Why 90% of X370 motherboards Only Have ONE M.2 Slot?

[TechyInAZ]

Just out of curiosity, why are most 1st gen ryzen motherboards only including one m.2 slot? Most of the 2xx series motherboards for Kaby Lake had two M.2 slots at that time which is why I'm confused.

[Crunchy Dragon]

That's actually really odd, my B350 board has dual M.2 slots....

[TechyInAZ]

That's not fair LOL. My Hero only has one M.2.

[Sakkura]

Ryzen has 16 PCIe 3.0 lanes (mainly) for graphics cards, which can be split to two ports with 8 lanes each. That's not usable for M.2 (maximum of 4 lanes). It also has a dedicated 4 PCIe 3.0 lanes for an NVMe M.2 drive.

 

The rest goes through the chipset (X370 etc), which only offers PCIe 2.0 lanes, not 3.0, which would hold back performance for faster NVMe drives.

 

Z270, on the other hand, does have PCIe 3.0 lanes on the chipset. And while Kaby Lake CPUs do not actually have dedicated PCIe lanes for NVMe M.2, it can split its 16 lanes for graphics into x8/x4/x4, the latter being usable for M.2 slots.

 

That said, more X470 boards do seem to have moved to dual M.2 slots, despite nothing really changing in the CPUs or chipset.