n Most computer systems support a very large virtual address space u 32 to 64 bits are used for logical addresses u If (only) 32 bits are used with 4KB pages, a page table may have 2^{20} entries n The entire page table may take up too much main memory. Hence, page tables are often also stored in virtual memory and subjected to paging u When a process is running, part of its page table must be in main memory (including the page table entry of the currently executing page)