old nouveau wiki:
real VRAM address = VRAM_size - (ramin_address - (ramin_address % reversal_unit_size)) - reversal_unit_size + (ramin_address % reversal_unit_size) 

nv3=16 bytes

0x400000 - ((0x100000) - (0x100000 % 16)) - 16 + (0x100000 % 16) = 2ffff0


 