ROG Crosshair X870E Hero MB and M.2 PCIe Bus Sharing

25H2 OS Build 26200.7462

While scrutinizing the Manual for my just purchased ROG Crosshair X870E Hero I discovered that the PCIeX16_1 shares its bus with the M.2_2. "When M.2_2 is enabled, but M.2_3 is not in use, PCIeX16_1 will run in X8 Mode, and PCIeX16_2 will be disabled." It seems clear that I can have one X16 slot for my nVidia ROG X870E GPU and one M.2 for my 4TB SSD, *OR* a second M.2 and an ROG X870E that runs at 8X. Normally, I would run two SSD's - one for the OS and Programs, one larger SSD for frequently and currently used data, and rely upon External Drives for Movies, etc. as desired. This is a fairly expensive Gaming MB, and I did not expect this limitation. The MB actually has a total of 5 slots for M.2 devices, which seems unnecessary. Is this setup typical, even of higher end Game MBs?
My CPU is an AMD Ryzen 9950 X3D. Can anyone recommend a MB that would allow the PCIeX16 to run at 16, *and* use two M.2 SSDs without limitations?

Thanks in advance for any thoughts,

Tom

The M.2 slots on the ROG Crosshair X870E Hero all seem to be wired using the CPU's PCI-E lanes. The 9950X3D provides 28 lanes, 4 of which are used with the chipset. It's not obvious why the board doesn't permit X16 for the GPU slot, plus two X4 M.2 slots.

The MSI Mag X870E Tomahawk WiFi seems to have two M.2 slots off the CPU. Those slots are also PCI-E 5.0. It appears that the X16 slot doesn't fall back to X8 when those M.2 slots are populated. The board also has two more M.2 slots, using chipset PCI-E lanes. Those are both PCI-E 4.0.

Ther may be others, but that was one suggested by MS Copilot.

My ASrock X870E Taichi Lite works fine. 9900X

Edit to add....I think what you have to pay attention to is which slots are connected to the CPU and which ones are connected to the Chipset. Something @bobkin alluded to.

bobkn said:

The M.2 slots on the ROG Crosshair X870E Hero all seem to be wired using the CPU's PCI-E lanes. The 9950X3D provides 28 lanes, 4 of which are used with the chipset. It's not obvious why the board doesn't permit X16 for the GPU slot, plus two X4 M.2 slots.

The MSI Mag X870E Tomahawk WiFi seems to have two M.2 slots off the CPU. Those slots are also PCI-E 5.0. It appears that the X16 slot doesn't fall back to X8 when those M.2 slots are populated. The board also has two more M.2 slots, using chipset PCI-E lanes. Those are both PCI-E 4.0.

Ther may be others, but that was one suggested by MS Copilot.

Click to expand...

Thanks for the info.

After spending some time with the Manual, I discovered that on the above mentioned MSI Mag X870E Tomahawk WiFi, M.2_2 shares a PCIe 5.0x4 (from the CPU) with the two USB4/Thunderbolt connectors which have a theoretical capacity of 40Gbps. When a device is installed in the M.2_2 slot, each device runs at PCIe 5.0x2 or theoretically 8Gbps. The PCIe 5.0x16 as you state remains at x16.
This indicates that I can have one PCIe 5.0x16 for the GPU, one M.2 PCIe 5.0x4 for an SSD, and *either* a second M.2 PCIe 5.0x4 for an SSD, *or* the two USB4 40Gbps type C ports from the ASM2242; but *not* both simultaneously. The Manual later mentions these USB4 ports *are* used as outputs to DisplayPort 1.4a at 4k@60hz.
I have no experience with DisplayPort, so can anyone tell me if these USB4/Thunderbolt-rated ports can be used as DisplayPort outports and alternatively for a Thunderbolt SSD or are they limited to DisplayPort outlets only?

Thanks for any thoughts,

Tom

BamaInArk said:

My ASrock X870E Taichi Lite works fine. 9900X

Edit to add....I think what you have to pay attention to is which slots are connected to the CPU and which ones are connected to the Chipset. Something @bobkin alluded to.

Click to expand...

Thanks for the reply.

Found the following in the Manual:
CPU: 2x PCIe 5.0x16 slots
1x M.2 Gen 5.0x4 (128Gb/s)
Chipset: 3x M.2 PCIe Gen4x4 (64Gb/s)

I have a 4 TB Samsung 9100 Pro PCIe 5.0x4 M.2 SSD for the first M.2 slot. Samsung claims a "Read speed" of "up to 14,800 MB/s." The online conversion app coverts this to 115.6Gb/s. Of course in the "Real World" these numbers don't mean much, but if everyone lies equally, then a comparison is at least something to go on... If there's any truth here at all, a second 9100 Pro running in a 4.0x4 slot would not achieve the speed of the SSD in the slot 1 running in the PCIe 5.0x4 slot.

So far I've checked the Manuals for the following Mothers:
1. ASUS ROG Crosshair X870E Hero
2. ASUS ROG Crosshair X870E Extreme
3. ASUS ROG STRIX X870E GAMING WIFI
4. MSI Mag X870E Tomahawk WiFi
5. ASRock Phantom Gaming X870 Nova WiFi
6. ASRock X870E Taichi Lite

None of the above will allow me to install 2x M.2 PCI 5.0x4 without shifting the PCIe 5.0x16 down to x8.

If anyone can find me an AS5 socket MB that will allow 2x M.2 PCIe 5.0x4 I would greatly appreciate your help!

Tom

I thought that the MSI Mag X870E Tomahawk WiFi allowed two M.2 drives to be used in the CPU driven slots without causing the X16 slot to drop back to X8.

Ed McCauley said:

Thanks for the info.

After spending some time with the Manual, I discovered that on the above mentioned MSI Mag X870E Tomahawk WiFi, M.2_2 shares a PCIe 5.0x4 (from the CPU) with the two USB4/Thunderbolt connectors which have a theoretical capacity of 40Gbps. When a device is installed in the M.2_2 slot, each device runs at PCIe 5.0x2 or theoretically 8Gbps. The PCIe 5.0x16 as you state remains at x16.
This indicates that I can have one PCIe 5.0x16 for the GPU, one M.2 PCIe 5.0x4 for an SSD, and *either* a second M.2 PCIe 5.0x4 for an SSD, *or* the two USB4 40Gbps type C ports from the ASM2242; but *not* both simultaneously. The Manual later mentions these USB4 ports *are* used as outputs to DisplayPort 1.4a at 4k@60hz.
I have no experience with DisplayPort, so can anyone tell me if these USB4/Thunderbolt-rated ports can be used as DisplayPort outports and alternatively for a Thunderbolt SSD or are they limited to DisplayPort outlets only?

Thanks for any thoughts,

Tom

Click to expand...

I missed that. I didn't notice the 40Gbps ports, which run off the CPU.

I'm not very knowledgeable about USB/Thunderbolt, especially whether the ports which are described as 40Gbps USB are fully compatible with Thunderbolt 3 (40Gbps).

But if you want PCI-E 5.0 for the X16 graphics slot plus two M.2 X4 slots, there will be no CPU PCI-E lanes available for anything else.

In the let-them-eat-cake line, if you want lots of PCI-E 5.0 lanes, you want something more in the workstation line than a desktop PC. Like a Threadripper based system. It's only money.

Ed McCauley said:

None of the above will allow me to install 2x M.2 PCI 5.0x4 without shifting the PCIe 5.0x16 down to x8.

If anyone can find me an AS5 socket MB that will allow 2x M.2 PCIe 5.0x4 I would greatly appreciate your help!

Tom

Click to expand...

Yeah I was not aware you wanted multiple PCIe 5 SSDs. You'll have to wait for more advanced motherboards or go for super high end stuff. My current ASRock system has a PCIe 5.0 GPU(although there are 2 PCIe 5.0 slots) and ONE PCIe 5x4 SSD but that's all it supports @PCIe 5.0. All the other M.2 slots support Gen4x4 only.

You can run M2_1 (gen5) and M2_4 & 5 (gen 4) without touching the main pcie slot. The x870e has the same limitations no matter how you package it. Newer boards free up more lanes but there is always sacrifice and they're never gen 5. I have zero hesitation dropping to gen 5 8x when i need more storage as there is absolutely zero difference in games or benches.

I am planning to use all M2 slots and since i do not play games I am hoping i won't notice any difference. kciN your comments are promising.

kciN said:

I have zero hesitation dropping [MY GPU FROM GEN 5 x16 TO] gen 5 8x when i need more storage as there is absolutely zero difference in games or benches.

Click to expand...

As you state, according to nVidia my GE FORCE RTX5080 suffers no performance loss dropping from x16 to x8. For the RTX5090 users out there, nVidia says there is a 2% performance loss.

When the PCIe x16 #1 slot drops to x8, it frees up 8 lanes. M.2 #1 will continue to run at Gen5 x4, and these 8 free lanes would allow a second and a third NVME SSD in the #2 & #3 M.2 slot to also run at Gen 5x8. 8 + 4 + 4 + 4 = 20 CPU lanes used.

My Ryzen 9950X3D CPU has 24 Lanes in total. My ASUS X870 MB includes 2 USB-4 ports which are GEN5 and each use 2 Lanes to the CPU, so all 24 Lanes are in use. If I disabled these (through Device Manager???) I'd have four FREE Gen5 lanes - although I don't know how I could use them, it's something to consider..

I'm not sure that disabling the USB4 ports in the BIOS would make their PCI-E lanes available for other uses. I'd hope so, but I wouldn't bet on it.

We only wish it worked like that but I have no doubt some board level rewiring would be needed. Though like I said there is always sacrifice. Newer boards allow one to bite in to usb bandwidth to free up lanes but from what I have seen you cannot completely disable usb so they run at 2x/2x. Doesn’t look like asus has chosen to go down that road

bobkn said:

I'm not sure that disabling the USB4 ports in the BIOS would make their PCI-E lanes available for other uses. I'd hope so, but I wouldn't bet on it.

Click to expand...

I didn't word that very well - I meant I'd have 4 unused Lanes, that probably couldn't be used. After I built the computer I'll have to see what happens. It's going to be a while - the shipment of the DDR5s keeps getting delayed...