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Why does arch from Terminal return i386 which would seem to indicate I'm not in "64-bit mode" when I know based on my processor I'm running a 64-bit Mac? What does that actually mean and how am I supposed to use the arch command.

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Did you hold down the 6+4 keys when booting? Even when running a 64 bit capable Mac (which I am as well) Snow Leopard (I presume) defaults to a 32-bit kernel unless specifically instructed to load a 64-bit kernel, in order to maximize device driver compatibility for legacy devices. Mac Pros, I believe, will boot into 64-bit mode by default, as will the XServe.

See this link for details.

Edited to add:
John Siracusa over at Ars Technica has a great description of what's going on and why with the 64-bit kernel vs. the 32 bit kernel. The consensus seems to be that if you regularly run 64 bit apps, like Lightroom, you can see some performance gain by booting to 64-bit. But if you use chiefly 32 bit apps, you'll see next to no increase in performance. Also, if you use virtualization (Parallels, VMWare, VirtualBox) you need to make sure you have 64-bit KEXTs before you can use those programs in 64-bit mode. So, like with so many other things, YMMV. I personally haven't tried using any virtualization software under 64 bit so I don't know if I have the KEXTs or not, nor have I tried to find them. Anyone else given that a try?

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Having said that, it's important to note that you can run 64-bit applications while still running the 32-bit kernel. Also, if using gcc (e.g. for MacPorts) the default arch is x86_64. – fideli Jan 18 '10 at 19:42
I don't hold those keys, but I've also got a MacBook Pro, so I'd guess it's running in 64-bit mode. What I'm not clear on is what the arch command is telling me. Is it telling me I'm running in 32-bit mode or something different? – Bialecki Jan 18 '10 at 19:44
@fideli- Good points both. I should have mentioned that. @Bialecki, your MacBook Pro is running in 64 bit mode, in that you can operate 64 bit applications. But the kernel itself is running in 32-bit mode. See my edits above. – atroon Jan 18 '10 at 20:12
Also check out this previous discussion:… – fideli Jan 19 '10 at 3:22

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