The Free Software Foundation has condemned FSMLabs, which developed the embedded operating system RTLinux, for using software patents that it says violate a central pillar of the Linux world -- the GNU Public Licence (GPL). In a statement issued on Friday, the Foundation criticised FSMLabs for the surprise grant of a software patent for RTLinux last year, saying that not only are such patents wrong in general, but they also conflict with the GPL, which governs the distribution of the Linux operating system kernel. "The FSF opposes software patents, and in addition believes that Yodaiken's patent licence violates the GPL of the kernel named Linux," the organisation said. The FSF also charges -- as have other experts -- that the FSMLabs patent is unenforceable on purely technical grounds. "The patent covers real-time interrupt handling using a software emulation layer for interrupt masking, so that interrupts can be prioritised. There is significant prior art for this," the group said. According to the FSF, whose founder Richard Stallman originally developed the GPL, FSMLabs isn't the only company violating the free-software licence. "Anyone else redistributing a modified version of Linux under the restrictive patent licence that Yodaiken uses will also be violating the GPL," the company said. The FSF does not hold copyrights to the Linux kernel but does have copyrights on other software elements of the GNU/Linux operating system. Therefore the FSF says it will not take direct legal action, but "may well choose to support efforts by other companies to invalidate Yodaiken's patent in the courts, and we may also support actions taken by others to uphold the GPL," according to the statement. In August FSMLabs licensed the RTLinux technology to Lineo, its first licensee. Software patents in general are highly controversial, with some arguing that they are used to drive small competitors out of business. The furore is an example of the difficulties that can arise in reconciling the popular open-source development model with businesses based on intellectual property. GPL-based software is not owned by one company in the traditional sense; anyone who adds to it must pass on the improvements to the rest of the development community. However, companies like IBM insist that it is possible for "free" software to coexist with proprietary software, and for companies to make profits from Linux and its ilk. In recent weeks and months Microsoft executives have launched a propaganda campaign against Linux and particularly the GPL, saying that they are inherently against intellectual property. Complicating the matter is the fact that FSMLabs is not looking to profit from the patent, unless licensees are themselves making a profit. The distinction is important, because many developers use RTLinux or similar systems for non-commercial or research work. The patent licence is complex, but Yodaiken says that it aims to allow open-source and GNU Public Licence-covered development work to continue, without endangering FSMLabs' ability to charge licence fees for its intellectual property. "We would like to find some way of using the patent to reinforce GPL, so that people don't take this method and build proprietary systems on it, without some additional licence," Yodaiken said in an interview with ZDNet sister publication LinuxDevices.com last August. Some observers, if they do not oppose the patent on principle, wonder if it can be enforced. One of the features covered by the patent, for example, is the method of running an operating system as a program within another, very simple real-time operating system (RTOS). The operations of the first OS can then be interrupted by the RTOS whenever tasks need to be carried out in "real" time -- that is, with as little delay as possible. Software patents are also heatedly opposed on ideological grounds, although the most visible controversies have surrounded the patenting of software that facilitates business processes, rather than software that merely has a technical effect. The UK currently allows the latter, but not the former. The EU currently does not allow patenting of software, but is under pressure from business leaders and patent lawyers to bring its laws into line with those in the US. See the Linux Lounge for the latest GNU/Linux and open source headlines. See the Operating Systems News Section for the latest headlines on Linux, Windows and other operating systems. Have your say instantly, and see what others have said. Click on the TalkBack button and go to the Linux lounge forum Let the editors know what you think in the Mailroom. And read other letters.