Paging: Difference between revisions

A, or 'Accessed' is used to discover whether a page has been read or written to. If it has, then the bit is set, otherwise, it is not. Note that, this bit will not be cleared by the CPU, so that burden falls on the OS. (ie. if it needs this bit at all.)

 

 

W, the controls 'Write-Through' abilities of the page. If the bit is set, write-through caching is enabled. If not, then write-back is enabled instead.

U, the user\supervisor bit, controls access to the page based on privilege level. If the bit is set, then the page may be accessed by all; if the bit is not set, however, only the supervisor can access it.

 

 

P, or 'Presence', determines if the page is actually in physical memory at the moment. (eg. if a page only exists on the hard drive, it is not in physical memory.) If a page is called, but not present, a page fault will occur, and the OS should handle it. (See below.)