The patentability of software, often referred to by the expression software patent, is one of many legal aspects of computing, and one of many aspects of exclusive rights policy, often spoken of by the term intellectual property.
It is fair to say that many of those involved in the debate (see below) mean different things when they refer to software patents, and their views may change depending on what definition of the term is used.
There is no universally accepted definition of the expression "software patent" and no legal text defines what exactly is a software patent and what it is not.
The difficulty to draw a clear boundary between software patents and other patents may come from the fact that a patent claim can be written so as to embrace many different implementations (some using purely mechanical or electrical means, others using software), for instance by using functional features under certain jurisdictions (e.g. "means for controlling"). Additionally, under the so-called doctrine of equivalents and its analogues, a patent that on its face does not appear to require software can be infringed in certain circumstances if software is used as an equivalent of (i.e. to substitute for) a non-software element, making even more difficult to draw the boundary.
The term "computer-implemented invention" was put forward by the European Commission and defined as "any invention the performance of which involves the use of a computer, computer network or other programmable apparatus and having one or more prima facie novel features which are realised wholly or partly by means of a computer or computer programs."  (http://europa.eu.int/comm/internal_market/en/indprop/comp/com02-92en.pdf)
The national jurisdictions relating to software patents in Europe and in the European Union are not harmonized even though some harmonization has been brought into the national jurisdictions in the 1970s and 1980s. The EU Commission has proposed a Directive on the Patentability of Computer-Implemented Inventions to harmonize them.
Software patents under national laws:
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