Microsoft will make the specification for its virtualisation format software freely available.

The software, called Virtual Hard Drive (VHD), allows a single hardware device to have multiple operating systems and applications running simultaneously.

The decision to make it available is an extension of what Microsoft calls its Open Specification Promise, announced last month, when it made specifications for 35 of its Web services freely available.

Over 60 companies have licensed VHD from Microsoft in recent years. By making the software freely available, Microsoft expects 500,000 virtualized servers will be sold this year. According to forecasts by researcher IDC the number should rise to 1.2 billion by 2009.

"We are focused on delivering inter-operability by design," said Bob Muglia, senior vice president of the Server and Tools Business at Microsoft. "This means that customers have control over their data while vendors provide technologies that connect diverse systems. By having the VHD specification available under the OSP, the technology is viable for any development or business model."

Red Hat greeted the announcement. "Red Hat welcomes the extension of the Open Specification Promise to additional specifications, such as the VHD Image Format Specification," said its deputy general counsel Mark Webbink. But he cautioned that "while a positive step, there are still significant barriers to full inter-operability with Microsoft technologies that still need to be addressed, particularly with respect to inter-operability with technologies developed and licensed as open source."

The launch of the Open Specification Promise (OSP) last month was seen by some as a positive development in its relations with the open-source software community. "I see Microsoft’s introduction of the OSP as a good step by Microsoft to further enable collaboration between software vendors and the open-source community," said Lawrence Rosen of technology law firm Rosenlaw & Einschlag, and a lecturer in law at Stanford University.