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Subjectlost memory on a 4GB amd64

Hello,

A friend of mine has a new Opteron based machine (Tyan Tiger K8W with two
Opteron 24?) and 4GB main memory.

the problem is that about 512 MB of that memory is lost (AGP aperture and
stuff). Although everything is perfect otherwise.
As far as I understand, all the PCI/AGP hardware uses the top end of the
4GB address range to access their memory and there is just an
"overlapping" of the addresses. thus only the remaining 3.5 GB are
available.


Now there is an option in the BIOS called "Adjust Memory" which puts a
certain amount of memory (several choices between 64MB and 2GB) above the
4GB address range. I tried the 2GB setting which results in 2GB main
memory at addresses 0-2GB and 2GB memory at addresses 4GB-6GB.

the problem is that the kernel (2.6.3-9mdksmp and vanilla 2.6.8.1) crashes
if this option is enabled as soon as some memory expensive program is run
(e.g. X)

I've seen some postings on the net talking about some "kernel patch" for
some "memory split", but nothing more specific. Do I just need a certain
patch to get it working or is there more to it?



BTW, the memory map displayed at boot is

BIOS-e820: 0000000000000000 - 000000000009fc00 (usable)
BIOS-e820: 000000000009fc00 - 00000000000a0000 (reserved)
BIOS-e820: 00000000000e0000 - 0000000000100000 (reserved)
BIOS-e820: 0000000000100000 - 00000000d3ff0000 (usable)
BIOS-e820: 00000000d3ff0000 - 00000000d3fff000 (ACPI data)
BIOS-e820: 00000000d3fff000 - 00000000d4000000 (ACPI NVS)
BIOS-e820: 00000000ff780000 - 0000000100000000 (reserved)

if I leave the 4GB memory in one chunk and

BIOS-e820: 0000000000000000 - 000000000009fc00 (usable)
BIOS-e820: 000000000009fc00 - 00000000000a0000 (reserved)
BIOS-e820: 00000000000e0000 - 0000000000100000 (reserved)
BIOS-e820: 0000000000100000 - 000000007fff0000 (usable)
BIOS-e820: 000000007fff0000 - 000000007ffff000 (ACPI data)
BIOS-e820: 000000007ffff000 - 0000000080000000 (ACPI NVS)
BIOS-e820: 00000000ff780000 - 0000000100000000 (reserved)
BIOS-e820: 0000000100000000 - 0000000180000000 (usable)

if I enable the "adjust memory" option and split the memory in two 2GB
blocks.

Thanks in advance,

Sergei
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