I measured memory throughput of my computer with MemTest86+, and one result surprised me.
I use DDR3-8500 (533 Mhz) modules. I have one 4 GiB and two 2 GiB. The timings seem to be 7-7-7-20 fixed by the motherboard (a WMCP78M motherboard in an Acer X3300 with AMD Athlon II X2 215 Processor).
The motherboard has 4 memory slots, divided in two groups supporting dual channel.
A single module (2 GiB or 4 GiB) gives 3566 MB/S (I presume this means 3566 MiB/s).
Two identical modules (2 x 2 GiB) on separate slots give 3566 MB/S. I'm okay with that because dual channel is not active.
Two different modules (2 GiB + 4 GiB) on whatever slot give 3566 MB/S. Okay too.
Two identical modules (2 x 2 GiB) on adjacent slots gives 4122 MB/S. Better throughput because dual channel is in action.
Now if I put 2 x 2 GiB on adjacent slots, plus 1 x 4 GiB on another slot, I expect dual channel being active for half the memory. But I have 4122 MB/S. So dual channel is working even with the lonely 4 GiB module.
It could be a bug into MemTest86+: the throughput can be computed only on a part of the memory. So I inverted the modules, but same result (I don't think the motherboard relocates the slots depending which ones are in dual channel).
I updated the BIOS to latest officiel version.
I checked hidden BIOS settings with ctrl+F1 but nothing more appear.
Finally I used AMIBCP to open the BIOS's ROM and see if any option still remain invisible, but everything was already shown. Note that "everything" is not a lot because there is not a single setting about FSB, voltage or so.
What could be the explaination behind the fact I obtain a dual channel score when I use 3 memory modules ?