As jingle-pundits desperately try to denigrate Snow Leopard as a "Service Pack," Apple's new operating system reference release actually expands the reach of the Mac platform in several important and under-reported new directions. Here's the second in a series looking closer at some of Snow Leopard's well-known, but often misrepresented or misunderstood features. It seems fashionable to describe Snow Leopard's new 64-bit kernel as a problem for Mac users with 32-bit EFI (the startup firmware that launches the operating system). It's true, 64-bit Core2 Duo machines prior to 2008 still run Snow Leopard's 64-bit apps using a 32-bit kernel, because Apple's 64-bit kernel requires both a 64-bit processor (a Core2 Duo or better) and 64-bit EFI. However, running a 64-bit kernel on these machines is of limited benefit. While there are certain advantages with the move to a 64-bit kernel, including new security enhancements , the primary benefit of a 64-bit kernel is being able to directly work with significantly more than 4GB of RAM, something that most existing consumer Macs and generic PCs can't do anyway. Users who want to run the new 64-bit kernel on late modeled Macs (pretty much anything released after early 2008) can do so by booting with the 6 and 4 keys held down. If you're wondering whether your Mac has a 64-bit EFI firmware, you can type the command "ioreg -l -p IODeviceTree | grep firmware-abi" into the Terminal. The response will identify the machine as either having 32-bit or 64-bit EFI. What Snow Leopard does do is bring all Core2 Duo, 64-bit Macs (pretty much everything sold since 2007) up to speed with 64-bit system apps, from the Finder and Dock to iChat and Mail to background processes such as launchd and the system-wide spell checker. Running

Source: Apple Insider