Linked by moondevil on Wed 11th Jul 2012 22:49 UTC
Mac OS X Ars Technica is reporting that certain 64bit Mac models won't be able to run Mountain Lion. The problem is the graphic card drivers; these are still 32bit, and Apple is unwilling to update them to 64bit. A 64bit kernel can't load 32bit drivers, so that's that. Apple has a list of supported models on their Mountain Lion upgrade page, so you can easily check if your computer is capable of running Mountain Lion.
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Not graphic card drivers
by cm49 on Thu 12th Jul 2012 08:22 UTC
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From what I understand, these machines aren't supported because of the firmware. To boot the 64-bit kernel, the EFI needs to be 64-bit as well. So these no longer supported machines are the same machines that couldn't boot Snow Leopard or Lion in 64-bit mode.(I remember managing to do this on my unsupported iMac though. Had to use Boot Camp and the chameleon boot loader from OSx86).

So, this is not because of graphics drivers, but rather that Mountain Lion no longer contains a 32-bit kernel.
I don't know why the EFI needs to be 64-bit though, seeing as these macs boot both 64-bit Windows and Linux just fine using Boot Camp.

Edited 2012-07-12 08:25 UTC

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