Massbus - was: Re: VAX 11/750

Josh Dersch derschjo at
Thu Feb 25 14:37:25 CST 2021

On Thu, Feb 25, 2021 at 12:27 PM Al Kossow via cctalk <cctalk at>

> On 2/25/21 10:43 AM, Chris Zach via cctalk wrote:
> > Oh this is fun stuff. Is there a specification write-up anywhere on the
> MASSBUS overall?
> there is some documentation on bitsavers
> the register descriptions were intentionally obfuscated to prevent cloning
> that was figured out to make SIMH work

Interesting... Is this obfuscation documented anywhere (beyond looking at
SIMH code?)  I was working on an RH11/RPxx emulation for the Unibone that
was partially working (and that I should really get back to...)

- Josh

> the CDC drive interface was also slightly modified for the same purpose

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