Massbus - was: Re: VAX 11/750

Paul Koning paulkoning at
Thu Feb 25 15:26:49 CST 2021

> On Feb 25, 2021, at 4:15 PM, Al Kossow via cctalk <cctalk at> 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.


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