signal to the CPU,usually by way of system Bus.(HARDWARE INTERRUPT)
@ Sachin sir,

When the timer expires for currently executing process and the scheduler is scheduled, it decides which process to load that means get that process address , then it has to call dispatcher for actually loading that process so it now scheduler is off from cpu and dispatcher is in. The dispatcher will load and save register and PC values for new process to be loaded and hand over cpu to that new process.

In want to know as scheduler and dispatcher are also processes do they have there dedicated register set ??
please clarify separately what is scheduler & what is dispatcher ??
Nice explanation !!
No they don't have their dedicated register set.


Means when for scheduling decision scheduler is loaded by preempting process will not result in context switch actually..means lets say P1 running and only one in ready Q in RR and TQ expires then scheduler will be loaded to make decision and it will see only P1 hence do nothing no Context switch so time for this is very less..but still there will be SOME small time will be involved right ??

Short term scheduler selects which process to schedule next while dispatcher gives the control of cpu to the selected process.

@reena_kandari Does it mean dispatcher interrupts the running process, as and when a process of higher priority arrives in ready queue? I'm stuck at this point that what actually interrupts a running process when a process of higher priority comes?


@Saideepak Bejawada

The timer interrupt will invoke the scheduler and the scheduler will check if any higher priority process is there in the ready queue. That process will be selected for the CPU and the dispatcher will do the context switching and all that stuffs.

In the above question...How a device can interrupt  a running process?

The CPU cannot execute all of these processes simultaneously, so the operating system has to choose a particular process on the basis of the scheduling algorithm used. So, this procedure of selecting a process among various processes is done by the scheduler. Once the scheduler has selected a process from the queue, the dispatcher comes into the picture, and it is the dispatcher who takes that process from the ready queue and moves it into the running state. Therefore, the scheduler gives the dispatcher an ordered list of processes which the dispatcher moves to the CPU over time.

3 votes
Ans : None

A device can generate an Interrupt

Timer generates interrupt when time quantum is over in case of RR

Power failure will generate an interrupt

A scheduled process can also generate an interrupt in case of priority scheduling , when priority of scheduled process is more than Currently Running process


yes, agreed .

All can cause an interrupt.

A, B, D  -> Hardware interrupt.

C -> software interrupt.
edited by

@vijaycs option (C) is scheduler process (scheduler) not scheduled process in ACE past year booklet.

Please correct the question. Read this

according to this option D but i think all interrupt
so how to deal with this kind of thing ??
0 votes
A. Device will not interrupt a running process. When a device requests service, DMA will take over, so no interruption to the running process.
B. Timer obviously interrupts a running process.
C. Scheduler Process interrupts a running process and schedules some other process.
D. Power Failure definitely generates interrupt.
–4 votes
according to me option D is correct because power failure means shut down system its not interrupt generated


interrupt a running process means any kind of interruption...and during power failure whole sysytem fails..all the process inside it also if u were playing a game during power failure that will also stop..means a process interrupted...none of the option matches here
i think all interrupt generated so none of these

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