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Setting Up Paging: Difference between revisions
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The first step is to create a blank page directory. The page directory is blank because we have not yet created any page tables where the entries in the page directory can point.
Note that all of your paging structures need to be at page-aligned addresses (i.e. being a multiple of 4096). If you have already written a page frame allocator then you can use it to allocate the first free page after your kernel for the page directory. If you have not created a proper page allocator, simply finding the first free page-aligned address after the kernel will be fine, but you should write the page frame allocator as soon as possible. Another temporary solution (used in this tutorial) is to simply declare global objects with __attribute__((align(4096))). Note that this is a GCC extension. It allows you to declare data aligned with some mark, such as 4KiB here. We can use this because we are only using one page directory and one page table. Please note that on the real world, dynamic allocation is too basic to be missing, and paging structures are constantly being added, deleted, and modified. For now, just use static objects;▼
▲Note that all of your paging structures need to be at page-aligned addresses (i.e. being a multiple of 4096). If you have already written a page frame allocator then you can use it to allocate the first free page after your kernel for the page directory. If you have not created a proper page allocator, simply finding the first free page-aligned address after the kernel will be fine.
<source lang="c">
uint32_t page_directory[1024] __attribute__((align(4096)));
</source>
Now that we have a
<source lang="c">
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The second step is to create a basic page table. In this example we choose to fill up the whole first page table with addresses for the MMU. Because each page is 4 kilobytes large, and because each page table has exactly 1024 entries, filling up the whole table causes us to map 4 megabytes of memory. Also, the page directory is 1024 entries long, so everything can map up to 4GiB, the full 32-bit address space. Remembered the non-present page trick? Without it, we would use 16MiB per each paging structure. A single page directory needs 4KiB, but it can map some tables as non-present, effectively removing their space needs.
Now, its time to create a new page table.
<source lang="c">
uint32_t first_page_table[1024];
</source>
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