Page Fault occurs when

Virtual memory lets a process reference more address space than is physically resident by keeping only some pages in RAM at a time. Each page-table entry carries a valid/invalid (present) bit that records whether that page's frame currently holds the page's data.

When the CPU accesses an address whose page-table entry is marked not-present, the memory management unit raises a hardware trap. The operating system's page-fault handler then checks the page table, locates the page on secondary storage, allocates a free frame (evicting one if none is free), loads the page into it, updates the page-table entry, and resumes the faulting instruction. So the trigger for a page fault is that the referenced page's frame is not currently in RAM.

• "Page is in RAM" describes the normal case, where the memory reference is resolved directly from the existing mapping, with no trap and no fault.

• "Page is locked" describes a pinned, resident frame that the replacement algorithm cannot evict; the page remains present in memory, so locking by itself does not produce the automatic, hardware-detected trap that defines a page fault.

• "Page is swapped out manually" describes an administrative or scheduled operating-system action, not the automatic, hardware-detected condition that defines a page fault.