Use of modify bit and a present bit in page table

Present Bit:

The present bit says whether a particular page you are looking for is present. In case if it is not present, that is called Page Fault. It is set to 0 if the corresponding page is not in memory. Used to control page fault by the operating system to support virtual memory. Sometimes this bit is also known as valid bits.

Modify Bit:

Modified bit says whether the page has been modified or not. Modified means sometimes you might try to write something onto the page. If a page is modified, then whenever you should replace that page with some other page, then the modified information should be kept on the hard disk or it has to be written back or it has to be saved back. It is set to 1 by hardware on write-access to page which is used to avoid writing when swapped out. Sometimes this modified bit is also called the Dirty bit.