This stuff isn't limited to IO, but is also e.g. used to map VRAM
| Map IO address range| Bits 0-19: index of the first IO page to map; Bit 20: Map read-only (otherwise read-write). Another 0b11111111100x must follow this one to denote the (exclusive) end of the IO range to map.
| Map single IO page| Bits 0-19: IO page index to map; Bit 20: Map read-only (otherwise read-write)