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Now that Snow Leopard brings 64-bit computing to MacBook Pros with Core 2 Duos, does this increase the maximum amounts of ram that can be installed?

I currently have 4GB of RAM in my MBP 3,1 - could I use more ram if I ran the MBP in 64-bit kernel mode?

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A 64-bit kernel can access more RAM without using Physical Address Extension (PAE), and allow processes to use more than 4GB of actual memory without having to map some of it into virtual memory. So no, it does not actually increase the maximum amount of RAM you can use in a MacBook Pro. A Mac Pro or an Xserve it does to a point (32GB. Greater than 32GB and you'll want to use the 64-bit kernel)

If your MacBook Pro physically supported more than 4GB of RAM both Leopard and Snow Leopard would be able to see it. According to MacTracker your motherboard actually supports up to 6GB of RAM instead of the 4GB maximum Apple says it does.

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  • Will the Apple firmware allow me to access that extra 2GB of RAM?
    – Darren Newton
    Sep 13, 2009 at 22:46
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    Technically, we say that it can "address" more ram.
    – Nippysaurus
    Sep 14, 2009 at 0:43
  • The hardware itself can access 6GB of RAM - and Mac OS X 10.5 and 10.6 will address it.
    – Chealion
    Sep 14, 2009 at 2:23
  • MacRumors has more information on this here: forums.macrumors.com/showthread.php?t=573906 - Chealion is correct that MBPs from 3,1 up can handle 6GB or more, although 8GB is apparently problematic.
    – Darren Newton
    Sep 14, 2009 at 12:40
  • @knweiss: Ooops. Thanks I meant to write 'without'. Thanks for catching that.
    – Chealion
    Sep 19, 2009 at 16:33
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It can but you are still limited to your chipset. My MBP is limited to 3GB nothing 10.6 can do about that.

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  • Unfortunately, I am in the same situation :(
    – Alexis Hirst
    Sep 13, 2009 at 23:04
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No, you already can use more than 4 GB ram with a 32-bit kernel that uses Physical Address Extension (PAE) (Mac OS X's kernel does). However, there are several other advantages of a 64-bit kernel. One of them being that much more memory can be used for kernel caches because of the larger (kernel) address space.

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There are two limits:

  • Hardware limit. This is the amount of memory which the hardware can handle. Unfortunately 10.6 can not upgrade your hardware.

  • Software limit. The upgrade to 10.6 will allow the operating system to access more of the memory which your hardware can provide, but if your hardware cannot handle the ram, then neither can your operating system.

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