[Simh] Adding a third massbus to the VAX780 emulator

[Johnny Billquist]

I think this is totally the wrong strategy. If anything, you should try 
to model this as close to how the actual hardware worked, and not do 
some special thing that happens to just match your very specific 
configuration.

What you need is to split the massbus handling, as such, from the 
specific disks. You should then be able to create massbusses for you 
machine, and then you attach disks to the massbus. It is a hierarchy, 
and the config should reflect that.

I think simh's current approach of presenting things in a very flat way 
is a bit too simplistic. I got annoyed the other day when I wanted more 
than one DZ11 on a configuration, and realized I could not configure the 
controllers as such at all, but instead configured the total number of 
serial lines, with an implicit overflow from one controller to the next.
In real life, you might have a machine with four serial lines, but all 
attached to separate DZ11.

Just specifying the endpoints without the ability to specify the 
configuration in the middle layers will cause issues like the ones you 
have, because simh then makes assumptions for you. (Such as having just 
one massbus, and all disks attached to that one.)

In fact, I wonder if this will actually totally break something like 
RSX-11M, which do not allow mixed device massbuses. Ie. in 11M you 
cannot have both RM and RP disks on the same massbus.
M+ allows this, but then, M+ is rather more advanced... :-)

	Johnny

On 2012-11-21 21:27, Peter Allan wrote:
> 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 <mailto:petermallan at gmail.com>
>
>
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