Question: I have installed 4GB of PC2-5300 DDR2 memory in my Intel 945PM chipset based laptop but it only reports 3GB. Why?
The memory controller found within the Intel 945PM mobile chipset codenamed Calistoga does indeed state support for up to 4GB RAM, this however is where the good news ends. Unfortunately, while this is true it is also a minor error made by Intel. The Intel 945PM mobile chipset cannot address all the 4GB of installed memory if there are other devices in the system also sharing the same memory addressing space (something that is true in every respect). As a result, if you wish to upgrade to 4GB RAM, the practical amount the system will see thus use is around the 3GB mark. This will not change regardless whether you install a 32bit or 64bit Operating System.
For example, if you install 4GB RAM (2x 2GB DDR2 PC2-5300 200pin SODIMM) and your system reports 3GB, 3.25GB, 3.5GB etc. then this is exactly how much memory you'll be able to use. The remaining amount will be invisible. How much your system sees is largely dependant on how much memory your graphics card features as it also needs to share the same addressing space.
Luckily, the successor to the Intel 945PM mobile chipset, namely the Intel PM965 mobile chipset codenamed Crestline supports memory remapping thus can utilise the full 4GB if and when installed.
Learn more about the aspects that limit how much you may upgrade your laptop's memory in part 4 of the Laptop Memory Upgrades Guide.
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