What is "page fault"

n. (context computing English) An alert (such as an interrupt or exception) indicating that a page of memory was accessed without being loaded.

A page fault (sometimes called #PF, PF or hard fault) is a type of interrupt, called trap, raised by computer hardware when a running program accesses a memory page that is mapped into the virtual address space, but not actually loaded into main memory. The hardware that detects a page fault is the processor's memory management unit (MMU), while the exception handling software that handles page faults is generally a part of the operating system kernel. When handling a page fault, the operating system generally tries to make the required page accessible at the location in physical memory, or terminates the program in case of an illegal memory access.

Contrary to what "fault" might suggest, valid page faults are not errors, and are common and necessary to increase the amount of memory available to programs in any operating system that utilizes virtual memory, including OpenVMS, Microsoft Windows, Unix-like systems (including Mac OS X, Linux, *BSD, Solaris, AIX, and HP-UX), and z/OS.