First, `unibus' is too easily confused with the DEC UNIBUS/Q-bus.
Hayakawa-san is renaming it to `rbus', since its really just a trampoline.

What rbus adds is the ability to take an underlying bus -- in this
case, PCI -- and further restrict the memory and I/O windows which can
be mapped by that bus-space implementation.  There's also some mixins
from PCMCIA methods, but that's the gist of it.

The canonical example is that, while PCI can give you a handle which
maps a range anywhere in the machine's PCI space, CardBus bridges only
handle a small fixed-size subset of the total PCI space.
(anyone familiar with Unibus windows 


If theres' a cleaner way to do (in C++ terms) subclsassing which
restricts mappable bus-windows, I'm all ears. So is Hayakawa-san.