NOTES

Base virtual address of higher-half kernel stack: FFFF FEFF FFFF FFF8 (it grows down)

int 15h memory map is placed at 0x500

Usage/mapping of physical memory at kernel load and jmp:
0x00000000 - 0x000FFFFF: Identity-mapped, 1 megabyte
0x00100000 - 0x008FFFFF: Kernel stack, eight megabytes
0x00900000 - 0x010FFFFF: Kernel + heap, eight megabytes from 0xFFFF800000000000 - 0xFFFF8000007FFFFF   

Once MM is set up:
Direct map starts at virtual address FFFFA00000000000, limit 32TB (this is the
max physical memory CosmOS currently supports).

The first part of the direct map, as much as can safely be put in the ID-mapped
first megabyte without overwriting critical data, is known as the "lower direct
map."  It is used to create enough of a page table to make it possible for us to
start putting the rest of the direct map at the start of a block of contiguous
physical memory sufficiently large to hold the rest of the direct map + the page
directory.  This part is known as the "upper direct map."