The system requirements for the OS X 10.8 (Mountain Lion) developer preview released last week indicated that quite a few "modern," 64-bit capable Macs would not be able to upgrade to the new OS. How Apple determined the cutoff still remains fuzzy, but according to our research, we believe the issue is related to the graphics capabilities of those machines. Apple has suggested to Ars that the list is not set in stone, however, so some of the earlier Core 2 Duo Macs may end up making the final cut after all.

Why are some Macs unsupported?

When Lion was released last year, it was Intel-only and fully 64-bit, removing support for 32-bit Macs. That made the cutoff easy to determine: if you had a 64-bit Intel Mac with a Core 2 Duo or newer processor, you could upgrade to Lion.

The list of supported Macs for Mountain Lion's developer preview, however, left out many of the early Core 2 Duo models, and the reason seems to be due to graphics. Mountain Lion, according to Apple, includes a revamped graphics subsystem with additional reliance on OpenCL. "A new graphics infrastructure underpins OpenGL and OpenCL," and "new video APIs deliver modern 64-bit replacements for low-level QuickTime APIs," according to Apple's press release.

A major overhaul to OS X's graphics capabilities has been a long time coming. Macs were stuck on OpenGL 2.1 for far too long, though some Snow Leopard updates added extensions that supported functions from OpenGL 3. Lion finally added support for OpenGL 3.2, but that was after OpenGL had been updated to 4.1 and was already supported on Windows and Linux.

Our own Dave Girard noted last year that Lion's OpenGL 3.2 support "is playing catch-up to Windows and Linux, and it's pretty sad that we were stuck with 2.1 for as long as we were." He did say, however, that "the good news is that we won't be waiting as long for OpenGL 4.1 support," suggesting that Apple had plans to accelerate support for newer versions of OpenGL.

Whatever improvements Apple is making to the graphics subsystem in Mountain Lion, though, support for OpenGL 4.1 isn't there yet. A source with access to the developer preview told Ars that API calls are still limited to targeting "legacy" (OpenGL 2.1) or "3.2core." Developers using GLKit, an API that originated on iOS 5 for simplified OpenGL development, will require OpenGL 3.2 support, however.

Looking at the graphics hardware differences between Mac models that are currently supported in Mountain Lion, at least one clear pattern emerges: older Intel integrated graphics aren't going to be part of OS X's future going forward. MacBook, MacBook Air, and Mac mini models with integrated Intel GMA 950 or X3100 seem to be out. Newer models with NVIDIA 9400M, NVIDIA 320M, or Intel HD3000 appear ready for the new hotness.

The division for the iMac also appears to be pretty straightforward. The earliest Core 2 Duo models, which had a white polycarbonate enclosure, featured discrete GPUs with a 128MB of VRAM. The later aluminum enclosure iMacs, which are supported under Mountain Lion's developer preview, include GPUs with 256MB or more of VRAM.

The 13" MacBook Pro, which relies on integrated graphics, was first launched with an NVIDIA 9400M, so those are in the clear so far. With respect to 15" and 17" MacBook Pros, which have discrete GPUs from NVIDIA or AMD, the cutoff seems to happen with the transition to "Santa Rosa" CPUs from Intel. Those machines included an improved, faster front-side bus. Whereas the iMac has a clear 128MB/256MB VRAM split, though, some pre-Santa Rosa MacBook Pros have 256MB of VRAM, while one low-end 15" Santa Rosa MacBook Pro has only 128MB. Our best guess is that the improved bus speed might make up for the difference in VRAM, assuming it is a limiting factor.

The picture is even less clear with the Mac Pro, however. Even the earliest Mac Pros have 64-bit capable Xeon CPUs, fast front-side buses, and graphics cards with 256MB minimum VRAM. Why the earliest Mac Pro models were left out of the developer preview is a mystery.

To Be Determined

The evidence so far is inconclusive. There is clearly some connection to graphics capabilities--of that much we can be sure. However, when we spoke to Apple, the company cautioned that Mountain Lion is still in the early stages of readying for release this summer, and so far those limits only apply to the developer preview. "It's still early, and the system requirements for the final version have not yet been determined," an Apple spokesperson told Ars.

That's no guarantee that some of the earlier 64-bit Intel Macs will be able to upgrade to Lion, but how exactly Apple will make the cutoff will most likely relate to the new graphics subsystem, and whether or not it can be backported to those models that are currently excluded. From the information we have, it seems technically possible, at least for some Mac Pros and MacBook Pros.

But doing so may present support exigencies that Apple isn't prepared to meet. Snow Leopard dropped PowerPC support four years after the last G5-equipped machine rolled off the assembly line, and Lion dropped support for Rosetta and 32-bit Macs. It's clear that Apple is ready to leave older hardware behind when it comes to OS X upgrades, and Mountain Lion doesn't seem to be any exception.

UPDATE: Some readers have asked about EFI support, as some earlier 64-bit Macs are limited to a 32-bit EFI. As far as we can determine, this doesn't cut a clear line across the models Apple has listed as incompatible with the Mountain Lion developer preview. Furthermore, firmware updates could potentially address issues for those particular machines.

However, Andrew Cunningham over at AnandTech had some additional insights regarding the graphics capabilities to share. The earlier GPU models that didn't make the cut for Mountain Lion DP, including NVIDIA GeForce 7300 and ATI/AMD Radeon X1900, among others, were no longer getting active manufacturer driver support after Apple switched to a 64-bit kernel for Lion. Again, this may make supporting the new graphics subsystem on some of those older Macs tricky, if not outright impossible, and Apple may decide that resources needed to update the drivers for these GPUs might be better utilized elsewhere.