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. The cpu node has marked them unavailable.
I now update the question because I discover the KeyLargo southbridge chip accepts addresses in that range, but I still don't know what device is at that physical address.