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Honestly, I am really confused with this particular virtual memory related concept.

Q1) When a page fault occurs, does the processor first finishes the execution of the current instruction and then moves the IP register contents (address of next instruction) to the stack? Or, it aborts current instruction being executed and moves the contents of instruction pointer register to stack?

Q2) If the second case is true, then how does it resume the instruction which was aborted because when if it resumes, the stack contains the instruction pointer value which is nothing but the address of the next instruction. So it will never resume the instruction where the page fault occurred.

What I think
I think the second case sounds wrong. The confusion occurred while i was reading Operating System Principles by Silbershatz and Galvin. In that they have written

when a page fault occurs, we will have to bring in the desired page, correct page table and restart the instruction.

But the instruction pointer always points to the address of the next instruction so it means, according to what this book is trying to convey, we are decrementing the value of IP just to restart the execution of the instruction where the page fault occurred?

Bhavyanshu
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  • So how would it complete the instruction that generated the page fault? The page is not there. It cannot happen. – harold Nov 24 '14 at 21:34
  • @harold Okay, so you are saying that we abort execution and decrement IP value just to restart after handling page fault trap? Because as far as I know normally the interrupts are ignored until the execution of current instruction is completed. Once it is completed, then the context switching takes place. – Bhavyanshu Nov 24 '14 at 21:37
  • Well let me quote the manual on that. This is for x86, right? There might be other solutions, particularly on machines where it is possible to decrement the instruction pointer (on x86, stepping back can be ambiguous) – harold Nov 24 '14 at 21:41
  • Okay, but can you please help me understand on how it is done on x86 if decrementing is not the right way? Probably point me to some online resource that you know of? – Bhavyanshu Nov 24 '14 at 21:46

1 Answers1

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In the Intel System Programming guide, chapter 6.5, it says

Faults — A fault is an exception that can generally be corrected and that, once corrected, allows the program to be restarted with no loss of continuity. When a fault is reported, the processor restores the machine state to the state prior to the beginning of execution of the faulting instruction. The return address (saved contents of the CS and EIP registers) for the fault handler points to the faulting instruction, rather than to the instruction following the faulting instruction.

A page fault is classified as a fault (no surprises there), so when a page fault happened you're in the state "before it ever happened" - well not really, because you're in the fault handler (so EIP and ESP are definitely different, also CR2 contains the address), but when you return it'll be the state before the ever happened, only with changes made by the handler (so, put there page there, or kill the process)

harold
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  • Ok so I was making a mistake thinking it in terms of how normally interrupts are handled? Hm this makes much sense now. Thanks a lot! – Bhavyanshu Nov 24 '14 at 21:51