[bunderbunder]

Oldest patent I see that looks like it could be for MS-DOS is #4,779,187, "Method and operating system for executing programs in a multi-mode microprocessor", which was issued in October 1988. That wouldn't apply to DOS 2.0, of course.

The bigger concern would be copyright. That might be a fairly plausible argument for why FreeDOS developers shouldn't be reading this source. Though realistically the odds that Microsoft would actually pursue such a case are remote at best.

[einhverfr]

I would call the odds "beyond remote."

The chance that you are going to get MSDOS expressive structures in FreeDOS in a way that is even arguably beyond deminimis is nil. You don't usually just move code between implementations easily, and chances are pretty good that the FreeDOS folks use fairly different approaches than the MSDOS folks. The argument is that if you read the code you might be tainted, but I don't see this as a significant argument here for software that's 30 years old.

[rbanffy]

Issue date is not the only relevant date - patents get extensions and expand in scope. I would imagine the FAT patent that Microsoft uses to get some revenue from every SD card sold comes from the dawn of personal computing.

[ekianjo]

> patents get extensions and expand in scope

Yeah, but usually you cannot get extensions THAT easily, and extensions are limited in time. I don't think you can expect extensions to last too long either (at most you can add a couple of years of exclusivity).

[danelectro]

Most of the FAT patents of the '90's finally expired in November.

Now exFAT is much more recently encumbered.