The role of individual bits in page table entries (page descriptors)

The role of individual bits in page table entries (page descriptors):
1. Page number
2. Block Number (page box number)
3. Interrupt bit: Used to determine whether the page is in memory, if it is 0, indicates that the page is not in memory, it will cause a missing pages interrupt
4. Protection bit (access control bit): Used to indicate what type of access the page allows, if one is used to identify it: 1 is read-only and 0 is read-write
5. Modify bits (Dirty bit): For the page swap out, if a page has been modified (that is, dirty), when the page is retired, it must be written back to disk, conversely, you can directly discard the page
6. Access bit: Both Read and write (get or set), the system sets the access bit of the page, and its value is used to help the operating system to select the page to be retired when a fault is broken, that is, for page replacement
7. Cache Disable bit (secondary address bit): Useful for pages that map to device registers instead of regular memory, it is important to assume that the operating system is looping through an I/O device to respond to its instructions, and that it is necessary to keep the hardware constantly reading data from the device rather than accessing a copy in an old cache. That is, for page entry.