Re: Address of second TMSCP controller

[Johnny Billquist <bqt%update.uu.se@localhost>]

On 2021-01-26 14:06, Nigel Johnson wrote:
>

> > Thanks, Mouse.  Yes, as an FE I used to have to figure out those 
> > floating assignment from charts - some DEC systems (RSTS, RT11) could 
> > not be told where to look - they had to be in the right place. IIRC, 
> > there was a strict ranking, and to make it start looking for a device 
> > of the next rank, you had to skip the number of registers that each 
> > device took so it would get a bus timeout and then look for the next 
> > device in the list! Seems like it was intended to save core memory 
> > which was so expensive in those days!  I do remember that if you got 
> > it wrong, it would treat whatever it found as the device that should 
> > be there, used the wrong driver, and crashed!

It didn't have anything to do with core memory.
The "problem" is that there are potentially many more address 
assignments needed to cover all kind of combinations you can think of 
than the address space available in the I/O page of the Unibus.

Just think of it. There are many different serial interfaces, and you 
could have many of each type. If you fixed the addresses for as many of 
each as you could imagine, the amount of address space you would need 
would be very large.

So a more dynamic address assignment was needed. So floating space 
essentially is a free for all. However, if you want to be able to just 
plug and play, and have software figure out what actually is installed, 
you then need some kind of a schema. And that is what the DEC designed 
allocation of addresses in the floating space address. And yes, it is 
essentially just putting a defined order on all controllers. So you 
probe for the first device until you get a bus timeout, then move to the 
next. Existence of any controller is just based on it responding to the 
CSRs. There never were any standard way of reading out the type of any 
controller, so it was just figured out implicitly by the ordering, and 
at which address it then responded.

But of course, this is only needed in order to do some kind of automatic 
detection of what is installed on a Unibus. If you explicitly assign 
addresses both on hardware and in software, this scheme for the floating 
address space can be totally ignored.

RSX basically do explicit address assignment, and only use 
autoconfiguration in one explicit place in the SYSGEN process, to find 
out what hardware is installed on your system, to provide some default 
answers for the SYSGEN process itself. It's not used at any other time.

I think RSTS/E made more use of the probing at runtime. Not sure about 
RT-11.

And how you probe the I/O space is almost as the description above, but 
not quite. But if someone wants to, we could go over how it is done.

  Johnny

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