In a computer system that employs virtual memory, multiple versions of a
given page are stored: a directory version, a table version, and a data
version. The data version contains the data that a software object
believes to be stored in the page. The directory and table versions of
the page contains versions of the page's contents that have been modified
in some manner to comply with a restriction on the address translation
map employed by the virtual address system. When a page is being used by
the virtual address system as a directory or table, then the directory or
table versions, respectively, of that page are used. When a page is the
target of a read request, the data version of the page is used.