We previously reported that while the MacBook Pro Core 2 Duo can physically accomodate two 2 GB RAM modules for a total of 4 GB, Apple's specs list the model as able to recognize a maximum of 3 GB of RAM. Today we have some some additional information regarding why the units can only recognize 3 GB, and the questionable nature of other manufacturers' claims that their Core 2 Duo-based portables can address up to 4 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 that 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.
This is not a Mac-specific issue, and it calls into question the claims some other manufacturers are making about 945PM chipset-based models being able to accept 4 GB of RAM. For example, HP's Compaq NC8430 makes use of the Intel 945PM chipset, and claims a maximum of 4 GB RAM with no caveats listed.
UPDATE: Some readers noted that HP does provide a technical specifications page for the NC8430 stating that "all memory may not be available" above 3 GB. Still, the online HP store allows users to purchase 4 GB of RAM as a shipping option with no apparent indication of the limitation.
In essence, Apple's 3 GB limit appears to be accommodation on Apple's part, discouraging users from installing expensive memory that is not addressable by the system and refraining from falsely advertising a 4 GB limit.
- MacBook Pro Core 2 Duo (Late-2006) (#2): Expensive 2 GB modules required for maximum RAM, cannot be matched
- MacBook Pros updated with Core 2 Duo processors