Apple's OS X 10.6 operating system Snow Leopard, released last week, by default loads with a 32-bit kernel, despite running 64-bit applications.
While it ships with a number of 64-bit native applications, Apple's kernel itself defaults to 32-bit, unless the user holds down the "6" and "4" keys during boot time, at which point the 64-bit kernel is loaded. Only Apple's X-Serve products, using Snow Leopard Server, boot into a 64-bit kernel by default.
"For the most part, everything that they experience on the Mac, from the 64-bit point of view, the applications, the operating system, is all going to be 64-bit," Stuart Harris, software product marketing manager at Apple Australia said.
Harris said that at this stage there were very few things, such as device drivers, that required 64-bit mode at the kernel level but the option was available. The decision to boot in 32-bit was to ease the transition to 64-bit, mainly for third party drivers that may currently be 32-bit only, such as those for printers and scanners.
"But we're trying to make it as smooth as possible, so people don't end up finding that 'oh, that doesn't work' because it's not available yet," he said.
There appears to be no way within the GUI to permanently boot the 64-bit kernel, requiring the editing of the com.apple.Boot.plist file to make the change--a text-based configuration file. Users have already released applications to address this issue.
Older Macs with a 32-bit EFI chipset are prevented from loading the 64-bit kernel, although there are claims that this is an arbitrary decision by Apple rather than a technical concern, with a hack using the Chameleon boot loader devised to get around the lock out.
It is unknown at this stage what sort of performance implications running 64-bit applications on a 32-bit kernel will have compared to a 64-bit kernel.
While a large portion of the OS has been optimized and updated, some applications, such as DVD player, Front Row, Grapher and iTunes are still 32-bit only, and some extensions are still run as a Universal Binary, despite the PPC architecture no longer being supported.
Apple also released Snow Leopard Server last week, but were unable to detail the reasoning behind dropping ZFS support for the operating system, a much touted feature during the development stage.