Two Level Page Table In Os

A page offset consisting of 12 bits.

Two level page table in os. This size is only needed for mapping the whole 4gb address space. Multi level page tables. So size of outer page table 2 10 4b 4kb. It converts the page number of the logical address to the frame number of the physical address.

A logical address on 32 bit machine with 4k page size is divided into. Pae and the virtual address bits supported by the processor current amd64 processors support up to 48 bit. If there is less memory installed the page tables can be smaller with invalid or unmapped entries in the first level page table. Each page table including the top one has only 1024 entries few enough to fit comfortably within a 4k page.

Thus here our outer page table page table 2 can be stored in one frame. A multitasking os need to manage different memory maps for the different running applications so there can be several copies of the page tables. The size of a page depends on the processor mode protected compatibility or long mode the extensions used e g. A page number consisting of 20 bits.

The offset remains same in both the addresses. Prerequisite paging multilevel paging is a paging scheme which consist of two or more levels of page tables in a hierarchical manner. 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. 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 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. One for each block of 2nd level page table. Of pages of the page table 2 outer page table 2 22 2 12 2 10 pages.

Page tables can be limited to one page more easily be paged out and multiple page faults possible. 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. The logical address 12345678 16 has been translated to the ba9678 16 physical address.

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. The sizes of the page table directory the table directory and the page are equal. What s the size of a single page.

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