THE OFFICIAL BLOG OF THE SBS "DIVA"

When is 4 gigs of ram not 4 gigs of ram?

Based on my research and experiences, this issue occurs because the
available system RAM on a 32-bit computer is reduced to allow space within
the first 4 GB of address space for PCI hardware configuration
requirements. This behavior is expected on 32-bit computers that have 4 GB
of RAM installed.

Below is a workaround for this issue:

To enable a 32-bit version of Windows Vista to access more of the installed
RAM below 4GB that is reserved for hardware, enable Physical Addressing
Extension (PAE) in Windows Vista boot settings. First, your computer must
meet these requirements:

1. The chipset must support at least 8 GB address space. Chipsets with this
capability include the following chipsets:  Intel 975X, P965, and 955X on
Socket 775. 
2. Chipsets that support AMD processors that use sockets F, 940, 939 & AM2.
These are AMD socket/CPU combinations where the memory controller resides
in the CPU. 
3. The processor (CPU) must support the x64 instruction set. AMD64 and
Intel EM64T CPUs support the x64 instruction set. 
4. The BIOS must support the Memory Remapping feature where the segment of
system RAM previously overwritten by PCI configuration space is remapped
above the 4 GB address line. This feature must be enabled in the system's
BIOS configuration utility. See the product documentation for the computer
for information about how to enable this feature. Many consumer-oriented
computers may not support Memory Remapping. There is no standard
terminology for the feature in documentation or BIOS configuration screens.
Therefore, you may have to read the descriptions of the various BIOS
configuration settings that are available to determine which, if any,
result in this feature being enabled.

Contact the manufacturer or retailer to determine whether the computer
meets these hardware requirements. If your computer meets these
requirements, you can use the /PAE option in the Boot Configuration
Database (BCD) file. To do this, log on to the computer by uning an account
that has administrator privileges, and then type the following command at
the command prompt: "Bcdedit /set ID PAE ForceEnable" (without quotation)

The resolution of this issue is that to access 4 GB of RAM or more in
Windows Vista, make sure that the computer is running a 64-bit version of
Windows Vista.

Note: Some drivers might fail to load if PAE is enabled, because the device
might be unable to perform 64-bit addressing or the drivers might assume
that PAE mode requires more than 4 GB of RAM. For more information, see the
More Information section.

For more information, please refer to the following KB article:

Windows Vista or Windows Server 2003 may report less memory than you expect
http://support.microsoft.com/kb/929580