Sebastian Soeiro (sebastian_2896.delete@this.hotmail.com) on August 11, 2013 9:20 am wrote:
[snip]
> - So the page walker finds the physical address associated with a virtual
> address... But how is this association "known" to the page walker?

The page tables are set up the by operating system. With a hardware page table walker, the format used for each entry in the page table and how the entries are indexed are both defined by the architecture.

With the common hierarchical page table format (x86, ARM, and now Release 5 of MIPS), a (physical) page table base address is provided in a special purpose register (whose value must be changed by the OS when the address space changes). A selection of more significant virtual bits are used to index off of this base address. (Often a number of the most significant bits are ignored and forced to match the most significant bit in this selection.) The entry at that index provides the physical address for the next layer of the page table. A selection of virtual address bits starting from the bit just less significant than the earlier selection is used to index from the retrieved base.

For 32-bit address spaces with 4KiB pages and page table entries 32 bits in size, bits 22 through 31 are used to index the table pointed to by the special purpose register (with two zero bits appended to provide a 32-bit aligned address). The address value in the indexed entry (typically 20 bits in size, one bit is used as a valid bit, and the other 11 bits can be used for permissions or other metadata) is used as the base address for a second table. This second table is indexed with bits 12 through 21 of the virtual address. The entry at this index provides the physical address of the page for the virtual address. Bits 0 through 11 of the virtual provide the offset within the page (these bits are the same for virtual and physical addresses).