> > I don't know whether FreeDOS is a 100% free OS.
>
> Its license is GPL.
>
> Is it complete and self-hosting?
Yes.
I spent a while investigating Emacs on FreeDOS. The results go way beyond the scope of this bug report, so I'll file other bugs, or write to gnu-prog-discuss as appropriate. In summary, though:
1. Emacs 23.3, as distributed for DJGPP, works fine on FreeDOS.
2. The MS-DOS build system was broken in Emacs 24.3, i.e. the most recent release. (There are some problems that may be specific to FreeDOS, such as an inability to understand "@call" in config.bat, but the lack of the msdos/autogen/ directory in the tarball means that the build won't work; it still doesn't work after copying that directory, though I think the remaining problems are down to the lack of long file-name support in some of the native FreeDOS utilities.)
3. Rather than try to repair the MS-DOS build system (which remains, AFAICS, the only documented way to build Emacs for MS-DOS), it seems more productive to get the normal build system working. Given that DJGPP distributes a reasonably comprehensive and up-to-date set of GNU tools, this shouldn't be too hard. I built a distribution of Bazaar head, and after hacking it to accept the msdosdjgpp OS name, and a couple of other small hacks that I'll write up, it gets all the way through the configure script, crashing with an error I've not yet been able to diagnose when it tries to write config.status.