Two Level Page Table Scheme

A page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses virtual addresses are used by the program executed by the accessing process while physical addresses are used by the hardware or more specifically by the ram subsystem.

Two level page table scheme.

Even two level paging scheme not sufficient if page size is 4 kb 212 then page table has 252 entries if two level scheme inner page tables could be 210 4 byte entries address would look like outer page table has 242 entries or 244 bytes one solution is to add a 2nd outer page table. Roughly sketch out the format of a complete page table. Of pages of the page table 2 outer page table 2 22 2 12 2 10 pages. 4 kilobytes 2 12 bytes which means we need 12 bits to address a specific byte within the page.

So size of outer page table 2 10 4b 4kb. The page table is a key component of virtual address. The entries of the level 1 page table are pointers to a level 2 page table and entries of the level 2 page tables are pointers to a level 3 page table and so on. This is two level paging because here we got 2 page tables.

Prerequisite paging multilevel paging is a paging scheme which consist of two or more levels of page tables in a hierarchical manner. Consider a two level paging scheme. 220 descriptors 1 descriptor for each virtual page blocked into 2 10 blocks of 2 descriptors each 0 1 220 1 210 descriptors per block page of the page table 210 such blocks pages of the page table 210 entries. It is also known as hierarchical paging.

If so we could repeat this process by paging the top level page table thus introducing another layer of page table. To look up an address in a hierarchical paging scheme we use the first 10 bits to index into the top level page. What is the format of a 32 bit virtual address. The size of outer page table is 4 kb.

Thus here our outer page table page table 2 can be stored in one frame. Page tables caches and tlbs page tables suppose that we have a two level page translation scheme with 4k byte pages and 4 byte page table entries. Finding data with hierarchical paging. Thus we can stop here.

With different numbers we could have a very large top level page table. Outer page table 2 is required to keep track of the frames storing the pages of outer page table 1.