At the Day 2 keynote for its BUILD developer conference, Microsoft touted its vision for a cloud OS, which will combine the capabilities of its on-premises and hosted Windows Server 2012 and its cloud-based Windows Azure services. As you might expect from a developer show, much of the focus was on how programmers can access cloud OS functionality from a variety of developer tools and technologies.

“Just as we have reimagined Windows and Windows devices from the chip to the developer platform and the user experience, we have also reimagined Windows from the back end; we call this the Cloud OS,” Microsoft Server & Tools President Satya Nadella wrote in a post to the Official Microsoft Blog. “Our focus is to make sure we’re creating an experience that makes it easy for developers to do what they do best: build apps.”

Microsoft announced an update to Windows Azure Mobile Services, providing support for Windows Phone 8 and Windows 8. It also previewed the Windows Azure Store, which helps customers “discover, purchase, and provision premium services for their cloud applications.” And it released the Windows Azure hosted Team Foundation Service to manufacturing.

It also put the muscle of Halo 4 behind its Azure promotions, noting that the coming console blockbuster’s multiplayer experience will be powered entirely by Windows Azure, providing the ability to host more than 2 million players concurrently. Microsoft says that the Halo 4 team reduced costs by over 60 percent compared with the previous Halo release, in part by moving to Azure.