On 2018-04-10 03:28, Mark Pizzolato wrote:
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
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 Unit
Plug.  Did these exist on DEC MSCP drives?

Yes they did.

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 unit
numbers.  This may have been how third party MSCP controllers worked.

I've never seen any controller doing this approach.
The SCSI controllers I've seen and used have a configuration tool built in, which allows me to map the SCSI unit number to an arbitrary MSCP unit number for each unit.

Which of these best reflects how the real hardware worked?

I'd say option 1.


Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: b...@softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol
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