Re: The original open source?
Yes, they were all the same, but different...
I remember programming a system for DEC CP/M86, HP125 (CP/M), HP150 (MS-DOS), HP Vectra (MS-DOS) and IBM PC (PC-DOS). The code was pretty much the same, only different. Each used its own video card escape sequences, for example. Converting the core code was quick, it was the UI modules that took the work.
Then there were the exotics, like BTOS.
I remember at one stage, my desk had a Burroughs BTOS workstation, DEC Rainbow, DEC VT100 (connected to a Gandalf running in our data center), Apple Mac Plus, HP 150 and a Vectra, with an IBM-PC XT on another desk at the side of the room. I was supporting systems on all of them. All slightly (or very) different.