Build notes leaked on the web of a prerelease version of Mac OS X 10.6 Snow Leopard indicate that the software only supports enabling its new 64-bit kernel on certain machines, including the Xserve, Mac Pro, and MacBook Pro, but this does not mean Snow Leopard's kernel will be limited to 32-bit operation on consumer machines.
A 64-bit kernel requires all of its extensions to also be 64-bit. Kernel extensions or KEXTs include drivers for audio hardware, graphics adapters, networking, certain printing components, and other devices on the logic board or attached as peripherals. Until Apple delivers 64-bit versions of the nearly 300 extensions it ships with Mac OS X (not all of which will need to be supported on 64-bit Macs; many are legacy), it is limiting official 64-bit kernel support to a subset of Macs in prerelease builds of the new operating system.
Snow Leopard will deliver the first 64-bit kernel for Mac OS X. Earlier versions of the operating system, including today's Leopard, can run 64-bit software but do so using a 32-bit kernel. More accurately, whether running on 32 and 64-bit CPUs, Mac OS X loads the same kernel image and run it as a 32-bit process, although when run on 64-bit hardware, the 32-bit kernel switches into "long mode compatibility mode."
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