High Quality Content by WIKIPEDIA articles! Macintosh Business Unit (informally known as Mac BU or MacBU) is an operating unit of Microsoft that, as of 2009, is the largest software developer outside of Apple Inc. for the Mac OS operating system. Formed on January 7, 1997 (with release of Mac OS 7.6), it was formerly situated within Microsoft`s Specialized Devices and Applications Group inside the Entertainment and Devices Division., with it now being inside the Microsoft Office division.