Re: Massbus - was: Re: VAX 11/750

[Paul Koning via cctalk]

> On Feb 25, 2021, at 4:15 PM, Al Kossow via cctalk <cctalk@classiccmp.org> 
> wrote:
> 
> On 2/25/21 12:37 PM, Josh Dersch wrote:
> 
>> Interesting... Is this obfuscation documented anywhere (beyond looking at 
>> SIMH code?)
> Not that I know of. Thinking about it some more I think that it was the 
> massbus device addresses
> behind the controllers that weren't documented

That rings a bell, for the PDP-11 case.  I think there is a mapping from 
RH11/RH70 CSR offsets to Massbus control bus register addresses; it isn't the 
natural 1:1 mapping of the low order Unibus address bits being used directly as 
Massbus register addresses.

From the software point of view that won't matter, but if you're trying to 
build a Massbus device you'd have to know it.  I'm pretty sure I've seen that 
mapping somewhere (in the past 10 years), at least in part, but I no longer 
remember where.

        paul