GATE CSE 2018 | Question: 9

Question

The following are some events that occur after a device controller issues an interrupt while process $L$ is under execution.

• P. The processor pushes the process status of $L$ onto the control stack
• Q. The processor finishes the execution of the current instruction
• R. The processor executes the interrupt service routine
• S. The processor pops the process status of $L$ from the control stack
• T. The processor loads the new PC value based on the interrupt

Which of the following is the correct order in which the events above occur?

• A. QPTRS
• B. PTRSQ
• C. TRPQS
• D. QTPRS

Comments

 it is not always the case that the processor always completes the execution of current program and then service the interrupt. 

You must mean "current instruction", Not "current program".

So, in that case, yes, you are right.

It is not always the case that the processor always completes the execution of current instruction and then service the interrupt.  

I found the following in a book "The HCS12 / 9S12: An Introduction to Software and Hardware Interfacing By Han-Way Huang". Read the last paragraph in the following image:

Image Source

So, your doubt is valid and good But for the given question it seems that answer will be Option A because NMIs are rare.

A Non-Maskable Interrupt (NMI) is a type of hardware interrupt that cannot be ignored (or "masked") by the processor. These interrupts are typically reserved for high-priority tasks that require immediate attention, such as critical hardware failures.

In most cases, NMIs (Nonmaskable Interrupts) are infrequent and only used when a system needs to handle emergencies that cannot wait for normal interrupt processing.

There is No reason to consider NMIs for this question, as NMIs are rare. If the question explicitely talks about NMI then we should consider it, else the normal interrupt processing must be considered. 

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Can someone please address , What is the difference between control stack and program stack? Also In OS it it mentioned that PC value and PSW is stored in PCB of a process, so is storing in control stack is equivalent of storing in PCB? 

@arjun sir, @Deepakk Poonia (Dee) sir.

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its crazy how often this question is repeated

Answer 1

Answer should be A.

Comments

@Debapaul

It is not always the case that the processor always completes the execution of current instruction and then service the interrupt.  

I found the following in a book "The HCS12 / 9S12: An Introduction to Software and Hardware Interfacing By Han-Way Huang". Read the last paragraph:

Image Source

A Non-Maskable Interrupt (NMI) is a type of hardware interrupt that cannot be ignored (or "masked") by the processor. These interrupts are typically reserved for high-priority tasks that require immediate attention, such as critical hardware failures.

In most cases, NMIs (Nonmaskable Interrupts) are infrequent and only used when a system needs to handle emergencies that cannot wait for normal interrupt processing.

There is No reason to consider NMIs for this question, as NMIs are rare. If the question explicitely talks about NMI then we should consider it, else the normal interrupt processing must be considered. 

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@Deepakk Poonia (Dee)

As per my understanding, in order to store the status of the instruction, we will have to make sure the current instruction is completely executed first. Please correct me if I'm wrong.

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From all the "standard" resources that I have gone through it seems like it is not the case Necessarily. It might be done most of the times. Refer the image that I have shared.

Moreover, It is not this easy. What do we do to the pipelined instructions which have already executed partially??

In pipeline, at a time multiple instructions are in partially executed state.

https://stackoverflow.com/questions/8902132/when-an-interrupt-occurs-what-happens-to-instructions-in-the-pipeline

Answer 2

the answer should be A  not D because we should first store the process status info into the stack and then load the new value of PC into the program counter as the process status includes the PC of the process as well... if we directly overwrite the new value of PC into the PC register we will lose the old value  hence we should copy it into the stack first..

Comments

Even I have the same query, Please clear my doubts. Please.

Answer 3

It should be $A$

Answer 4

The sequence of execution in the case of an interrupt while executing a process L is:
(Q) The processor finishes the execution of the current instruction
(P) The processor pushes the process status of L onto the control stack
(T) The processor loads the new PC value based on the interrupt
(R) The processor executes the interrupt service routine
(S) The processor pops the process status of L from the control stack
This is the sequence because when a process is in the middle of execution if an interrupt comes then that process execution is completed then the interrupt is serviced.