Microsoft Tuesday made available for download the latest beta of its virtualization add-on for Windows Server 2008.

The company said Hyper-V Release Candidate (RC) 1 is feature complete, but it did add on additional Windows guest operating system support and integration features.

A Release Candidate signifies the software is out for final beta testing before final release. The previous RC, RC0, was released two months ago.

"This is the last planned Hyper-V milestone before RTM," says Jeff Woolsey, senior program manager for Windows Server virtualization. RTM stands for Release to Manufacturing, which signifies the software development process is complete.

Microsoft said in April that it was ahead of schedule on the new timetable for release of Hyper-V and that the technology would likely show up in June or July.

The company has said since the May 2007 decision to cut Hyper-V from Windows Server 2008 that the virtualization technology would ship within 180 days of the release of the server, which was February 28.

"We are feeling pretty good that it won't be up to the full 180 days," said Dai Vu, director of virtualization products and solutions in Microsoft's server and tools division, at Microsoft's Management Summit in April.

Microsoft also confirmed that its MSDN and TechNet Web sites have been running in production on Hyper-V for several weeks.

The RC1 code of Hyper-V adds support for Windows 2000 Server SP4 and Windows 2000 Advanced Server SP4. Both of those configurations are one-way and support emulation mode only.

Hyper-V supports a number of guest operating systems: Windows Server 2003 SP2, Novell SUSE Linux Enterprise Server 10 SP1, Windows Vista SP1 (x86) and Windows XP SP3 (x86).

In addition, RC1 simplifies the process for installing integration components for Windows Server 2008 guest systems. Integration components are sets of drivers and services that help ensure virtual machines have a more consistent state and perform better.

Microsoft also said it is releasing mouse integration support for SUSE 10 in conjunction with RC1.

The RC1 upgrade is optional for current testers given that it is not compatible with the current beta of Virtual Machine Manager (VMM) 2008, the Microsoft tools for managing virtual environments. Microsoft says it will release an upgrade of the VMM 2008 beta to support RC1.

In addition, users running earlier betas of Hyper-V on x64 versions of Windows Server 2008 can get RC1 via Windows Update beginning May 27.

The final release of VMM is slated to ship 30 to 60 days after Hyper-V hits its RTM stage.

When Hyper-V ships, Microsoft will be providing users a third hypervisor option to go along with those already available from VMware and Xen-based derivatives marketed by Citrix, Oracle, Red Hat and Novell.

Hypervisor technology is a base technology layer that acts as the virtualization foundation for guest operating systems.