Hmm, this is quite interesting, but I had different impression of the definition of "open" at the time: it seemed like what people were saying when they said that Unix was "open" was much less about the source code, but rather about the interfaces and APIs; in particular especially after the standards bodies got together and starting writing down how things were supposed to work. This led to vendor independence (to some extent, anyway) and was a distinction from closed systems which were defined by a single vendor who controlled everything about them (though presumably modulated by customer demand), including the OS (since this was usually written in-house for each platform. This even makes historical sense: Unix was written by a third party who didn't design the hardware).

Consider DEC: In 1981, they had at least three hardware platforms intended for the timesharing market, each running multiple operating systems: PDP-11 running RSX-11*, RT-11, RSTS/E and Ultrix-11 (Unix); PDP-10 running TOPS-10 and TOPS-20; VAX running VMS and Ultrix-32 (Unix). And this isn't to mention any of the other stuff they were selling/supporting (PDP-8's, etc). Of those software systems it's easy to see what Ultrix-11 and Ultrix-32 have in common; one has a reasonable shot at getting software written for one running on the other. Contrast with RT-11 and VMS, or even RT-11 and RSX. Similarly with IBM, CDC, HP, GE, etc.

In other words, the "openness" in "open systems" wasn't about code *for the system itself*; it was about freedom from software lock-in to a particular hardware vendor. Or, perhaps, openness to multiple system vendors supporting the same customer-written code. "Open" in a sense closer to what we now call "open source" (meaning the source was available for inspection) came much later.

        - Dan C.