GATE CSE 2009 | Question: 8, UGCNET-June2012-III: 58

Question

A CPU generally handles an interrupt by executing an interrupt service routine:

• A. As soon as an interrupt is raised.

• B. By checking the interrupt register at the end of fetch cycle.

• C. By checking the interrupt register after finishing the execution of the current instruction.

• D. By checking the interrupt register at fixed time intervals.

Answer 1

It will be (C).

After finishing the execution of each instruction the CPU reads the interrupt pins to recognize the interrupts.

INTR $= 1 =$ Interrupt is present.(Service the Interrupt)

         $= 0 =$ Interrupt is not present.(Goto next Instruction fetch from user program)

Comments

@Overflow04 bro , Option A does not make any sense because the CPU will always finish execution of current execution and then it will check the interuppt register if any interuppt has been raised or not . 

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Does a will be true in any type of interrupt question.

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obviously yes , when CPU is idle :) 😉

and also in cases of interrupts caused by exceptions the  CPU can stop execution of the current instruction mid way and service the interrupt first.

For example , A user process trying to write to other processes memory then it will stop the execution midway

Answer 2

Answer: (C)

Explanation: Hardware detects interrupt immediately, but CPU acts only after its current instruction. This is followed to ensure integrity of instructions.

Answer 3

I think it will act differently for hardware and software interrupts for hardware interrupts are the urgent or given the higher priority but we're as the software interrupts are given less priority so it d pends on the interrupt if it is hardware it is immediately addressed but we're as the software interrupts it can wait  

 

According to me if it is an hardware interrupt option a is correct

Software interrupts remaining three are applicable na all the 3 are methods to address the the interrupts irrespective of there performance and feasibility

 if I take for software interrupts ABC all options are ways to handle an interrupt request how are u chooseing that one of the option is best but if we look all options are ways to handle a request

 Integrity is maintained in every option (except option a because it may lead to error as it executes only half instruction it may lead to errors

: I wanted to know why did you think that option c is more appropriate (Iam not able to figure out how to answer this question because every option seems to be valid for me and how what is generally mean in the question it depends on the type of interrupt as I said hardware and software )

Comments

i'm not satisfied with the level of explanation of the answer , can someone explain it with shedding light on all options ?

Answer 4

CPU after completing the execution of a instruction checks the status of interrupt if there is a interrupt then CPU executes the interrupt service routine to service it else CPU stats executing next instruction.