The Coherent operating system was a Seventh Edition Unix clone by the now-defunct Mark Williams Company, originally produced for the PDP-11 in 1980. A port was introduced in 1983 as the first Unix-like system for IBM PC compatible computers.
Coherent was able to run on most Intel-based PCs with Intel 8088, 286, 386, and 486 processors. Coherent version 3 for Intel-based PCs required at least a 286, Coherent version 4 for Intel-based PCs required at least a 386. Like a true Unix, Coherent was able to multitask and support multiple users. From version 4 on Coherent also had support for X11 and MGR windowing systems.
Much of the operating system was written by alumni from the University of Waterloo: Tom Duff, Dave Conroy, Randall Howard, Johann George, and Trevor John Thompson. Significant contributions were also made by people such as Nigel Bree (from Auckland, New Zealand). The manual was entirely written by Fred Butzen and was praised because of its quality.
Dennis Ritchie provides an anecdote about a visit to the Mark Williams Company to determine if any original Unix source code has been used to create Coherent:
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.
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.
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.
In the event, "we" (i.e. AT&T) backed off, possibly after other thinking and investigation that I'd wasn't involved in.