For reading, most operating systems use demand paging. This means that pages are only read from the disk into physical memory when they are needed. In the page table, there is a resident status bit, which says whether or not a valid page resides in physical memory. If the MMU tries to get a physical page number for a valid page which is not resident in physical memory, it issues a pagefault to the operating system. The OS then loads that page from disk, and then returns to the MMU to ?nish the translation.
In addition, many operating systems make some use of pre-fetching, which is called pre-paging when used for pages. The OS guesses which page will be needed next, and begins loading it in the background, to avoid future pagefaults. This depends heavily on locality of accesses, namely that future accesses will be near recent accesses, and this often true.