The inverted page table keeps the size of the page table reasonable. Note, too, that there is only one page
table for the whole system??”not one page table per process. The only problem is that searching through the
page table for a match on process ID and logical page number can require many memory references itself.
Once again, the TLB delivers a workable solution. Programs still show strong locality, so the TLB continues to
resolve most address lookups at high speed. TLB misses do require a lengthy search, but engineering means
making sensible tradeoffs.
Finally on the subject of page tables, page table entries have additional status bits associated with them.
Earlier we mentioned the valid bit, used for signaling whether the page was in memory or not. In addition, read
and write permission bits allow the system to provide protection to the contents of memory, page by page.
There are also bits to indicate whether the page has been accessed and whether it has been modified. When
it comes time to replace the mapping of one page with another, the accessed bit allows the OS to replace a page
that isn??™t being accessed very often instead of one which is.
The modified bit also helps during page replacement decisions.
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