CODEWITHSUNDEEP
assemblymemory-managementx86paging
St.Antario
St.Antario

Reputation: 27455

Understanding page directories

I read some manuals about page directories and page tables and still very confused about these concepts. For example here I found that making only a single level paging is wasting memory. So in x86 architecture we use 2-level paging.

cr3 --> PageDirectory --> PageTable --> Page

So consider the following linear address. 

01001010101 1010101101 10101110101010 
 page tbl     page      offset

Using page tbl bits we find the page directory entry physical address. PD = cr3 + L * (page tbl) which contains page table physical address PT. Now we are looking for our page P = PT + L * page. So in single level we had one page table 4MiB. Now we have 1024 page tables 4KiB = 4MiB in total. Don't see memory economy.

What did I miss?

Upvotes: 2

Views: 1479

Answers (1)

fuz
fuz

Reputation: 93172

Normally, the page table is far from full. With a flat page table, if only the first few and the last few entries are needed, still the entire table needs to be allocated. With page directories, all but the first and last page table can be omitted, thus saving a lot of memory. The gains seem to be small in today's age of vast RAM, but it was really a lot back when computers might only have 4 MiB of RAM in total.

Upvotes: 5

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