Where the program counter goes after returning the interrupt handler?

3
votes

Hi I am wondering where the program counter goes when the program returns from interrupt service routine? I know when the interrupt event occurs the PC is pushed into the stack. But what address is pushed into the stack the next one or the same one(just executed one)?When we have 

first instruction;
interrupt event here;
go inside the ISR;
exit ISR;
second instruction;

OR

first instruction;
interrupt event here;
go inside the ISR
exit ISR;
first instruction;

I mean, does the PC points to the instruction executed prior to the ISR entrance, or it goes to the next instruction(after return from the ISR)? I hope you get the idea.

4
assemblyinterruptprogram-counter
What does this have to do with C? – OldProgrammer
The CPU will push the return address of the next instruction that should be executed after the function call. Or if you will: push the program counter and increment it by one instruction. – Lundin
It pretty much depends on the specific interrupt/exception. E.g: a trap/syscall/software interrupt will obviously push the address of the next instruction, but a page fault/TLB miss exception will push the address of the offending instruction so it can be retried. – ninjalj
Another case where the interrupt returns to the first instruction can happen on a X86 for repeat instruction sequences, like | rep movsb |, but this is because PC is not advanced until the repeat instruction sequence has completed. The interrupt will take place after the current iteration of a repeat sequence completes (registers are updated before the interrupt takes place). – rcgldr

4 Answers

4
votes

Since an interrupt can't be handled until the CPU is in a precise fixed state, if the interrupt is raised in the middle of an instruction then the jump to the interrupt vector procedure will occur after the execution of the instruction.

So when coming back from the interrupt procedure the PC will be pointing to the instruction after the first.

first instruction fetch (PC is updated meanwhile)
first instruction decode
interrupt is raised
first instruction execution
** now and only now the CPU checks for a possible interrupt **
interrupt is present, pushing PC on stack and other things
jump to interrupt address
execution of interrupt
return from interrupt (pop of PC and other things)
second instruction fetch 
...
2
votes

When an instruction is executing then the program counter keeps the address of the next instruction to be executed. When an interrupt occurs, then processor do the following:

  • Suspends the execution of the program being executed and save its context. This means it saves the address of the next instruction to be executed, i.e, the value of the program counter and other relevant data.

  • Update the program counter with the starting address of that interrupt handler routine.

When the interrupt handler routine is completed, CPU can resume execution of the program at the point of interruption.

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Interrupt is occurred at instruction i and after completion user program resumes its execution from i+1 instruction.

1
votes

Exact behaviour of interrupts are hardware-specific, but the CPU will simply wait until first_instruction is finished. After that, it will push CPU state onto stack (or save it in other way) and start ISR. That means that your ISR will not be executed immediately - there is a tiny delay, which may become an issue in hard realtime applications.

1
votes

There are 2 types of interrupts : a) Software interrupt- caused due to some critical problems like division by zero during execution of an instruction (say i th instruction) in a program(say program is Divide 2 number).

How is this handled by CPU?

Very much like an exception in Java. In this case the interrupt request is immediately handled(current i th instruction is not finished). current PC value( pointing to address of i+1 th instruction) is saved in some location. Interrupt is processed and after serving interrupt,it returns for of execution division program to finish i+1 th instruction and rest of instructions.

b) Hardware interrupt - processor is running a program when some input arrived from a keyboard (for example) or some other hardware.

How is this handled by CPU?

In this case CPU doesn't immediately serve interrupt request .It first completes the execution of current i th instruction, saves PC current value (pointing to address of i+1 th instruction) in some location . Then it listens to that interrupt, finish it and later comes back to old program instruction i+1 .