The hardware that detects this situation is the memory management unit in a processor. The exception handling software that handles the page fault is generally part of an operating system. The operating system tries to handle the page fault by making the required page accessible at a location in physical memory or kills the program in case it is an illegal access.
Contrary to what their name might suggest, page faults are not necessarily fatal and are common and necessary to increase the amount of memory available to programs in any operating system that utilizes virtual memory, including Microsoft Windows, Mac OS X, Linux and Unix.
The closely related exception known as the protection fault is generated for page accesses where:
Protection fault can also be generated for many other invalid accesses not related to paging.
On the x86 architecture, accesses to pages that are not present and accesses to pages that do not conform to the permission attributes for a given page (protection faults as described above) are both reported via the page fault processor exception. Internally, the processor hardware provides information to the page fault handler that indicates what sort of access triggered the fault, so that these scenarios may be differentiated from the perspective of the operating system. The usage of the term protection fault (when speaking in relation to page faults) is thus not to be confused with the general protection fault exception, which is used to signal segmentation-based memory access violations, as well as a variety of other general protection related violations (such as the use of an instruction that is not valid at the current privilege level).
|Page Mapped||Page Loaded||Page Status||Page access valid||Exception|
|yes||--||not present||yes||Page fault|
A program trying to access memory that belongs to another process is attempting 'illegal access' which generates a protection fault. The operating system handles this protection fault much like an invalid page fault: The operating system temporarily blocks 'write permission' until a page fault is generated, then creates new access information to allow the program to successfully find this memory.
Note that in terms of the x86 architecture, memory protection violations reported via the page fault mechanism (protection faults, as described in this article) are distinct from memory protection violations reported via the general protection fault mechanism. The general protection fault mechanism is used to report segmentation-based memory access violations as well as other classes of protection violations. The page fault mechanism is used to report paging-based memory protection violations.
Operating systems such as Windows and UNIX (and other UNIX-like systems) provide differing mechanisms for reporting errors caused by page faults. Windows uses structured exception handling to report page fault-based invalid accesses as access violation exceptions, and UNIX (and UNIX-like) systems typically use signals, such as SIGSEGV, to report these error conditions to programs.
If the program that received the error does not handle it, then the operating system typically performs some sort of default action, typically involving the termination of the running process that caused the error condition, and a notification to the user that the program has malfunctioned. In this vein, recent versions of Windows often report such problems with less technical error messages simply stating something along the lines of "this program must close" (an experienced user or programmer with access to a debugger can still retrieve detailed information, if necessary). Additionally, recent Windows versions also write a minidump (similar in principle to a core dump) describing the state of the crashed process for later analysis alongside such less-technical error messages. UNIX and UNIX-like operating systems typically report these conditions to the user with error messages such as "segmentation violation", or "bus error".
How much does a page fault reduce the performance of your computer? An average hard disk has an average latency of 3ms, seek-time of 5ms and a transfer-time of 0.05ms/page. So the total time for paging comes in near 8ms (8 000 000 ns). If the memory access time is 200ns, then the page fault would make the system about 40000 times slower.
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