Massbus - was: Re: VAX 11/750

Paul Koning paulkoning at
Thu Feb 25 12:38:52 CST 2021

> On Feb 25, 2021, at 1:30 PM, Noel Chiappa via cctalk <cctalk at> 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
> data).
> 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.

Yes, I meant the RH70.  The point is that, without Massbus, the only I/O bus you have in PDP-11 systems is the Unibus (pre-Qbus that is) with all its limitations.  The Massbus enabled the creation of the RH70 with its direct memory connection both for more bandwidth and more address bits.  So, for example, you can use an RM03 rather than being limited to an RM02 because of the bus performance.  And your disks and tapes don't have to fight all the other I/O devices for unibus map slots.


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