The Midterm Solution - Suraj @ LUMS - Lahore University of ...

29.[10 marks]SP(StackPointer)Consider that an interrupt takes place when themicroprocessor is executing an instruction stored atlocation N. The status flags at this instant contain thevalue ABCDh. The corresponding interrupt serviceroutine (ISR) is stored in memory starting from location Xand X + C (both inclusive)a) What is the address of the instruction that will beexecuted after the interrupt occurs. [2]XXstartb) Where will the program branch to after the completionof the interrupt? [2]N + 1c) How will the microprocessor know where to branchafter the completion of the interrupt? [3]X+C endPrior to branching to the ISR the microprocessorstores the address of the current value of theInstruction pointer onto the stack. On completion ofthe subroutine it pops this address off the stack andloads this into the instruction pointer and thusbranches back to the correct location.UserProgramNMAINMEMORYd) What will happen if a second interrupt takes placewhile the first one is being serviced? [3]If a second interrupt took place, then if it were ofhigher priority and the interrupt process was enabledthe microprocessor would once again push the currentcontext onto the stack and branch to the new ISR andon completing it branch back and complete the firstISR by popping off the context from the stack. If theinterrupt process was disabled or if the new interruptwas of a lower or the same priority as the currentinterrupt it would remain pending.Page 16 of 16