[Simh] Adding a third massbus to the VAX780 emulator

[Peter Allan]

Many, many years ago, I used to be the system manager of a VAX-11/780
system that was part of the Starlink astronomical data processing network
in the UK. One of my pet projects is to have an emulation of this as close
to the original as I can get it. The system had two RP06 disk drives and
two System Industries drives that emulated RM05 drives. Since the System
Industries drives interfaced to the 780 through their own massbus adapter,
the RP06 drives appeared as devices DBA0 and DBA1, and the emulated RM05
drives appeared as DRA0 and DRA1.

I can create a similar system using simh, but with all of the drives on a
single massbus, the drives appear as devices DBA0, DBA1, DRA2 and DRA3.
This apparently trivial difference annoys me so I am looking for a way of
having both the RP06 and RM05 drives appear as units 0 and 1.

I presume that this will require modifying simh to add a third massbus
adapter. (Please do tell me if there is an easier way.)

Having looked at the simh code, I am reasonably confident of being able to
add a third massbus. However, how can I attach RP drives to one massbus and
RM drives to another? Both drives are handled by the same function, i.e.
pdp11_rp.c. My thoughts at the moment are that I should copy pdp11_rp.c to
a new file pdp11_rm.c and remove the code that handles the RM disk from
pdp11_rp.c and vice versa for the RP code in pdp11_rm.c

I can see that there is more work to do in terms of splitting the handing
of interrupts and the like, but before diving in, does my basic strategy
seem correct? It would be more like real life (I think) to be able to add
RM and RP drives to both massbuses that handle disks, but I don't see how
to do that without a lot more work.

And before anyone says it, I know I am mad to be trying to do this. The
effort is clearly disproportionate to the benefit, but it is a hobby after
all.

All comments gratefully received.

Peter Allan,  petermallan at gmail.com
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