It turns out that there really is a reason why Apple states on their site that the MacBook Pros can only handle up to 3 GB of RAM. The reason actually has to do with the chipset on which the MacBook Pro (and various other laptops for that matter) run.

The Intel 945PM chipset that the MacBook Pro can physically handle the 4GB of RAM that you’d expect, but there are problems with doing this. A certain amount of the memory address space is set aside for various system functions aside from memory access. Since the chipset only has 4GB of address space, and up to 1GB of it is used to access devices other than memory, it is possible to have a conflict that prevents you from accessing all of your available memory. While other manufacturers have kept quiet about this limitation and continued to sell up to 4 GB of RAM in their configurations (with no guarantee of all 4 GB actually being usable), Apple decided to be upfront about it and say that the laptop only supports 3 GB of RAM, because that’s all your guaranteed to have access to with the chipset.

More on why units are limited to 3 GB RAM [MacFixIt via ZDNet]

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