[Simh] Limits on MSCP controllers

[Johnny Billquist]

On 2019-06-24 01:37, Johnny Billquist wrote:
> On 2019-06-23 19:30, Bob Supnik wrote:
>> 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.

Timothe Litt clearly knows this better than I do. His mention of the 
attention message jogged my memory a bit. There are just notifications 
going the other way about available disks. And ports are not really 
relevant. In RSX, they are then obviously matched against what you 
configured in your SYSGEN. Disks which do not match any configured 
device are just going to be ignored. No dynamic adding of disks 
possible. And unit numbers can be totally unrelated between one 
controller and the next, and have nothing to do with the device 
numbering inside RSX. So DU5: can map to unit #42 on the third 
controller. RSX will happily do that mapping.

And by the way, if the goal of simh was to try and act just like 
existing hardware, then you should not be allowed to have more than one 
tape drive per tmscp controller, since that was the way of all DEC tmscp 
controllers. But tmscp itself also certainly do not have such a 
limitation, and in this case simh do deviate from what the real hardware 
did. One TQK50 (or TUK50) per TK50, and so on...

   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