We've arrived at the start of the customary "internals" section. Snow Leopard is all about internal changes, and this is reflected in the content of this review. If you're only interested in the user-visible changes, you canskip ahead, but you'll be missing out on the meat of this review and the heart of Apple's new OS.
As we'll see, all that yellow in the Snow Leopard diagram represents its capability, not necessarily its default mode of operation.
Snow Leopard is the first version of Mac OS X to ship with a 64-bit kernel ("K64" in Apple's parlance), but it's not enabled by default on most systems. The reason for this this is simple. Recall that there's no "mixed mode" in Mac OS X. At runtime, a process is either 32-bit or 64-bit, and can only load other code—libraries, plug-ins, etc.—of the same kind.
An important class of plug-ins loaded by the kernel is device drivers. Were Snow Leopard to default to the 64-bit kernel, only 64-bit device drivers would load. And seeing as Snow Leopard is the first version of Mac OS X to include a 64-bit kernel, there'd be precious few of those on customers' systems on launch day.
And so, by default, Snow Leopard boots with a 64-bit kernel only on Xserves from 2008 or later. I guess the assumption is that all of the devices commonly attached to an Xserve will be supported by 64-bit drivers supplied by Apple in Snow Leopard itself.
Perhaps surprisingly, not all Macs with 64-bit processors are even able to boot into the 64-bit kernel. Though this may change in subsequent point releases of Snow Leopard, the table below lists all the Macs that are either capable of or default to booting K64. (To find the "Model name" of your Mac, select "About This Mac" from the Apple menu, then click the "More info…" button and read the "Model Identifier" line in the window that appears.)
Early 2008 Mac Pro
Early 2008 Xserve
MacBook Pro 15"/17"
UniBody MacBook Pro 15"
UniBody MacBook Pro 17"
Early 2009 Xserve
For all K64-capable Macs, boot while holding down "6" and "4" keys simultaneously to select the 64-bit kernel. For a more permanent solution, use the nvram command to add arch=x86_64 to your boot-args string, or edit the file /Library/Preferences/SystemConfiguration/com.apple.Boot.plist and addarch=x86_64 to the Kernel Flags string: