[chucky]

It might not be a problem for DOS 4, but often the source code of software that was only ever meant to be published as closed source contains source code that was licensed from 3rd parties. This license may not allow publishing the source code.

Doing an investigation of what licensed software was used and possibly trying to get permission from the relevant rights holders (if you can even figure out who owns the rights so many years later) can be a big and expensive task, unfortunately. I understand why companies might not want to take that on (even though it sucks).

[skissane]

For DOS, I believe the core was only ever Microsoft or IBM. Some DOS versions bundled add-ons by third parties, but they are hardly essential for operation - e.g. MS-DOS 6 included DEFRAG and MSBACKUP (both licensed from Symantec) and MSAV (licensed from Central Point Software)

Similarly, with Windows, the third-party components are generally inessentials such as certain device drivers, games, some optional system components like the ZIP file support in Windows Explorer-you would still have a usable OS with these bits ripped out. Parts of NTVDM are third-party licensed, although I believe that’s mainly the software CPU emulator used on RISC platforms, I think x86 was mostly Microsoft’s own code

[lproven]

Agreed.

From MS-DOS 6, remove the defrag, backup and antivirus programs, and DoubleSpace/DriveSpace, and that should I think cover all external code.

If I remember correctly, it didn't include CD-ROM drivers, just MSCDEX to run on top of one... and the network stack was an optional extra. I'm not even 100% sure it includes a mouse driver as standard.

IBM PC DOS 6.3, 7.0 and 7.1 include some additional IBM code: Rexx in place of QBASIC, the IBM E editor, but not much else.

[rezonant]

Isn't the zip support in explorer the stuff written by Dave Plummer? I would imagine MS has the rights to that already, and if they don't, I'd imagine Dave would, and I'm sure he'd be fine with it being released.

He has lots of YouTube videos about the zip stuff.

[chucky]

Yeah, but it's the "mostly" and "I think" that will cause lawyers to start sweating and force someone to do a bigger and more expensive investigation.