Two Level Page Table In Os

A multitasking os need to manage different memory maps for the different running applications so there can be several copies of the page tables.

Two level page table in os.

Page tables can be limited to one page more easily be paged out and multiple page faults possible. The sizes of the page table directory the table directory and the page are equal. The size of a page depends on the processor mode protected compatibility or long mode the extensions used e g. If there is less memory installed the page tables can be smaller with invalid or unmapped entries in the first level page table.

Thus we can stop here. 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. The logical address 12345678 16 has been translated to the ba9678 16 physical address. This is two level paging because here we got 2 page tables.

A logical address on 32 bit machine with 4k page size is divided into. It is also known as hierarchical paging. One for each block of 2nd level page table. A page offset consisting of 12 bits.

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 page table stores all the frame numbers corresponding to the page numbers of the page table. Multi level page tables.

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. A page number consisting of 20 bits. Of pages of the page table 2 outer page table 2 22 2 12 2 10 pages. It converts the page number of the logical address to the frame number of the physical address.

Pae and the virtual address bits supported by the processor current amd64 processors support up to 48 bit. This size is only needed for mapping the whole 4gb address space. What s the size of a single page. Each page table including the top one has only 1024 entries few enough to fit comfortably within a 4k page.

The cpu has two level paging and the logical and physical addresses are of 34 bits size each. Thus here our outer page table page table 2 can be stored in one frame. The offset remains same in both the addresses. A simple technique is a two level page table.

So size of outer page table 2 10 4b 4kb.