I've installed 2x 1 GB and 4x 512 MB memory into my GA-8KNXP system which would sum up to 4 GB. The specification from the manual says:
Maximum memory support: 4 GB. If all six slots are utilized, slot 5+6 may only equipped with single-sided RAM modules.
And so I did.
Anyway: The BIOS counts up to 3.5 GB (minus AGP aperture size) and finishes there. Also my Linux system reports only 3.5 GB of memory although 4 GB memory support is activated in the kernel.
So I suppose this is a memory mapping issue or a hardware issue. I've tried removing only on of the 512 MB memory modules leaving 5 modules in place. But that just stopped the system from powering on correctly (screen stays black although fans and LEDs come to live). Dual Channel was detected and enabled so the system technically found all 6 modules.
dmidecode in Linux reports only memory in slots 1 to 4 and ignores slots 5+6, so it only detects 3 GB of memory. It also says the system would support up to 16 GB of memory with 4 GB modules per slot. I think technically the chipset should be able to offer and utilize the complete 4 GB memory range.
Any clues what else I could check? Or do I have just to live with 0.5 GB wasted memory?
Update: It seems that the first mapped memory (my 256 MB graphic card) has 256 MB memory mapped at 0xE0000000, which imho resolves as the 3.5 GB address. But it also seems this is not mapped as a memory hole but every memory range following that range is simply ignored by the BIOS. The BIOS revision is already latest (F13). Here's the
01:00.0 VGA compatible controller: nVidia Corporation G70 [GeForce 7800 GS] (rev a2) (prog-if 00 [VGA controller]) Subsystem: nVidia Corporation Device 035d Flags: bus master, 66MHz, medium devsel, latency 248, IRQ 16 Memory at f4000000 (32-bit, non-prefetchable) [size=16M] Memory at e0000000 (32-bit, prefetchable) [size=256M] Memory at f5000000 (32-bit, non-prefetchable) [size=16M] [virtual] Expansion ROM at f6000000 [disabled] [size=128K] Capabilities:  Power Management version 2 Capabilities:  AGP version 3.0 Kernel driver in use: nvidia Kernel modules: nvidia