If you thought having a modern 64-bit capable Mac would be enough to ensure that Mountain Lion would run on your computer, you'll be disappointed.
Some 64-bit capable Macs will not be able to run Mountain Lion, due to graphics limitations.
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). Read more here.
It is thought that Mountain Lion will not download from the Mac App Store on Macs that do not meet the specifications.
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