Some 64-bit capable Macs will not be able to run Mountain Lion, due to graphics limitations.
Mountain Lion is expected to launch on 25 July. The Gold Master is now with developers.
The reason for the limitations are related to an updated graphics architecture that is designed to improve OS X’s graphics subsystem, according to Ars Technica. Specifically, it’s the connection to 32-bit graphics drivers that’s the problem.
The report claims that, while Mountain Lion is compatible with any Mac capable of running a 64-bit kernel, the kernel does not support loading 32-bit kernel extensions (KEXTs).
It is thought that Mountain Lion will not download from the Mac App Store on Macs that do not meet the specifications.