I'm trying to get a clearer picture of how the PCIe bus works, specifically for the Gigabyte B850 Aorus Elite motherboard. The manual indicates on page 5 that the PCIe x16 slot and the PCIe M.2 slot labeled M2A_CPU share the same PCIe 5.0 bus. It also mentions that there's a x4 AMD chipset connection and another PCIe 4.0 connection for M2B_CPU. Considering that both the 9900x and 9800x3d CPUs support 24 PCIe 5.0 lanes, does this mean that at least 4 of those lanes are shared between the x16 GPU slot and the x4 M2A_CPU slot? This would leave 8 lanes for the other two x4 PCIe interfaces.
First off, is my understanding of this bus diagram correct? Secondly, would sharing the bus like this impact performance, such as how quickly textures load from the M.2 drive for the GPU? Would it be better to use a dedicated PCIe 5.0 M.2 port instead of sharing it with the GPU?
1 Answer
You’ve got it mostly right! The GPU gets 16 lanes, while the M2A_CPU and M2B_CPU slots receive 4 lanes each. The motherboard's physical slots for those 'x4' connections are only wired for x1; most of the additional lanes come from the motherboard chipset, which typically supports 8 lanes total (with a max of 4 more if the manufacturer doesn’t wire for SATA). So, if you’re planning to add power-hungry devices like a Thunderbolt card or a high-performance capture card, this board might not be the best choice since it lacks a dedicated x4 slot for such upgrades.
Thanks for breaking that down! I was a bit confused about the chipset lanes not being counted in the 24 CPU lanes, so I see now that it can total 28 lanes. I appreciate the tip about the x4 slots as well! I'm definitely on the lookout for good motherboard recommendations.