It may come as a surprise that the extension of patent law to software is still legally questionable. It rests on an extreme interpretation of a particular 1981 Supreme Court decision, Diamond vs. Deihr.
Traditionally, the only kinds of processes that could be patented were those for transforming matter (such as, for transforming iron into steel). Many other activities which we would consider processes were entirely excluded from patents, including business methods, data analysis, and "mental steps." This was called the "subject matter" doctrine.
Diamond vs. Deihr has been interpreted by the Patent Office as a reversal of this doctrine, but the court did not explicitly reject it. The case concerned a process for curing rubber--a transformation of matter. The issue at hand was whether the use of a computer program in the process was enough to render it unpatentable, and the court ruled that it was not. The Patent Office took this narrow decision as a green light for unlimited patenting of software techniques, and even for the use of software to perform specific well-known and customary activities.
Most patent lawyers have embraced the change, saying that the new boundaries of patents should be defined over decades by a series of expensive court cases. Such a course of action will certainly be good for patent lawyers, but it is unlikely to be good for software developers and users.