Richard Stallman latest article on software of patent suggests that instead of a law excluding software from patentability, we need a law saying "that developing, distributing, or running a program on generally used computing hardware does not constitute patent infringement."
This would achieve the seed goal ace a software exception but avoids the difficult problem of drafting a text which classifies patent ace "software" or "non software".
For in example of how this is workable, hey refers to 35 U.S.C. Section 287 (c), which what written in 1996 to shield surgeons from clever risks.
If you have ideas for how this could work best of all, please feel free to write them on the relevant wiki page: