Consider a system that has a 30-bit address space and 512 byte pages (assume PTEs and PDEs are each 4 bytes). Why should we use more than two levels for the page table?

Question

Answer

Because the page table is too big to fit inside a single page, so we must add an additional page directory.

Answer

If we use a two-level page table, then the page directory doesn't fit on one page. In this case, we will still be able to save memory by only allocating pages of the page table with valid PTEs, but we have to allocate the entire page directory, even entire pages without valid PDEs. Therefore, it is best to use more levels (and in this case, a third level will be sufficient) so that we can save memory by only allocating the pages of the page directory with valid PDEs.

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There are too many bits of the 30-bit virtual address space left over from the VPN to just use it as a page table index, it must be broken into more pieces.

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We have 128 PTEs per page, which is too many to account for without a page directory.

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Consider a system that has a 30-bit address space and 512 byte pages (assume PTE...

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