Massbus - was: Re: VAX 11/750
cz at alembic.crystel.com
Thu Feb 25 12:43:17 CST 2021
Oh this is fun stuff. Is there a specification write-up anywhere on the
For example I wonder if the RH70 could do a transfer >128kb at a time.
Another is around the RH11: There were two models, the traditional RH11
(which could only do so many words on a DMA transfer) and the RH11-C
which could do more words per transaction by basically running the
Unibus in "Hog mode". That allowed the 2020 to run RM03's at a full 3600
RPM (and I assume can allow the 2020 to run things like RM80's).
Another item I always wondered about was the RH11's support of two
unibuses. I think the idea was to do the data transfers on the second
bus right to an 11/45's FASTBUS memory without worrying about DMA
timeouts while running the control and status registers on the normal
UNIBUS (which wouldn't block other devices). I wonder, does the MBA on
an 11/70 use similar cards to an RH11?
On 2/25/2021 1:30 PM, Noel Chiappa via cctalk wrote:
> > From: Paul Koning
> > There's a good reason why the big disks on many DEC machines were Massbus
> > devices until MSCP arrived. It's quite clear on Unibus PDP-11s, which
> > needed Massbus both for speed and for a cleaner answer to more-than-18
> > bit addressing.
> I follow the first sentence, but I'm confused by the second, especially "a
> cleaner answer to more-than-18 bit addressing". The UNIBUS MASSBUS
> controller/adapter, the RH11, only has 18-bit addressing on the main memory
> side. It does have more than 18-bit addressing on the device side, but so does
> the RP11 (sort of). Are you thinking of the RH70? That does have access to
> more than 2^18 bytes of main memory, but that's because it connects to the
> -11/70 memory bus (as well as the UNIBUS, which is only used for control, not
> Similar questions about the speed point; passing data through an RH11 doesn't
> increase the speed of the UNIBUS? Yes, the RH70 is faster, but that's because
> of its connection to the -11/70 memory bus.
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