Microsoft is opening up its virtualisation server software by including the hypercall API for Viridian as part of its Open Specification Promise.

Developers will be able to use the hypercall API for Viridian to develop their own virtualisation elements that work with Windows Server 2008.

By opening up the software, Microsoft is giving up any claim to licensing fees and removing the threat of litigation over breach of the company’s patents.

An early version of the hypercall API has been posted on the Microsoft website.

The full version of the API will be released when Windows Server virtualization goes into production at the end of 2008.

The API was first given out to those attending the 2006 Windows Hardware Engineering Conference.

Microsoft launched the Open Specification Promise in September this year.