This articl addresses at one of the most common problems when dealing with virtual memory – the Page Fault. A page fault occurs when a program requests an address on a page that is not in the current set of memory resident pages. What happens when a page fault occurs is that the thread that experienced the page fault is put into a Wait state while the operating system finds the specific page on disk and restores it to physical memory.
When a thread attempts to reference a nonresident memory page, a hardware interrupt occurs that halts the executing program. The instruction that referenced the page fails and generates an addressing exception that generates an interrupt. There is an Interrupt Service Routine that gains control at this point and determines that the address is valid, but that the page is not resident. The OS then locates a copy of the desired page on the page file, and copies the page from disk into a free page in RAM. Once the copy has completed successfully, the OS allows the program thread to continue on. One quick note here – if the program accesses an invalid memory location due to a logic error an addressing exception similar to a page fault occurs. The same hardware interrupt is raised. It is up to the Memory Manager’s Interrupt Service Routine that gets control to distinguish between the two situations.