18 Mar
2015
18 Mar
'15
8:28 a.m.
On Mar 18, 2015, at 11:16 AM, Noel Chiappa <jnc at
mercury.lcs.mit.edu> wrote:
Oops. Yes.
From: Paul Koning
if the CPU generates an address in the range
17000000 to 17577777, it
lands on the Unibus and is then mapped by the Unibus map.
Umm, I think you meant "to 17 757 777", no? (Too many 7's - I find it
helpful
to break them up into groups of 3, the way DEC does, to prevent that kind of
confustion.) More precisely, it works that way for 11/44 and
11/70 - but not for the
J-11. That's a difference not documented in the PDP-11 Architecture
handbook model differences table
But, but, but... the J-11 doesn't _have_ a UNIBUS map! Well, the 11/84 and
11/94 do, but not other J-11 machines. (And the Architecture Handbook table
does not have separate columns for the 11/73, 11/84, etc.) So an 11/73 can't
have a reference in the range 17 000 000 to 17 757 777 go out the UNIBUS -
there isn't one! :-)
So what happens if one does a reference to something in the range 17 000 000
to 17 757 777 on the 11/84? The 11/84 _does_ support having memory on the
UNIBUS (up to 248KB), _but_ how it appears depends on how much there is. (See
section 3.13.2, EK-1184-TM-PR2.) There's a special register to configure it
(the 'KTJ11-B Memory Configuration Register', KMCR), which includes _how
much_ UNIBUS memory there is.
...
Basically, it seems like DEC was determined not to waste any address space on
the J-11/UNIBUS machines. Either it's configured as UNIBUS memory, or it's
PMI.
Interesting. I was referring to what I learned by reading the RSTS/E memory size
determination code. That code says that the main memory limit is 2M - 128kW for 11/44 and
11/70, and 2M - 4kW for J-11 Unibus. So it looks like RSTS/E doesn?t support the mixed
case you mentioned, only the ?just PMI? flavor.
paul