in operating systems that are not single address space operating systems, address space or process id information is necessary so the virtual memory management system knows what pages to associate to what process. two processes may use two identical virtual addresses for different purposes. the page table must supply different virtual memory mappings for the two processes. this can be done by assigning the two processes distinct address map identifiers, or by using process ids. associating process ids with virtual memory pages can also aid in selection of pages to page out, as pages associated with inactive processes, particularly processes whose main code pages have been paged out, are less likely to be needed immediately than pages belonging to active processes.