The 3+ GB limit on RAM
As RAM has become cheaper and cheaper and Vista has gobbled up available RAM. increasing numbers of PC users have been installing a pair of 2 GB sticks of RAM. Then they find that the system only reports that they have 3.1 GB or even less and they wonder what happened. I touched on this briefly when I posted about 64-bit systems. The upper limit on address space for 32-bit systems is 4 GB but some of that is used by various system processes or graphics. Here is an excerpt from a Microsoft knowledge base article:
This behavior is the expected result of certain hardware and software factors.
Various devices in a typical computer require memory-mapped access. This is known as memory-mapped I/O (MMIO). For the MMIO space to be available to 32-bit operating systems, the MMIO space must reside within the first 4 GB of address space.
For example, if you have a video card that has 256 MB of onboard memory, that memory must be mapped within the first 4 GB of address space. If 4 GB of system memory is already installed, part of that address space must be reserved by the graphics memory mapping. Graphics memory mapping overwrites a part of the system memory. These conditions reduce the total amount of system memory that is available to the operating system.
The reduction in available system memory depends on the devices that are installed in the computer. However, to avoid potential driver compatibility issues, the 32-bit versions of Windows Vista limit the total available memory to 3.12 GB. See the “More information” section for information about potential driver compatibility issues.
If a computer has many installed devices, the available memory may be reduced to 3 GB or less. However, the maximum memory available in 32-bit versions of Windows Vista is typically 3.12 GB.
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