Apple has said that Mac OS X 10.6 Snow Leopard will run in full 64-bit mode on any 64-bit capable Intel processor. In fact, Snow Leopard's 64-bitness is one of its highly touted under-the-hood features. But so far, the purported golden master build still won't boot in 64-bit mode on a number of 64-bit capable models.

That build, 10a432, included release notes that detail running Snow Leopard in 64-bit mode, meaning the underlying kernel is taking full advantage of the 64-bit capabilities of the hardware. According to the document, Apple is still asking developers to develop and test 64-bit versions of any kernel extensions—most commonly device drivers—as Apple has been asking developers to do for some time. Snow Leopard running in 64-bit mode can only load drivers that are 64-bit, and though Apple provides many common hardware drivers, some devices simply won't work without a custom one.

Only Intel Xserves will boot 64-bit by default, and Apple lists a number of models that are capable of booting in 64-bit by holding down the '6' and '4' keys while starting up. Even still, not all machines with 64-bit processors will boot into 64-bit mode. Specifically, some earlier Core 2 Duo-based Macs have a 32-bit EFI firmware, and despite the fact that a 32-bit EFI can load a 64-bit kernel, Apple (so far) limits these machines from doing so. Ironically, a tool to enable hackintoshes to boot Mac OS X can enable Macs with 32-bit EFI firmware to boot Snow Leopard in 64-bit mode. Luckily, even if Snow Leopard boots into 32-bit mode, it can still load and run 64-bit applications just fine. And, oddly, Apple notes that Snow Leopard Server builds boot into 64-bit mode by default—why the client version still has the limitation isn't clear.

These details suggests to us in Orbiting HQ a couple of things, though. Apple says that Snow Leopard is part of its transition to 64-bit. While Snow Leopard is a huge step in that direction—all the included applications in Snow Leopard are 64-bit, for instance—not all of Apple's machines in the last few years are 64-bit capable (like the MacBook1,1 I'm typing on right now). Also, as of build 10a432, it's possible that even Apple might not have all the necessary device drivers for all 64-bit-capable Macs fully tested and ready. Still, since Apple lists a 64-bit Intel processor as the only requirement for 64-bit mode, we're sure Apple will have this sorted out before Snow Leopard officially launches.

Of course, whether booting every 64-bit capable machine into 64-bit mode by default is the right thing for users is still not certain. There are a number of performances gains to be had from doing so, but if it comes with compatibility headaches, Apple may be better off letting those will older machines decide for themselves to force 64-bit mode if they desire. Less savvy users will be none the wiser, as it will boot in 32-bit mode and run 64-bit applications without issue.