Setting Up Paging: Difference between revisions

Now that we have a page directory, we need to blank it. The page directory should have exactly 1024 entries. We will set each entry to not present so that if the MMU looks for that page table, it will see that it is not there (...yet. We will add the first page table in a moment).

 

//set each entry to not present

int i;

page_directory[i] = 0x00000002;

}

 

A page is "not present" when it's not (intended to be) used. If the MMU finds one, it will Page Fault. Non-present pages are useful for techniques such as Lazy Loading. It's also used when a page has been swapped to disk, so the Page Fault is not interpreted as an error by the OS. To the OS, it means someone needs a page it swapped to disk, so it is restored. A page fault over a page that was never swapped is a error by which the OS has a reason to kill the process.

We now need to fill each index in the table with an address to which the MMU will map that page. Index 0 (zero) holds the address from where the first page will be mapped. Likewise, index 1 (one) holds the address for the second page and index 1023 holds the address of the 1024th page. That's for the first table. So, to get the page at which a certain index is mapped is as simple as (PageDirIndexOfTable * 1024) + PageTabIndexOfPage. If you multiply that by 4, you'll get the address (in KiB) at which the page will be loaded. For example, page index 123 in table index 456 will be mapped to (456 * 1024) + 123 = 467067. 467067 * 4 = 1868268 KiB = 1824.48046875 MiB = 1.781719207763671875 GiB. It's easy, right?

 

// holds the physical address where we want to start mapping these pages to.

// in this case, we want to map these pages to the very beginning of memory.

first_page_table[i] = (i * 0x1000) | 3; // attributes: supervisor level, read/write, present.

}

 

===Put the Page Table in the Page Directory===