Asus PRIME B650-PLUS motherboard: a disappointing experience

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I am retiring from academia, so I had to burn some remaining cash and settled on building a new computer. After the latest troubles with Intel processors, I decided to look around for an AMD processor and motherboard. I wanted to have a generous amount of RAM as I have been playing a lot with different IA projects. So, I chose a 4nm Ryzen 7 9700X processor (8 cores, 40 MB cache), and I trusted the good reviews of the inexpensive Asus PRIME B650-PLUS, which I checked supported 128 HB of DDR5 RAM. 

The build was easy, and I was quickly running Windows 11 and Ubuntu 24.04, but something fishy was going on: My DDR5 DRAM was only running at 3600 MHz, which the manufacturer marketed as 5200 MHz. I contacted the seller's support, and I was told that while the board could do 128 GB and 5200MHz memory, it could not do both at the same time, and the BIOS was choosing a more conservative frequency for the memory if I was using the four DRAM SIMMs (which I was).

I could have taken the loss and run my system's RAM at a lower frequency, but after some testing, I realized the extra RAM would not make much difference for IA loads, as the memory bandwidth was still pretty low (47 GB/s). So, I removed two RAM SIMSs to keep only 64 GB of RAM and got 5200 MHz of RAM frequency with the increased memory bandwidth (56 GB/s). 

The worst part was that the seller took no responsibility and refused to accept my return of the other 64 GB, now an expensive paperweight, on my desk. I was informed I was exceeding their 30-day return policy. I did not tell them they have screwed me as a customer, and my policy is to never buy again from them; there are plenty of other sellers to buy from. I really felt Asus screwed it here, as they are selling a motherboard that, in my opinion, it is not up to what they promised.

A bit more insight

I hate to be wrong, but if you want to know more, you'll often find yourself in that position. So I kept asking, and I have learned that the culprit is not the motherboard, as I thought, but the Ryzen processor, or its integrated memory controller (IMC). It may be made to work with specific x4 RAM kits, though, as mentioned in this Reddit post.

A quick check with AMD support

Trying to get to the bottom of it, I asked about the issue to AMD's support, and the answer confirmed my suspicions: That was the expected behavior and not a defect, though they did not call it a feature. I was told the information was written in the Connectivity section of the processor data on their website, where you could read:
Max Memory Speed
2x1R
DDR5-5600
2x2R
DDR5-5600
4x1R
DDR5-3600
4x2R
DDR5-3600

So now you know, be aware you are going to get lower memory bandwidth if you use four SIMMs

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