Page fault is a trap to the software gained by the hardware when a process accesses a page that is goes in the virtual address space, but not added in physical memory. In the hard case the operating system tries to operate the page fault by creating the required page accessible at a location in physical memory disk or kills the process in the case of an illegal access. The system that detects a page fault is the memory management unit in a processor. The exception handling software that operates the page fault is usually part of the operating system.
Contrary to what the name 'page fault' might suggest, page faults are not errors and are necessary and common to raise the amount of memory available to programs in any operating system that use virtual memory, including Unix-like, Microsoft Windows systems, and z/OS. Microsoft access the term hard fault in more recent parts of the source Monitor to mean 'page fault'