Microsoft has announced that a key API of its hypervisor technology will be open sourced.

Microsoft's Open Specification Promise saw the company vowing to drop intellectual-property and patent claims, making them available licence-free for anyone to use.

Microsoft said: "With the OSP, any individual or organisation is free to implement, commercialise and modify Microsoft’s virtualisation format technology for free, now and forever." The OSP initially involved 35 of its Web service protocols but now also includes its virtual hard disk image format, four security technologies, two XML file formats and a robotics protocol.

Now Redmond has added to the OSP the hypercall API that is central to Windows Server virtualisation (codenamed Viridian) which will be available when Windows Server virtualisation is released to manufacturing (RTM).

In the interim, Microsoft said that it has posted an updated draft of the hypercall API so that developers can gain early access to the interface. Microsoft first distributed hypercall API draft documentation to attendees of Windows Hardware Engineering Conference (WinHEC) 2006.

The hypercall API enables guest OSes to call for resources from the hypervisor. According to Microsoft, it enables development of software that builds on Viridian and is, said Microsoft, "available for use by any organisation seeking to integrate or extend their software with Windows Server 2008 and Windows Server virtualisation."

Microsoft continues to maintain that Viridian will RTM within 180 days of the RTM of Windows Server 2008, currently scheduled for Q1 2008.

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Two of Microsoft's close partners endorsed the OSP move. Both sell virtualisation technology based on Xen, which was originally open source, and whose creator, XenSource, was bought by Citrix earlier this year.

"The majority of our customers have mixed-source environments, and they want their platform vendors to make things work together," said Roger Levy, Novell's senior VP and open source general manager. "That's why we entered into a technical collaboration agreement with Microsoft. As a result, Novell is the first vendor to develop and ship technology that will allow a para-virtualised Windows Server 2008 to be hosted as a guest on the Xen hypervisor.

"Microsoft's decision to put the hypercall API under their Open Specifications Promise will make it even easier for Novell, our customers and partners, and the entire open source community to develop high-quality virtualisation solutions that deliver true interoperability between Windows and Linux."

“Citrix is committed to the delivery of value-added virtualisation solutions for the Windows platform, so interoperability with Microsoft's virtualisation solutions is key to our success. This is made possible by Microsoft's open and progressive approach to licensing key technologies such as its VHD image format and the Windows Server Virtualisation hypercall API," said Citrix CTO Simon Crosby. "This will allow us to ensure that virtual machines created on XenServer will be compatible with Microsoft WSV when it is delivered as a component of Windows Server 2008."

Microsoft applied the OSP to its virtual hard disk image format in October 2006, mirroring a similar initiative by market-leader VMware. VMware has also opened its virtual machine format, some of its management interfaces, and a number of its hypervisor APIs. VMware also pointed out that: "Microsoft’s licensing policy has some restrictions on transferring licences from one server to another. Under Microsoft licensing, Windows Server licences are tied to physical hosts and do not move with virtual machine environments."

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