STRUCTURE OF PAGE TABLE IN OPERATING SYSTEM
- 1. STRUCTURE OF THE PAGE TABLE
- 2. PAGE TABLE : • A PAGE TABLE is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses.
- 4. Common Techniques used for structuring the page table are : • Hierarchical paging • Hashed page tables • Inverted page tables
- 5. Hierarchical paging: • It is also known as multilevel paging. • The page table might be too big to fit in a contiguous space , so we may have a hierarchy with several levels. • So , we break up the logical address space into multiple page tables. • For this a simple techniques we can use are: Two level page table Three level page table
- 6. Two level paging: • A logical address (on 32-bit machine with 4k page size) is divided into: A page number consisting of 20 bits. A page offset consisting of 12 bits. • Since the page table is paged , the page number is further divided into: A 10-bit page number. A 10-bit page offset. • Thus a logical address is as follows: page number | page offset 10 10 12 pi p2 d
- 7. Address translation scheme for the two level paging is as follows:
- 8. Three level paging: • A logical address(on 64-bit machine with 4k page size) is divided into: outer page inner page offset 42 10 12 2nd outer page outer page inner page offset 32 10 10 12 Pi P2 d Pi P2 P3 d
- 9. Hashed page tables: • It is a common approach used when address space is >32 bits. • The virtual page number is hashed into a page table . This page table contains a chain of elements hashing to the same location. • Each element consists of three fields: 1. The virtual page number 2. The value of the mapped page frame 3. A pointer to the next element in the linked list.
- 10. Address translation scheme for the hashed page table is as follows:
- 11. Inverted page tables: • The inverted page table combines A page table and A frame table into one data structure. • One entry for each virtual page number & real page of memory. • Entry consists of the virtual address of the page stored in that real memory location , with information about the process that owns that page. • Decreases memory needed to store each page table , but increases time needed to search the table when a page reference occurs.
- 12. Address translation scheme for the Inverted page table is as follows: