How do the reference and modified bits of the page table entry affect paging?

Question

I am reading up on paging and memory management unit on wikipedia. How does reference and modified bit of the page table entry affects the operation of paging?

en.wikipedia.org/wiki/Paging http://en.wikipedia.org/wiki/Memory_management_unit http://wiki.osdev.org/Paging

Discovert

Answer 1

I'm assuming you are referring the accessed and dirty bits. These bits solely exist to aid the implementation of a memory manager and do not affect the working of the MMU.

From Intel's manual, Volume 3: System Programming, Section 3.6.4 (I've compacted two points into one since they are nearly identical, italic text holds only for the dirty bit):

Accessed (A) flag, bit 5 - Dirty (D) flag, bit 6

Indicates whether a page (or page table) has been accessed (written to) when set. (This flag is not used in page-directory entries that point to page tables.) Memory management software typically clears this flag when a page or page table is initially loaded into physical memory. The processor then sets this flag the first time a page (or page table) is accessed (for a write operation). This flag is a “sticky” flag, meaning that once set, the processor does not implicitly clear it. Only software can clear this flag. The accessed and dirty flags are provided for use by memory management software to manage the transfer of pages and page tables into and out of physical memory.

Pastern

Nice answer for a pretty old question. We could use more of these kinds of answers for old, unloved questions. – Cheree

@Job I am not able to understand Indicates whether a page (or page table) has been accessed (written to) when set. How a page can be set without being accessed? – Elwoodelwyn