Looking to build a new system and was looking at the MSI MPG B650 Carbon WiFi. This is for an AMD build, probably a 7900 or 7900X. I have a Radeon 7900XT GPU already.
So there’s a PCI-E gen 5 x16 slot on the motherboard for the GPU but on reading the motherboard specs it seems that this is shared with two of the M.2 slots and I WILL want to populate both of them. With those populated it seems the PCI-E slot will be running in x8.
Now my current set up is PCI-E gen 3 so even going at x8 the GPU should be getting more bandwidth than it is right now. In fact, PCI-E Gen 5 x8 should be as good as Gen 4 x16 from the motherboards point of view, but can the GPU actually function the same? Will the GPU be slower on Gen 5 x8 than it would be on Gen 4 x16 even if theoretically the motherboard is offering the same bandwidth?
Thanks for any replies. I’m not wedded to getting this specific motherboard. It just looks a good fit if this isn’t an issue.
If my board and slot is pcie 3.0 x16 and the card is only wired 4.0 x 8 isnt it still only using 8 3.0 lanes per the board/slot which is 4x 4.0 lanes essentially for the card?
Yeah, see that’s where I’m coming from. The AMD specifications on their webpage don’t say what PCI generation the GPU supports. Obviously it will be compatible, but can it function at gen 5 speeds just because the motherboard offers it. I don’t know.
Yes, the lanes on a PCIe card and slot are physical.
Companies are not going to pay for data lanes that are not being used. Same goes for some motherboards with x8 & x4 full-size slze slots.
Its pretty easy , lowest common denominators ends up determining the final connection parameters.
If you have x16 slot with G5 x8 bandwidth allocated, connected gpu will be able to connect at best @ x8 GEN4 link.
No magical redistribution of bandwidth is possible.
However this might not lead to noticeable performance loss, for 4090 wost case scenario is 20% perf loss when using GEN1 x16 link (effective x2 Gen4) instead of native x16 GEN4 . Reference here.
There are no gen5 consumer gpus on the market yet, nor would any available models benefit from extra bandwidth.
Well that’s where I was coming from. IF the GPU was capable of gen 5 speeds then it would benefit in this scenario because my dropping the x16 down to x8 would still give it the bandwidth of gen 4 x16. But as it seems my 7900XT is therefore not capable of gen 5 speeds, this motherboard would mean it was still only getting the same bandwidth as its current gen 3 x16.
It’s not the end of the world but I think that is a performance loss still, even if not a huge one. That’s a shame. But thank you for your help.
However, many PCIe cards are NOT PCIe 5.0, most are in fact PCIe 3.0. And therein lies the rub. Because that card will still just run on 3.0 bandwidth.
You cannot magically make a PCIe [email protected] card magically run PCIe [email protected], limiting the lanes to x4 will only make the card run [email protected]. Both card and motherboard will use the lowest PCIe version available, so if only one supports 4.0 then 3.0 it is, but if both are 4.0, then yes, you will get 4.0 speeds and an [email protected] will provide the same bandwidth as an [email protected].
Personally, I would like to claim that a high end motherboard in 2025 would need, at most, [email protected] PCIe and [email protected] m.2/NVMe slots, with another [email protected] to the chipset to power everything else. That would be 44 lanes in total and most consumer CPUs come with 28 lanes today. Just add 16 more and you would be golden. That, I could get behind. Heck, instead of a chipset taking care of things like SATA and fan headers, make NVMe expansion cards that include more of those, too, which would free up chipsets to basically just take care of a small subset of connectors.
All is mostly irrelevant as the main card is a gen3. But being the main difference vs a Gen2 and gen3 will be about 5-6 fps , you will not notice it. If you are on esport, you do not have that kind of system. So all is more than fine and no botlle neck at all.