I promised myself I would stay out of this one... and I tried.
You can
patent an invention or algorythm. You can
servicemark or
trademark a name. And you can
copyright an published work (including a computer program).
A copyright means no one can copy and claim as their own, your published work. Even changing it slightly (remember some of that homework you turned in?) does not remove the fact it is plagiarism.
However, a completely unique work, which may perform the same function/contains the same information can be copyrighted as a unique and different work. It is also possible to publish a work of a collection of works, copyrighted by others, and to get a copyright on that.
If you want more details I suggest you look up the Bourne Convention, which is the international agreement under which copyright is covered.
On the subject of emulators and legal cases, I came across this:
http://www.worldofspectrum.org/EmuFAQ2000/AppendixB.htm
As for DMCA... it is contrary to international agreements and contains components which are clearly unconstitutional. Don't look for it to survive in the courts.
http://anti-dmca.org/
http://slashdot.org/features/01/06/06/131232.shtml
http://www.ectnews.com/perl/story/20291.html
http://www.freedom-to-tinker.com/archives/000002.html[/i]