PDP-11 addressing question and a model round-up

[Mike Cesari]

On Feb 14, 2005, at 8:53 AM, Paul Koning wrote:

<snip>

>
> Life gets more complicated in 22-bit unibus machines, because there
> you have 22 bit physical (memory) addresses, yet 18 bit addresses on
> the I/O bus (the Unibus).  So those 18 bit addresses are virtual, they
> are mapped to physical addresses via the Unibus map.  Q-bus systems
> avoid this hassle by using full 22 bit physical addresses on the I/O
> bus.  (And Unibus systems do that partway -- Massbus adapters hook
> right into the memory so they handle 22 bit addresses directly, not
> via the Unibus map.)
>

WRT massbus -- 22 bit addresses only worked with RH70's on 11/70's. 
RH11's on the
unibus were strictly 18 bit.

<ramble>
Something I remembered regarding DZ11's and interrupt issues -- if you 
had the money,
you could use COMM-IOP-DZ. This consisted of up to 4 DZ11's, an M8206 
(KMC-11B) and
software to handle the charcter-at-a-time interrupt issue. This combo 
wound up being
better regarding system resources than DH11's in our testing at the 
time (early `80's).
We also used these on VAX78x's. It made DW780's not suck as much. (The 
DW780 was *slower*
than the 11/70 unibus. So we always had multiple DW's on each system.)
</ramble>

Mike