I'm trying to understand the PCIe lane behavior on my ASUS ROG Strix X870E-E Gaming WiFi system. I have an RTX 5080 installed in the primary PCIe 5.0 x16 slot, a Gen 4 M.2 SSD installed in the M.2_2 slot, and a Ryzen 7 9850X3D processor. GPU-Z reports the graphics link as PCIe 5.0 x16 @ x8 4.0. I initially assumed the lower speed was just an idle power-saving state, but even GPU-Z's Render Test leaves the connection at x8 Gen 4 instead of switching to Gen 5.
The motherboard documentation indicates that using certain M.2 slots splits the CPU lanes and reduces the graphics slot to x8 electrically, which I understand. What I don't understand is why the link is negotiating as Gen 4 rather than Gen 5 x8. Could the Gen 4 SSD in the shared lane group force the entire bifurcated connection to Gen 4, or is this more likely related to BIOS settings, signal quality, firmware, or another hardware issue?
If I disable the relevant M.2 slot in the BIOS without physically removing the SSD, would the board restore the GPU to x16, or would the populated slot continue to force the lane split?
3 Answers
Make sure the card is actually installed in the top CPU-connected slot. The lower full-length slot is chipset-connected and only supports PCIe 4.0, while processor support also matters. Your Ryzen 7 9850X3D should provide Gen 5 lanes, so with the top slot and the M.2_2 lane split, the expected result is generally Gen 5 x8 rather than Gen 4 x8.
For gaming, the practical performance difference between PCIe 5.0 x16 and x8 is likely very small, and even PCIe 4.0 x8 usually won’t noticeably limit an RTX 5080 in typical games. That doesn’t explain the unexpected link speed, though. The behavior is worth troubleshooting separately rather than assuming the x8 lane split is the problem.
The lane split itself should normally result in PCIe 5.0 x8, not PCIe 4.0 x8, assuming the CPU and the top graphics slot support Gen 5. Try manually setting the primary PCIe slot to Gen 5 in the BIOS instead of leaving it on Auto. If it still stays at Gen 4 under load, I’d investigate BIOS and chipset updates, reseating the graphics card, signal-integrity issues, or possible CPU/socket contact problems. A Gen 4 SSD in the other bifurcated x8 link should not automatically downgrade the GPU link generation.
The card is in the top PCIe x16 Gen 5 slot, and the processor is a Ryzen 7 9850X3D. I understand why the M.2_2 drive reduces the graphics connection to x8; I’m specifically trying to figure out why it also drops to Gen 4 instead of negotiating Gen 5.