I just heard this the other day..
That Snow Leopard does not run in true 64-bit mode unless you TELL it to...by holding down "6" and "4" while the machine boots.
Anyone else know about this?
It's true, but really depends on the Mac. "Most" intel Macs will require the reboot with the 6 and 4 keys held down to boot in 64bit mode. Some of the latest models or at least one of the latest models can boot natively in 64bit without holding down the keys.
The real question is, do you need to run in 64bit mode if you have less than 32GB of RAM?
You can never go wrong by doing the right thing.
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One of the main reasons is because while programs can run in 64 bit mode no matter of the kernel is running 32 or 64, device drivers and kernel extensions can't - they have to run in the same mode as the kernel. Since there are very few (if any) 64 bit device drivers/kernel extensions, it's safer to run in 32 bit mode as a default for now.
Any *program* that wants to run in 64-bit mode (almost everything Apple-suppied on the system, for instance) runs in 64 bit mode. Really, there are very few programs that can take advantage of 64 bit mode anyway besides large number-crunchers or large memory users (video/photography/CAD apps). But for most people, they'll never notice.
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