Massbus - was: Re: VAX 11/750

Noel Chiappa jnc at
Thu Feb 25 12:30:29 CST 2021

    > 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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