Two Level Page Table In Os

Each page table including the top one has only 1024 entries few enough to fit comfortably within a 4k page.

Two level page table in os.

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. For example let us take that 20 bit page number and split it into two 10 bit indices. Prerequisite paging multilevel paging is a paging scheme which consist of two or more levels of page tables in a hierarchical manner. It is also known as hierarchical paging.

A multitasking os need to manage different memory maps for the different running applications so there can be several copies of the page tables. A page offset consisting of 12 bits. A page number consisting of 20 bits. Thus we can stop here.

Of pages of the page table 2 outer page table 2 22 2 12 2 10 pages. The offset remains same in both the addresses. The cpu has two level paging and the logical and physical addresses are of 34 bits size each. 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. This size is only needed for mapping the whole 4gb address space. A simple technique is a two level page table. Thus here our outer page table page table 2 can be stored in one frame.

Pae and the virtual address bits supported by the processor current amd64 processors support up to 48 bit. The logical address 12345678 16 has been translated to the ba9678 16 physical address. 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.

If there is less memory installed the page tables can be smaller with invalid or unmapped entries in the first level page table. What s the size of a single page. Multi level page tables. It converts the page number of the logical address to the frame number of the physical address.

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. 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. This is two level paging because here we got 2 page tables. 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.