I upgraded the dual Opteron server (Tyan S2892 motherboard) from 2GB of RAM to 4GB, but the BIOS only finds 3GB of it. I’d put the memory on the second CPU. At first I thought perhaps one of the DIMMs wasn’t seated properly, so I reseated both. Still no go. Then it occurred to me that maybe there was a problem with one of the DIMM sockets or the memory controller on that CPU, so I tried putting the new DIMMs in the other pair of sockets on the second CPU, and I tried putting all four on the first CPU, but in all cases the BIOS only found 3GB.
I looked through the BIOS settings. There wasn’t anything obvious that would limit the memory size, though I did try changing a few things.
Today a Google search turned up a posting in an AMD support forum in which someone else had exactly the same problem. They found out from Tyan support that the “Memory hole” setting needs to be changed from “Hardware” to “Software”. I thought I ‘d tried that last night, but I’ll check it tonight.
This should really be in the motherboard manual, or in an addendum. Better yet, the BIOS could give an on-screen warning if it finds 3GB and the memory hole is set to hardware, telling the user that there might be more memory and to try setting the memory hole to software.
Update: There are three settings, “hardware”, “software”, and “disabled”. I originally had it set to “hardare”, and had tried setting it to “disabled”. Both resulted in the memory being limited to 3GB. Setting it to “software” solved the problem. The BIOS settings section of the manual offers this enlightening description of the “Memhole mapping” setting:
Remapping scheme for PCI memory hole.
It would be nice to know what each setting actually does; if they can’t explain it in the manual, they should at least reference some other document that does. It must be in a tech manual somewhere, or the people writing the BIOS wouldn’t have known how to control it.