I filled all 4 slots with memory and wanted to play a little with DRAM OC. But I would like to do that with only 2 modules in order to have better chances.
I saw that you can deactivate individual modules in the BIOS so I deactivated Channel A DIMM0 and Channel B DIMM0 but then the computer no longer starts. But if I remove the two modules Channel A DIMM0 and Channel B DIMM0, the computer starts again.
Yeah, that's the way it behaves unfortunately. Those settings are not meant for troubleshooting or disabling slots that are occupied, they are meant for disabling empty slots to prevent signal reflections from unterminated lines.
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hasnt worked as you woulda hoped for a while now, the data bus is shared with the two dimms in the channel, and even though the option ignores the other dimm, with that other dram not initialized, there is no on die termination and becomes quite a parasite on the signal integrity on the bus for the dimm that is actually trying to initialize.