64-bit Snow Leopard defaults to 32-bit kernel

Apple's updated OS ships with a number of 64-bit native applications, but Apple's kernel itself defaults to 32-bit, unless the user holds down the "6" and "4" keys during boot time.

Craig Simms
Craig Simms Special to CNET News
Craig was sucked into the endless vortex of tech at an early age, only to be spat back out babbling things like "phase-locked-loop crystal oscillators!". Mostly this receives a pat on the head from the listener, followed closely by a question about what laptop they should buy.
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Apple's Snow Leopard operating system, released Friday, by default loads with a 32-bit kernel, despite running 64-bit applications.

While Mac OS X version 10.6 ships with a number of 64-bit native applications, the 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 is available.

"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 make this change permanent, 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 lockout.

An older mid-2007 MacBook with a 32-bit EFI chip can't register the full 4GB RAM. Screenshot by CBS Interactive
This raises another problem: if equipped with 4GB RAM, those with a 32-bit EFI chip are not capable of making use of the full amount. Our mid-2007 MacBook only showed 3GB available for system use through Activity Monitor despite having 4GB installed, not delivering the true 64-bit experience. Newer machines equipped with a 64-bit EFI chip running a 32-bit kernel seemed capable, with Activity Monitor reporting 3.75GB RAM available, as 256MB was put aside for the GeForce 9400M graphics chip.

It is unknown at this stage what sort of performance implications running 64-bit applications on a 32-bit kernel will have compared with a 64-bit kernel.

While a large portion of the OS has been optimized and updated, some applications, such as the 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 on Friday, but was unable to detail the reasoning behind dropping ZFS support for the operating system, a much-touted feature during the development stage.

Some processes are still running as Universal Binary. Screenshot by CBS Interactive

Craig Simms of ZDNet Australia reported from Sydney.