B550 Mobo

[poppasilk]

I'm just trying to make sure I understand that with B550 mobo if you have gpu and m.2, the m.2 will still be able to be pcie 4?

 

Thanks 

[Staidey]

yes. It is going to be PCIe 4.0, no matter how many peripherals you connect

[DailyProcrastinator]

Yes, the primary (usually the top) m.2 slot will have PCIe 4.0, the second m.2 slot will not. 

[bowrilla]

1 hour ago, poppasilk said:

I'm just trying to make sure I understand that with B550 mobo if you have gpu and m.2, the m.2 will still be able to be pcie 4?

 

Thanks 

Zen2 and Zen3 have 24 PCIe gen 4.0 lanes: 16+4+4. 16 lanes for the GPU (usually the top most slot), 4 lanes for the primary M.2 slot and 4 lanes for the chipset downlink. Since B550 does not make use of any PCIe gen4.0 that downlink will be downgraded to PCIe gen3.0 on a B550 board (like pluggin a PCIe gen3.0 GPU into an PCIe gen4.0 capable slot with a PCIe gen4.0 capable CPU in the system).

 

You can however use PCIe bifurcation, basically splitting up the lanes going to one slot. Some B550 boards do that to give you more than 1x PCIe gen4.0 4x M.2 slots. In that case the 16 lanes (hence 16x) meant for the GPU are being split up, most likely into 8+4+4 so you will additionally 2x PCIe gen4.0 4x M.2 slots. Wether you need to do that manually in BIOS or wether this happens automatically probably depends on the specific board.

 

The chipset downlink with only 4 lanes also explains why additional M.2 slots and some SATA ports share bandwitch (no matter wether it's B550 or X570).