Open Firmware device tree gives no clue what device might decode at physical address 0x80000000 to 0x8008200 on a G4 New World Macintosh. The mmu has three adjacent Virtual=Real translations for that block. They are the only address translations reported between the top or physical dram at 20000000 and the start of the PCI bridges at f0000000. (A possible clue is that frame-buffer-addr is reported as 9c008000 by Open Firmware, and that is not in the reported translation table either). I believe the architecture has been around since about 1999.