The modified bit indicates whether the contents of the page have been altered and, if so,
the page must be rewritten to secondary storage when it is swapped out before its page
frame is released. (A page frame with contents that have not been modified can be overwritten
directly, thereby saving a step.) That is because when a page is swapped into
memory it isn’t removed from secondary storage. The page is merely copied—the original
remains intact in secondary storage. Therefore, if the page isn’t altered while it is in
main memory (in which case the modified bit remains unchanged, zero), the page needn’t
be copied back to secondary storage when it is swapped out of memory—the page
that is already there is correct. However, if modifications were made to the page, the new
version of the page must be written over the older version—and that takes time.