I'm not an assembly programmer and not very familiar with how that world works, but even then, if the two OSs were for the same architecture (x86), couldn't you still have a cross OS main part and then specific parts that deal with operating system things? I normally think of compiled languages like c being an abstraction over cpu architecture, not operating system api.

> Keep in mind this predates basically ANY kind of source control.

It might be before they were ported to DOS or OS/2, but it definitely wasn't before source control existed (SCCS and RCS were both definitely earlier.)

3x the work may still fall under reasonable cost.

If architectured properly (big if) you can split up the project appropriately so there is a common core and individual parts for specific OS.

Is it extra effort? Sure. Impossible? Definitely not.

Uh? Quite wrong.

SCCS was created in 1973. We're talking about over a decade later.

Also primitive forking, diffing and merging could be (painfully) done even with crude tools, which did exist.