While OS X 10.8 Mountain Lion will usher in a new day for many Mac owners, for others it will mean the end of the line as they discover that the OS isn't compatible with their existing system.

Apple confirmed on its official Mountain Lion upgrade page what was rumored back in February - older Macs will not support the upgrade despite being 64-bit systems.

The list of supported systems includes:

More Macs left out in the cold

Unfortunately, this means that many older Macs are still left out, including older Mac Pro, Mac Mini, and even the original MacBook Air systems.

The culprit is suspected to be the GPUs in older Macs, which Ars Technica points out are still based on 32-bit graphics drivers created before 64-bit became standard.

OS X 10.6 Snow Leopard saw a similar culling of older Macs, phasing out support for PowerPC systems.

OS X 10.8 Mountain Lion will come packed with a number of new features bringing the experience closer to that of iOS with integration of iCloud and iMessage, as well as AirPlay mirroring, Share Sheets, an improved Safari, and Gatekeeper security.