>    I'm still wondering whether UNIBUS has a 22-bit version or not.

The Unibus itself is always 18 (or 16) bit addresses.  But whenever a Unibus
is bolted onto a CPU with a wider address bus - a 11/70, 11/44, 11/84,
VAX 11/750, etc. - it's done through a "Unibus Map" that maps CPU/memory
address to Unibus addresses on a page-like basis.  If you look in a
PDP Unibus processor handbook or in a 11/750 or 11/780 technical manual,
this is all explained in great detail.

It's very analogous to the way that Microvax II and later Q-bus CPU's
map the 22-bit (4 Mbyte) Q-bus address space into the 16 Mbyte (or larger)
memory address space so DMA can be done.  Considering your pontificiation
about how drivers work, I'm very surprised you don't know these basic
details which anyone who has written a device driver must know about.
Have you even written a VAX device driver?

> And what
> about Q18 vs. Q22? How compatible are these two?

Depends on the hardware you're talking about.  Some Q18 peripherals (like
the RXV21) are "well-behaved" and the driver just has to be careful
to keep them DMA to/from them in the lower 256 kByte space they can address.
Others - like the RLV11 - are "badly-behaved" and are guaranteed to screw
up a 22-bit system.  Some Q18 backplanes run various odd voltages on
the 4 "unused" address lines which will burn out the bus drivers on a Q22
option unless the connection to these lines are cut.

All of this is well-documented in the various technical manuals that
came with the Q-bus peripherals.  But for someone like you who is in
need of a general introduction to the subject, you should check out
Micronote 5, "Q22 compatible options", at

http://sunsite.unc.edu/pub/academic/computer-science/history/pdp-11/hardware/
     micronotes/numerical/

Tim. (shoppa@triumf.ca)