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.

If there is less memory installed the page tables can be smaller with invalid or unmapped entries in the first level page table. The offset remains same in both the addresses. For example let us take that 20 bit page number and split it into two 10 bit indices. This size is only needed for mapping the whole 4gb address space.

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 is also known as hierarchical paging. Each page table including the top one has only 1024 entries few enough to fit comfortably within a 4k page. The sizes of the page table directory the table directory and the page are equal.

This is two level paging because here we got 2 page tables. Page tables can be limited to one page more easily be paged out and multiple page faults possible. Multi level page tables. Thus we can stop here.

A multitasking os need to manage different memory maps for the different running applications so there can be several copies of the page tables. Thus here our outer page table page table 2 can be stored in one frame. It converts the page number of the logical address to the frame number of the physical address. A page number consisting of 20 bits.

What s the size of a single page. The size of a page depends on the processor mode protected compatibility or long mode the extensions used e g. So size of outer page table 2 10 4b 4kb. A page offset consisting of 12 bits.

One for each block of 2nd level page table. To perform this task memory management unit needs a special kind of mapping which is done by page table. 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 page table stores all the frame numbers corresponding to the page numbers of the page table.

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. Of pages of the page table 2 outer page table 2 22 2 12 2 10 pages. The logical address 12345678 16 has been translated to the ba9678 16 physical address. The cpu has two level paging and the logical and physical addresses are of 34 bits size each.

Prerequisite paging multilevel paging is a paging scheme which consist of two or more levels of page tables in a hierarchical manner.