Two Level Page Table In Os

Pae and the virtual address bits supported by the processor current amd64 processors support up to 48 bit.

Two level page table in os.

The offset remains same in both the addresses. A logical address on 32 bit machine with 4k page size is divided into. A page number consisting of 20 bits. Each page table including the top one has only 1024 entries few enough to fit comfortably within a 4k page.

A multitasking os need to manage different memory maps for the different running applications so there can be several copies of the page tables. This is two level paging because here we got 2 page tables. If there is less memory installed the page tables can be smaller with invalid or unmapped entries in the first level page table. A simple technique is a two level page table.

This size is only needed for mapping the whole 4gb address space. One for each block of 2nd level page table. For example let us take that 20 bit page number and split it into two 10 bit indices. The cpu has two level paging and the logical and physical addresses are of 34 bits size each.

Of pages of the page table 2 outer page table 2 22 2 12 2 10 pages. It is also known as hierarchical paging. The page tables or page map levels are used to map each virtual page to a corresponding physical page zero or more virtual pages can correspond to the same physical page. To perform this task memory management unit needs a special kind of mapping which is done by page table.

We look up the first page table to find the second table then look up the second table to find the frame in which the page is stored. It converts the page number of the logical address to the frame number of the physical address. Page tables can be limited to one page more easily be paged out and multiple page faults possible. The page table stores all the frame numbers corresponding to the page numbers of the page table.

What s the size of a single page. A page offset consisting of 12 bits. The sizes of the page table directory the table directory and the page are equal. Prerequisite paging multilevel paging is a paging scheme which consist of two or more levels of page tables in a hierarchical manner.

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. The logical address 12345678 16 has been translated to the ba9678 16 physical 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. Multi level page tables.

Thus we can stop here.