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So I hear that Snow Leopard is not booting to "full" 64 bit mode by default, for compatibility reasons, and that one can bypass this by booting with the 6 and 4 keys pressed.

Is the 6+4 trick required in order to use more than 4 gb of RAM, or does the default mode handle this as well?

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You can access more than 4 GB of RAM even in Leopard. The 6+4 trick is not needed to access more than 4 GB in Mac OS X. The Mac OS X kernel is a 32 bit/64 bit hybrid, allowing it to bypass the 4 GB limitation.

I don't know if you've ever done something like this, but you can go to the Apple Store and just max out the RAM in a Mac Pro; you can add about 32 GB of RAM. This option was also available in Leopard, where the 6+4 trick was useless.

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Thanks! I know macs have shipped with more than 4 gb before, but I also read that the xserves ship with the 64-bit mode as default so I thought perhaps this was true for any machine ordered with more than 4 gb of RAM. Happy it's not so! –  Console Oct 13 '09 at 14:27
    
While you don't need 64 bit to access more than 4GB of ram in OS X you may benefit from 64 bit if you're doing a lot of intensive work such as 3d modeling, image processing (at very high resolutions) and so on. –  Jasarien Oct 14 '09 at 19:49
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You only need 64bit addressing if any single process needs more than 4GB worth of memory all for itself.

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