Question about RAM speed, don't make sense to me
If you want to actually study this look up something like RAM topology.
But if you want a super simple analogy, you can always imagine the traces of your motherboard like highways with cars that all go at the same speed (the speed being a stand-in for the frequency). Each car can transport one data package.
Since every car goes at the same speed, for everything to work out in the dual channel setup, you have to make sure, that the highways from your CPU to the first memory model is exactly the same length as the distance from your CPU to the second module.
In a daisy chain topology (which most motherboards use), the first and second RAM slot and the third and fourth slot are linked after another. Which causes some problems. Because as soon as you insert another RAM stick into one of the daisy chained slots, the cars don't have to drive as fast anymore. To compensate for that everything has to go slower, so the cars that go to the second module can catch up.
Since there isn't much the board makers can do to alleviate that problem, they just accept that in case of a 4 DIMM scenario they all go slower. So they only really optimize the 2 DIMM version. And for that they chose the two slots at the end of the signal trace. So if you run the scenario that doesn't work for you, you are likely running into some signal integrity issues and your motherboard falls back into one of the slower JEDEC standards.
One option for all 4 slots to have the same distance to the CPU is using a T topology, but since most people run with two DIMMs anyway, daisy chain is more popular and it is also better to optimize just two trace runs instead of four.
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