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.

Macs supported by Mountain Lion iMac (Mid 2007 or newer)

MacBook (Late 2008 Aluminum, or Early 2009 or newer)

MacBook Pro (Mid/Late 2007 or newer)

MacBook Air (Late 2008 or newer)

Mac mini (Early 2009 or newer)

Mac Pro (Early 2008 or newer)

Xserve (Early 2009)

Apple declined to tell us the reasoning behind leaving some of these models out of potential Mountain Lion upgrades, but we suspected 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 more specifically 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 does not support loading 32-bit kernel extensions (KEXTs). Furthermore, Macs with older EFI versions that are not 64-bit clean won't load Mountain Lion's 64-bit only kernel.

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.