Update on zero UNIBUS interrupt vector problem

[Dave Martindale]

The "glitch" sounds like some device is grabbing the bus and then
refusing to release it.  The Select Time Out error means that the
unibus adapter couldn't get access to the unibus to do an NPR cycle
to read or write a unibus address.  The DZ's show up in the FUBAR
probably because they are polled 60 times a second by the clock
interrupt, so if the bus locks up they are almost certainly the first
thing to be referenced.

Finding out what happens during the lockup is an exercise in guessing
and experimentation.  Try to look at the state of the bus next time
it locks up (you'll have to disable the unibus reset code to prevent
an automatic reset...).  If a device is continually holding down BBSY,
that will be visible.  If something is holding down NPR, or pulling it
down very frequently, that will be visible too.  Assuming that something
is requesting NPR cycles continuously, you can tell which device is
accepting the grants (and thus probably doing the requesting) by
following the NPG line from slot to slot until you find the device
which is blocking it.