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Several years ago, I tried to get two identical pairs of DDR4 RAM to run together in my computer. I have since bought a set of four, but I've always wondered why the RAM didn't work. Its been long enough that I don't have images from the MemTest, but I keenly recall the errors the test was throwing constantly from the moment the test began.

It expected a certain address, and got an address that was different, but when I looked at the addresses, which were reporting so quickly that I can only believe it was reporting on every address given how there were probably millions after less than a minute, I found that the only difference was just one bit that returned different.

To help visualize the issue, imagine Pair A is in Channel A, and Pair B is in Channel B. Say that MemTest is expecting an address like F73D-9A74 or something (as a real address is very long) and the address it got back was F75D-9A74 instead, just one bit was consistently the exact same amount higher, and it was always the same bit in the address. Then, when I swapped the RAM modules so Pair A was in Channel B and Pair B was in Channel A, instead the error reported that it was expecting F75D-9A74 but got F73D-9A74, the same difference in the same bit of the address but the same amount lower rather than higher as it was before.

The reason I'm asking this on Computer Science is because I'd like to know what about the architecture of DDR4 and modern motherboards that makes identical but unpaired RAM modules unable to function together, when you can mix differently-sized modules and differently timed modules and have them work, albeit bottlenecked. Is it because RAM that is identical but from different production batches are too similar and thus throw errors since the motherboard cannot correct for a small but consistent deviation? Is it something related to planned obsolescence? Or is it something else? This question has been burning in the back of my mind for nearly two years now.

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