On Tue, Sep 27, 2005 at 06:37:50AM -0700, Max wrote:
I'm suffering the same problem on S2875. While is it possible to make Linux
seeing all 4GB of memory available, that causes kernel traps on attempts to
use higher memory regions. So I'm forced to live with smaller amount of
memory visible in favor of system stability.
Max
From the official Tyan FAQ:
===
Why does my OS see less than the total memory installed when I install 4GB
or more of memory (typically 512MB less)?
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.
Is there a solution for the missing memory when using 4GB of total memory?
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.
What a load of crap. They don't have to decide which way to optimize
it. They should let the end user pick that with a bios option just as
almost every other athlon 64/opteron board has done.
Time for them to make a bios update with proper options in it if that is
really the current state of their bios.
Len Sorensen