2007 Intel chipset should remove effective 3 GB RAM limit on some Macs

We previously reported that while the Core 2 Duo-based MacBook Pro (Late 2006) can physically accommodate 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. The same limitation applies to Apple's Core 2 Duo-based iMacs.

The 3 GB spec results from Apple's attempt to protect users against purchasing memory that will not be usable by their systems.

Current Core 2 Duo-based Macs (MacBook Pro, iMac) share a chipset that -- while capable of carrying 4 GB of RAM -- can recognize and make use of only slightly more than 3 GB. This is because the currently used chipset assigns to critical system functions and components (the PCI bus, etc) a ~700 MB chunk of memory space in the 3-4 GB memory range that cannot be used by the operating system.

Note: Apple's documentation [MacBook Pro or iMac developer notes] does not specify which chipset is used in its Core 2 Duo Macs. However, the chipsets used in currently shipping MacBook Pros and iMacs are in the Intel 945 series.

In systems with less than 3 GB of RAM, the occupation of this space is trivial -- no physical RAM exists there, so the devices happily occupy an otherwise empty range. In systems with more than about 3.3 GB of RAM, a conflict arises. The Intel 945 chipset is 32-bit. A 32-bit processor/chipset can address a maximum 4 GB of RAM, so the motherboard is built to accept only 4 GB. However, all system functions have registers that occupy mapped memory space. Many of these functions occupy tiny portions of space -- 4 KB for the Memory Control Hub MMIO, for instance. Some take significantly larger portions: The PCI Express Register Range occupies 256 MB. In aggregate, these items use about 700 MB of memory space -- yielding the ~3.3 GB ceiling for OS-addressable RAM.

Speculation holds that this limitation will be overcome when the Core 2 Duo (Merom) gets a chipset better suited for its 64-bit capabilities, and with a memory addressing scheme that does not require virtual space for system functions/components between 3 GB and 4 GB. This more apt chipset should show up in the first half of 2007, and holds the moniker Santa Rosa.

Santa Rosa will be the chipset for which the Core 2 Duo (Merom) was designed (the 945 chipset is better suited to the original Core Duo, but can accept the Core 2 Duo as well). It eliminates the ~3 GB memory cap, and more importantly, sports the capability to access memory in the 64-bit range, opening the door for portables that can carry 8 or 16 GB of RAM (or more), power considerations aside.

According to Intel's press materials, Santa Rosa will also provide: "an improved graphics chipset, codenamed Crestline, an IEEE* 802.11n Wi?Fi adapter, codenamed Kedron, as well as Intel?optimized advanced management and security solutions. The platform will also include Intel's NAND flash?based platform accelerator, codenamed Robson, which enables much more rapid boot?up time and power savings"

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