[...]
> 
> i dont see why we should create another layer to put between nexi and 
> anything. a SHAC is at the same level and the same privilege as the bus
> adapetors. 
> 
That's what I'm (trying to) propose :-)

mvbus is the top-level of a microvax.
sgec, shac and uba is attached to the mvbus.

In the same way as sbi is the top level on a 780, or cmi on a 750, 
or vsbus on all vaxstations.

The "mainbus" is just the union of all vaxen, and used just as a 
carrier for all top-level buses. Mainbus only have buses attached to it,
not devices.

-- Ragge