On Monday, April 9, 2018 at 4:56 PM, Paul Koning wrote:
> > On Apr 9, 2018, at 6:25 PM, Mark Pizzolato <m...@infocomm.com> wrote:
> >
> > ...
> >> ...
> >> I'd like to have more than 4 disks on one MSCP controller. There is
> absolutely
> >> no reason for the limit of 4. That's just an implementation detail on some 
> >> of
> >> the existing MSCP controllers, but there are MSCP controllers who also take
> >> more than 4 disks.
> >>
> >> And that exists in real life, and I cannot do that (and a bunch of other
> >> setups) in simh, so I'd say that simh is rather more limited than real 
> >> life. :-)
> >>
> >> Same story for TMSCP.
> >
> > Please identify Qbus or Unibus controllers that had hardware support for
> > connection of more than 4 units and I'll include those controllers (with
> > their limits) in the simulator.  A pointer to the documentation for these
> > devices would be helpful.
> I don't remember controllers with more than 4 units per device, but MSCP unit
> numbers are arbitrary 8 bit values.  So a limit of 4 units is proper, but a
> limitation on those unit numbers being 0..3 is not accurate.  It matters in 
> some
> OS.  For example, VMS has type/controller/unit addressing (DUA3 etc.) but
> RSTS does not.  So on RSTS, if you have more than one MSCP controller, they
> must have disjoint unit numbers.  Since the unit numbers are 8 bits, that's 
> easy
> to do.  But I don't see how I can do that in SIMH.

I see 2 approaches to adding support for different unit numbers:
1) A unit number is settable uniquely for each of the 4 drives that can be 
attached to any of these controllers.  This would map to a conceptual Drive 
Plug.  Did these exist on DEC MSCP drives?
2) A 'base' unit number can be set on a controller and each drive on that 
would then have unit numbers equal to the base plus their existing internal 
numbers.  This may have been how third party MSCP controllers worked.

Which of these best reflects how the real hardware worked?

- Mark

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