[Submitter here]

> I am curious to the attraction of Coherent now.

TBH I am not sure that it has one.

> Anyone that used it or wrote code for it hated it.

{{citation needed}}

I knew a few owners. They loved it. Until Linux, Coherent was the cheapest way to get a UNIX OS on a PC.

> It was not compatible with any version of Unix.

That is not fair or reasonable or accurate for the time.

This was a 1980s product. No 1980s Unix was compatible with any other. That's why the iBCS existed:

https://en.wikipedia.org/wiki/Intel_Binary_Compatibility_Sta...

They were all different, even though all were based on some version of the same AT&T kernel or code derived from it.

Coherent was so close and so compatible that AT&T believed it must have been based on stolen code and sent Dennis Ritchie himself to visit the company and check.

Here are his comments from that visit:

https://groups.google.com/g/alt.folklore.computers/c/_ZaYeY4...

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An anecdote: sometime fairly early after the Mark Williams company
started offering their Coherent system (a Unix clone), some AT&T
legal people asked me to visit Mark Williams for purposes of determining
whether what they were offering was a rip-off (i.e. essentially
a copy) of the currently licensed Unix done by us. I find it
hard to reconstruct the date this happened, but it was a long
time ago; probably early 1980s. I went to Chicago with Otis
Wilson, who was then involved in Unix licensing.

It was a rather strange experience. The Mark Williams company
was a paint producer, and I was given to understand that
the subsidiary that was doing Coherent was, approximately,
a corporation arranged by a father who, approaching
retirement, had more or less shut down the older business
and was using the corporate name and legal setup to help
his son in a new venture.

Otis and I visited the offices of Mark Williams on the outskirts
of Chicago and were received with courtesy and some deference.
We talked to the father and the son (Bob Swartz, i.e. the guy
behind Coherent). There had been communication before, and
from their point of view we were like the IRS auditors coming
in. From my point of view, I felt the same, except that playing
that role was a new, and not particularly welcome, experience.
The locale of the company was in an industrial section and
it definitely retained the flavor of a the offices
of a paint company being recycled.

What I actually did was to play around with Coherent and look for
peculiarities, bugs, etc. that I knew about in the Unix distributions
of the time. Whatever legal stuff had been talked about in the
letters between MWC and AT&T didn't allow us to look at their source.
I'd made some notes about things to look for.

I concluded two things:

First, that it was very hard to believe that Coherent and its basic
applications were not created without considerable study of the
OS code and details of its applications.

Second, that looking at various corners convinced me that I couldn't
find anything that was copied. It might have been that some parts were
written with our source nearby, but at least the effort had been
made to rewrite. If it came to it, I could never honestly testify
that my opinion was that what they generated was irreproducible from
the manual.

I wrote up a detailed description of this. I can't find it, probably
because at the time I was advised that it was privileged lawyer/client
material. Partly at the time, partly thereafter, I learned that
a variety of Unix enthusiasts (several from U. Toronto) had spent
time there.

In the event, "we" (=AT&T) backed off, possibly after other
thinking and investigation that I'd wasn't involved in.

So far as I know, after that MWC and Coherent were free to offer
their system and allow it to succeed or fail in the market.

I suppose there's a second story about the suit by USL against
BSDI and then UCB, but my own involvement was far tinier
and didn't get me a trip to Falls Church or Berkeley to snoop.
What advice I offered in this situation was exactly in line with
that about MWC/Coherent, and as it turned out the resolution
(though more costly for all) was pretty much the same.

(As a capper, Bob Swartz came by Bell Labs a week or so ago,
and we had a pleasant social visit.)

Dennis

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