Past exam of the mathematics course of the University of Cambridge / 2006 / ia / Paper 5 / 7 / a / ii / Solution

Emulate reference observations using page table protection and page faults. When resetting a page's reference state, temporarily revoke access to its resident mapping and invalidate any corresponding translation lookaside buffer entry. The first subsequent access traps. The handler recognizes an intentionally protected resident page, sets a software reference flag and restores the mapping without performing a disk read. A later CLOCK page replacement scan can use and clear this software flag.

One protection fault records the first access after each reset, rather than trapping on every access. All mappings that can access the frame must be considered. The method requires ordinary memory-protection traps, even though hardware reference bits are absent, and adds a fault cost that native reference flags avoid.