[Simh] Limits on MSCP controllers

[Johnny Billquist]

On 2019-06-23 19:30, Bob Supnik wrote:
> The four ports is not arbitrary. SimH simulates actual hardware. DEC 
> never built a backplane MSCP controller with more than four ports.

I'm not entirely sure about that. What about the DSSI controller which 
talks MSCP on the Qbus? I can't remember the name of it. But I'm not 
sure if that one was limited to just four disks.

Anyway, the only reason for the 4 disk limit was really just because the 
physical hardware only have 4 ports.

> If you want to extend the current RQ simulator to include third party 
> boards (either SMD-based emulators or SCSI-based emulators), feel free 
> to add an appropriate mode switch. I don't know what controller ID these 
> third party boards returned, though, nor do I know how VMS determined 
> the number of ports per controller.

Those 3rd party controllers usually identifies themselves as UDA50 or 
KDA50, if I remember right.
I can check in a day or three, when I bring my 11/93 back up running, 
since it has a CMD controller.
Those controllers also usually lie about disk types, commonly saying 
that disks are RA81 or similar, no matter what capacity they have. And 
since software should actually query about size, and not make any 
assumptions based on the id a disk returns, that should also be fine.

As for how to determine the number of ports, I don't know how VMS does 
things in general. In RSX, you tell which disks are expected to be found 
at which controllers when you do a SYSGEN, and you just define as many 
disks you want on a controller. At boot, the OS just tries to locate the 
disk on the controller without trying to do anything specific about 
which port it might be sitting on.
I can go digging into more detail if you want me to.

> I think it would be better to understand why VMS is waiting to mount 
> additional discs. Alternately, just create bigger discs and have fewer 
> of them.

Agreed that it would be interesting to understand what the problem is 
with VAX or VMS here, since I certainly do not observe anything similar 
with a PDP-11 and RSX.

Larger disks is also always a good option.

   Johnny

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