64-bit Snow Leopard defaults to 32-bit kernel

Apple's OS X 10.6 operating system Snow Leopard by default loads with a 32-bit kernel, despite running 64-bit applications.

Apple's OS X 10.6 operating system Snow Leopard, released today, 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.

See also: Snow Leopard: Special Report

"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.

"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.


An older mid-2007 MacBook with a 32-bit EFI chip, forced to a 32-bit kernel can't register the full 4GB RAM. (Credit: CBS Interactive)

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 lock out.

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 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.

Snow Leopard
Some processes are still running as Universal Binaries. (Credit: CBS Interactive)

Apple also released Snow Leopard Server today, but were unable to detail the reasoning behind dropping ZFS support for the operating system, a much touted feature during the development stage.

This article was originally posted on ZDNet Australia.