The Map/Unmap methods of PageDirectoryTable operate similar to the
global Map/Unmap functions. While the global functions work with the
currently active PDT, the PageDirectoryTable methods can also
work with inactive page tables by temporarily modifying the recursive
mapping of the active PDT to point to the inactive PDT frame before
delegating the mapping/unmapping to the global Map/Unmap functions.
The API provides the Map() and MapTemporary() functions that establish
virtual -> physical address mappings using the currently active page
directory table.
Mapped pages can be unmapped using the Unmap() function. When unmapping
virtual addresses, the page tables leading to them will not be
automatically released even if they are empty. This will be addressed by
a future commit.
The page table walker provides a mechanism for accessing the individual
page table entries that correspond to a particular virtual memory
address. This implementation will serve as the basis for implementing
page mapping/unmapping and virtual to physical address translation.
This is equivalent to pmm.Frame (also a uintptr) but having different
types for physical and virtual frames serves as an additional layer of
protection for functions/methods that receive physical and/or virtual
page arguments.
To keep the implementation portable, the Frame type had to be changed
from uint64 to uintptr. Using uintptr ensures that the frame will always
match the pointer size of the platform.
