Page fault

A page fault (English page fault ) occurs in operating systems with virtual memory management and paging when a program accesses a memory area that just is not in main memory, but for example, was swapped to the hard drive or when to the relevant address currently no description entry is available in the MMU. As a direct consequence of the page fault, there is a synchronous program interruption (English: trap ). The operating system then ensures that the requested memory area is loaded into the main memory or the lack of MMU entry is reloaded, so that the program can access it. A page fault is therefore not an actual error. The user feels this process of nothing more than a slow-down of the program that caused the page fault, because the page loading or editing the process requires a brief moment. Other programs or processes are not affected.

The page fault is not to be confused with the segmentation fault, which is a protection violation when accessing non-existent or protected memory areas.

Soft Page Fault

The Soft Page Fault behaves similarly to the page fault and occurs in the following cases:

• The process tries to write to the page, the read-only bit in the PTE of the MMU and the copy- on-write bit in the memory management of the operating system for this page are set.
• The page is in memory, but does not belong to the working set of the process but is, for example, a part of the free page list.
• The process calls for a new blank page.