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.