This is an excerpt from old thread about 4GB RAM limitation:
"I got this message from Tyan support regarding the 4GB memory issue:
Q. Why does my OS see less than the total memory installed when I install
4GB or more of memory (typically 512MB less)?
A. The BIOS needs to overlay the APIC, ACPI Table, AGP Aperture and PCI MMIO
(Memory-mapped I/O [see PCI Spec 2.3, Section 3.2.2 for more information])
over the last 512MB of the 4GB physical address space. OS accessible memory
and these structures cannot both exist at the same place and this portion of
DRAM is hidden and unavailable to the OS.
Q. Is there a solution for the missing memory when using 4GB of total
memory?
A. Not easily, the theoretical possibility exists that the BIOS can map all
of the addresses attached to one DIMM module above the 4GB limit, but the
BIOS cannot move smaller address ranges piece by piece. Mapping a whole DIMM
is a new concept, unproven in real world testing. It also penalizes 32-bit
OS's that cannot use more than 4GB. Since the BIOS does not know what OS you
have when it does the memory assignments, it has to optimize for the common
case, which is likely a 32-bit OS you may or may not want to use. In a
system with less than 4GB the BIOS must choose between providing as much as
possible below 4GB to benefit 32-bit legacy OS users or raise one whole DIMM
module above the 4GB ceiling to benefit 64-bit OS 's at the loss of DRAM to
a much more memory limited 32-bit OS. The current release of the the S2885
bios incorporates the benefit to a 32 bit OS in regards to memory
allocation."
You should research more AMD forums and decide whether this explanation is
true or false.
