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Why are the MacBook Pro "Core 2 Duo" models limited to 3 GB of memory? If I install dual 2 GB modules, why won't I be able to use 4 GB of RAM?
Please note that this answer refers to the "Late 2006" MacBook Pro "Core 2 Duo" series. These models were replaced by the "Mid-2007" MacBook Pro Core 2 Duo "Santa Rosa" series on June 5, 2007. The "Mid-2007" and subsequently released MacBook Pro models officially are capable of supporting at least 4 GB of RAM.
If you install a 2 GB SO-DIMM in both the bottom and top memory slots of the computer, the About This Mac window and Apple System Profiler will both show that you have 4 GB of SDRAM installed. However, Activity Monitor and other similar applications will reveal that only 3 GB of SDRAM has been addressed for use by the computer.
This applies to the "Late 2006" iMac "Core 2 Duo" models -- the iMac "Core 2 Duo" 2.0 17-Inch, 2.16 20-Inch, and 2.16 24-Inch -- as well, as these systems also officially support a maximum of 3 GB of RAM.
The MacBook Pro Core 2 Duo presumably uses Intel's 945PM chipset, which can physically handle 4 GB of DDR2 RAM. However, a number of items must be stored in physical RAM space, and when RAM reaches 4 GB, there is some overlap.
In other words, in a 3 GB RAM configuration, there is no overlap with the memory ranges required for certain system functions. Between 3 GB and 4 GB, however, system memory attempts to occupy space that is already assigned to these functions. For instance, the PCI Express RAM allocation occurs at somewhere around 3.5 GB of RAM and requires 256 MB of RAM. Thus, the virtual space between 3.5 GB of RAM and 3.75 GB of RAM is occupied by PCI Express data. So in a system with 3 GB of RAM, nothing is being wasted because the memory space required by PCI Express is still between 3.5 and 3.75 GB, and the installed system RAM does not violate this space.
The net result is that at least 3 GB of RAM should be fully accessible, while when 4 GB of RAM installed, ~700 MB of of the RAM is overlapping critical system functions, making it non-addressable by the system.
Ultimately, 4 GB of RAM may be installed, but not all RAM in excess of 3 GB can be used due to the possibility of memory "overlap".
For highly technical details, one also may choose to refer to the Intel datasheet for the Mobile Intel 945 Express chipset. Chapter 9, starting on page 317, and in particular pages 324-325 may be of interest.