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→Page Directory: Bits 6 and 8 are available if PS=0
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* P, or ''''P'''resent'. If the bit is set, the page is actually in physical memory at the moment. For example, when a page is swapped out, it is not in physical memory and therefore not 'Present'. If a page is called, but not present, a page fault will occur, and the OS should handle it. (See below.)
The remaining bits 9 through 11 (if PS=0, also bits 6 & 8) are not used by the processor, and are free for the OS to store some of its own accounting information. In addition, when P is not set, the processor ignores the rest of the entry and you can use all remaining 31 bits for extra information, like recording where the page has ended up in swap space.
Setting the PS bit makes the page directory entry point directly to a 4-MiB page. There is no paging table involved in the address translation.
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