Demand Paging
Demand Paging refers to a technique where program pages are loaded from disk into memory as they are referenced.
Each reference to a page not previously touched causes a page fault.
The fault occurs because the reference found a page table entry with its valid bit off.
As a result of the page fault, the OS allocates a new page frame and reads the faulted page from the disk.
When the I/O completes, the OS fills in the PTE, sets its valid bit, and restarts the faulting process.