Where Are Page Tables Stored?

Page tables require large memory

Page tables can get terribly large, much bigger than the small segment table or base/bounds pair we have discussed previously. For example, imagine a typical 32-bit address space, with 4KB pages. This virtual address splits into a 20-bit VPN and 12-bit offset (recall that 10 bits would be needed for a 1KB page size, and just add two more to get to 4KB).

A 20-bit VPN implies that there are $2^{20}$ translations that the OS would have to manage for each process (that’s roughly a million); assuming we need 4 bytes per page table entry (PTE) to hold the physical translation plus any other useful stuff, we get an immense 4MB of memory needed for each page table! That is pretty large. Now imagine there are 100 processes running: this means the OS would need 400MB of memory just for all those address translations! Even in the modern era, where machines have gigabytes of memory, it seems a little crazy to use a large chunk of it just for translations, no? And we won’t even think about how big such a page table would be for a 64-bit address space; that would be too gruesome and perhaps scare you off entirely.

Page tables are stored in memory

Because page tables are so big, we don’t keep any special on-chip hardware in the MMU to store the page table of the currently-running process. Instead, we store the page table for each process in memory somewhere. Let’s assume for now that the page tables live in physical memory that the OS manages; later we’ll see that much of OS memory itself can be virtualized, and thus page tables can be stored in OS virtual memory (and even swapped to disk), but that is too confusing right now, so we’ll ignore it. In the figure below is a picture of a page table in OS memory; see the tiny set of translations in there?