Controlling address translation
Translation table format
Here we can see the different formats that are allowed for translation table entries:
Each entry is 64 bits and the bottom two bits determine the type of entry.
Notice that some of the table entries are only valid at specific levels. The maximum number of levels of tables is four, which is why there is no table descriptor for level 3 (or the fourth level), tables. Similarly, there are no Block descriptors or Page descriptors for level 0. Because level 0 entry covers a large region of virtual address space, it does not make sense to allow blocks.
Note: The encoding for the Table descriptor at levels 0-2 is the same as the Page descriptor at level 3. This encoding allows 'recursive tables', which point back to themselves. This is useful because it makes it easy to calculate the virtual address of a particular page table entry so that it can be updated.