ok, i get it.. 

still, shac and sgec are equals with the Qbus adapter on a microVAX,

mainbus
cpu at mainbus
shac at mainbus
sgec at mainbus
uba0 at mainbus

and so on.. 
to mirror the actual structure of the thing. all VAXen have a two-layer
structure to them, whatever the parts are called is immaterial. 
The nexus bus, and everything else. the nexus bus is a bus where only 
nexus devices can connect. nexus devices are all equal in power and rights
to a VAX processor and they all manipulate VAX page tables. SHAC and SGEC
are nexus devices, just as the Q/U adapter is, and they are electrically 
connected in the same way as the uba is on a microvax. In early VAXen, only
unibus and massbus adapters were ever connected in the nexus bus, aside of
course from processors. In microVAXen, the nexus bus is on the processor 
module itself physically, and aside from embedded devices on that board, 
the uba whic his also on that board is th eonly one that can be there. 
on later large vaxen, the XMI and BI buses are the nexus bus, and many 
things are nexus devices in that world, though you can also hang a bus 
adapter there , for example a uba or massbus adapter, and then hang 
unibus devices off of it. VAX 4000 has such a thing, though VAX 4000 
doesnt use XMI. The only aberrants to this in large VAXen are VAX 
7000,9000,and 10000, which are all more like the big AXP systems than 
VAXen. Those have yet another layer in them, and have a nexus-like bus 
to which XMI adapters connect to! I had confused thse a bit, and 
apologize.. 

isildur