- Recent computer system support a large logical address space ,the page table becomes large.
- It is difficult to allocate contiguous main memory for page table.
- To solve this problem,Break up the logical address space into multiple page tables
- A simple technique is a two-level page table
- In this scheme the page table is divided into number of smaller pieces.
- In this two level paging ,the page table itself is also paged.
Two level paging
- A logical address (on 32-bit machine with 1K page size) is divided into:
- a page number consisting of 22 bits
- a page offset consisting of 10 bits
- Since the page table is paged, the page number is further divided into:
- a 12-bit page number
- a 10-bit page offset
- Thus, a logical address is as follows:
pi | p2 | d
12 10 10
- where pi is an index into the outer page table, and p2 is the displacement within the page of the outer page table
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