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6GB RAM on a Tyan S2875 Tiger K8W - 500 MB missing

Hello *,

i upgraded the RAM of my workstation yesterday and have some problems
understanding what i'm seeing.

here is what i did: started with 4 GB of RAM, made up from 4x1GB
registered non-ECC modules. all working well. ripped apart a new server
that came with 2x2GB registered ECC modules (also worked well, so
hardware defect is out of the question). the plan was to put those 2x2
in my workstation, leaving 2x1 in place.

that's what i did and i saw only 4 GB, but that was kinda logical
because the 2x1 are non-ECC and i had to tell the BIOS about that. after
disabling ECC alltogether the machines boots with the BIOS reporting
57xx MB of memory. my kernel says:
Memory: 5746100k/7438336k available (2638k kernel code, 118684k
reserved, 996k data, 196k init)

here is what i dont get: 6x1024MB are 6144MB but BIOS and kernel report
only 5611MB - so where are my 533MB ?!

i first though this might be an due to the way i plugged the modules
into the DIMM slots and tried some other patterns. it turns out that the
ECC modules alone only work when put into DIMM1/DIMM2 or DIMM1/DIMM3 -
in combination  with the non-ECC modules it only works with the 2x2 in
DIMM1/DIMM2 and the 2x1 in DIMM3/DIMM4. again, ECC is disabled in the
BIOS completely - otherwise it wouldnt use the non-ECC modules at all.

so i'm thinking if this is some kind of artefact of some "memory hole"
i'm not aware of.

also, the manual of the board (Tyan Tiger K8W S2875) contains a little
chart that supposedly shows how 64bit (non-interleaved) and 128bit
(interleaved) memory configurations work. but with all i know about
computers i cannot interpret nor understand it:)

question is: where is my memory ?! it's too much to be a miscalculation
of some form (like the HDD manufacturers do it :) could anyone make
sense of my BIOS-provided physical RAM map if i posted it? what am i

thanks for any hint,


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