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Operating Systems => Mac OS X 64bit (Intel) => 10.8 Mountain Lion (Jul 2012) => Topic started by: chrisNova777 on July 12, 2020, 06:47:23 PM
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https://arstechnica.com/gadgets/2012/07/confirmed-mountain-lion-sends-some-64-bit-macs-gently-into-that-good-night/
On those older Mac Pros that shipped with unsupported graphics cards, Apple hasn't deigned to update their firmware to support 64-bit EFI. You can run this command in Terminal to see whether you have a 32-bit or 64-bit EFI:
ioreg -l -p IODeviceTree | grep firmware-abi
All of the Macs dropped by Mountain Lion are going to return "EFI32" - if you can hack around that in addition to upgrading your graphics card, you may indeed be able to get Mountain Lion running on an older Mac, but it's a bit of a long shot.