Sun Microsystems and Microsoft, after tentative explorations of cooperation, today said they have opened a joint Interoperability Center in Redmond, Wash.

Spokesmen for the two companies said the center has been under discussion since last fall when Sun announced it would resell Windows Server. The purpose of the center is house technical experts who will design and test interoperability between Windows Server 2008 and Sun's Sun Fire x64 systems and storage. Sun Fire servers are built on x86 instruction set chips, including AMD's Opteron and Intel's Xeon chips.

"The center will provide a setting for hands-on testing and tuning of Sun/Microsoft solutions," Bob Kelly, corporate VP of infrastructure server marketing at Microsoft, said in a statement. It will serve as a demonstration facility for Windows Server 2008 on Sun x64 servers and storage and a place where customers may test drive their custom applications on a Sun server/Windows combination. It will also act as a proving ground for Microsoft Windows and applications to work with Java Platform Enterprise Edition and Standard Edition.

The center will "ensure more seamless integration between the technologies," said Lisa Sieker, VP of systems marketing at Sun, in the joint announcement.

Part of the purpose of the center is to ensure cross-vendor server virtualization with either Windows Server 2008 Hyper-V hypervisor or Sun's xVM hypervisor.

In addition, Sun officials said they are seeking to guarantee Sun Ray thin client software supports a Windows environment on a virtual desktop.

Sun has had employees working on the Microsoft campus for three years as part of closer collaboration promised by the two companies as they buried the hatchet in a long list of disputes and legal actions. Former Sun CEO Scott McNealy shook hands and embraced Microsoft CEO Steve Ballmer in April 2004 and announced an agreement where Microsoft would pay Sun $1.9 billion for settling differences and engaging in a technology exchange.

The Interoperability Center illustrates how both hope to capitalize on the presence of both Sun and Microsoft systems at the same customer sites. They also recognize the expanding role of virtualization. Microsoft relies on expertise from XenSource, a Citrix Systems acquisition and Microsoft partner, to let its Hyper-V hypervisor work with Linux. Sun uses open source Xen as a basis for its xVM virtualization software product line. If their virtual machines can communicate or migrate more easily between systems, it will give them both a better chance of competing with virtualization market leader, VMware.