25 Feb
2021
25 Feb
'21
1:26 p.m.
On Feb 25, 2021, at 4:15 PM, Al Kossow via cctalk
<cctalk at classiccmp.org> wrote:
On 2/25/21 12:37 PM, Josh Dersch wrote:
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.
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 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