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Confirmed: Mountain Lion sends some 64-bit Macs gently into that good night

Rage if you might, but it appears the reason is related to graphics drivers.

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Mountain Lion won’t support all 64-bit Macs when it ships in late July.

Apple has now confirmed via its Mountain Lion upgrade page that previously published limitations on some 64-bit Macs will extend to the final release. This means that several otherwise 64-bit capable MacBook Pros, iMacs, and Mac Pros will indeed be restricted from upgrading to OS X 10.8 when it goes public later this month. And according to information found in the recently released golden master (GM), the limitation appears to be related to graphics, as we originally suspected.

When the first developer preview of Mountain Lion was seeded to developers earlier this year, the release notes listed hardware requirements showing that some early 64-bit Mac models were not compatible. (Lion is likewise 64-bit, and can run on any Core2 or newer 64-bit Intel processor.) As such, Mountain Lion developer previews would not run on the earliest Mac Pros, MacBook Pros, iMacs, and other hardware.

Apple declined to tell us the reasoning behind leaving some of these models out of potential Mountain Lion upgrades, but we suspect it is related to an updated graphics architecture that is designed to improve OS X’s graphics subsystem going forward. Our own Andrew Cunningham suspects the issue is related to graphics drivers, since the GPUs not supported under Mountain Lion have drivers that were written before 64-bit support was common.

Information included with the first Mountain Lion GM now corroborates the connection to 32-bit graphics drivers as the culprit. While Mountain Lion is compatible with any Mac capable of running a 64-bit kernel, the kernel no longer supports loading 32-bit kernel extensions (KEXTs).

As you might have already guessed, graphics drivers are KEXTs under OS X. And the GPUs in some of those early 64-bit Macs were deprecated before 64-bit KEXTs became common. Since those older drivers are 32-bit, Mountain Lion won’t load them. We believe Apple decided it was better to draw the line in the sand for some older machines rather than invest the resources into updating the drivers for these older GPUs.

While Apple had suggested that the hardware limitations were not set in stone back in February, it seems anyone hoping for additional support before Mountain Lion is released will be disappointed. Look on the bright side: both Snow Leopard and Lion are likely to get security fixes for at least the next year, so your machine should continue to hum along fine for now. If Mountain Lion contains updates that are useful to you, however, it may be time to start shopping for a newer Mac.

Rykin wrote:
So I wonder if they have programmed ML specifically not to run on these machines or if say you have an older Mac Pro that has been upgraded to a supported GPU if it will let you install. I am willing to bet that soon after or likely even before it ships there will be hacks to get it running on unsupported machines. XPostFacto makes a dramatic return ;)

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.

Confirmed: Mountain Lion sends some 64-bit Macs gently into that good night

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