ASUS ROG STRIX X870-I GAMING WIFI (w/AMD Ryzen 9 9950X) - UNDERSTANDING PCI LANE INFO

[HBSound]

So, I want to understand the PCI lanes of this motherboard. Below is an image shot of the Asus ROG STRIX X670E-I GAMING WIFI manual. As far as PCI lanes and assignments, I would think this newer board will follow the same PCI lanes and assignments with the CPU and Chipset.

I understand, there are 16 lanes tited to the CPU - check.
I understand there are 4 lanes tide to the CPU for the M.2_1 Slot - check.
I undersatnd there are 4 lanes titde to the AMD Chicpset - check

According to the AMD 9950X CPU, there are Native PCIe® Lanes (Total/Usable) = 28/24.

Question 1 - Why is there no complete access to all 28 lanes? How do you access those extra 4 PCI lanes?

 

[Toothless]

Four lanes are probably going to the chipset for USB, LAN, etc.

 

[RootinTootinPootin]

4 lanes for all the internal USB controller, LAN, WLAN, etc..its not "technically" exclusive just for the GPU or the PCI-E Storages..

 

[ir_cow]

Now that USB4 is mandatory for X870, that's where those lanes are going. Previously, you got either M.2 or USB4 with those 4 lanes. Some motherboards like the ASUS X670E Hero made it so the USB4 and M.2 shared the same lanes without noting it in the manual. Now there isn't a chance it these will be sharing lanes anymore....or at least I hope.

 

[LittleBro]

Now that USB4 is mandatory for X870, that's where those lanes are going. Previously, you got either M.2 or USB4 with those 4 lanes. Some motherboards like the ASUS X670E Hero made it so the USB4 and M.2 shared the same lanes without noting it in the manual. Now there isn't a chance it these will be sharing lanes anymore....or at least I hope.

This.

Take a look at this board:

ROG STRIX X870-A GAMING WIFI | Gaming motherboards｜ROG - Republic of Gamers｜ROG Global

The ROG Strix X870-A Gaming WiFi answers the call with rock-solid 16-stage power, hyperspeed DDR5 slots and next-gen PCIe® 5.0 connectivity. It features four M.2 SSD slots with heatsinks, USB4® suppor

rog.asus.com

M2_2 slot shares bandwith with PCIE16_1. When M2_2 is occupied, main GPU slot gets crippled to x8 so that the board can support USB 4.0.

 

[HBSound]

This.

Take a look at this board:

ROG STRIX X870-A GAMING WIFI | Gaming motherboards｜ROG - Republic of Gamers｜ROG Global

The ROG Strix X870-A Gaming WiFi answers the call with rock-solid 16-stage power, hyperspeed DDR5 slots and next-gen PCIe® 5.0 connectivity. It features four M.2 SSD slots with heatsinks, USB4® suppor

rog.asus.com

M2_2 slot shares bandwith with PCIE16_1. When M2_2 is occupied, main GPU slot gets crippled to x8 so that the board can support USB 4.0.

I am trying to see where it speaks about this on the ITX version of the motherboard. Am I missing it, or is it not in writing?

ROG STRIX X870-I GAMING WIFI | Gaming motherboards｜ROG - Republic of Gamers｜ROG Global

The ROG Strix X870-I Gaming WiFi packs a punch with 10-stage power, hyperspeed DDR5 slots and PCIe® 5.0 connectivity. It features two M.2 SSD slots with heatsin

rog.asus.com

 

[LittleBro]

There are two M.2 slots on that X870-I Gaming board, one connected to CPU (PCIe Gen 5.0 x4) and second connected to chipset (PCIe 4.0 x4).
Another slot would require additional space on the board. On the other hand, there is no M.2 slot that would cripple PCIE x16 slot to half speed.
So, you have PCIe x16 + M.2 x4 + 2x USB 4.0 = there are your 24 CPU lanes. Another 4 lanes CPU uses to communicate with chipset (PCIe 4.0 x4). That's 28 lanes in total.

The fact about USB 4.0 is not written anywhere, it's AMD's thing ... they specified for X870(E) boards to support USB 4.0.
One USB 4.0 port should require two PCIe Gen 5.0 lanes, although I don't know this for sure.
Logically, 40 Gbps of USB 4.0 cannot be fully utilized using just one PCIe Gen 5.0 lane (~ 32 Gbps).

My personal opinion is that USB 4.0 is not worth it. It may be worth it on PCIe Gen 6.0 but not now.
Then such stupid board designs like ASUS X870-A Gaming are made ...
When two PCIe x16 ports share bandwitdh of 16 lanes (thus can be split into x8/x8), that's perfectly fine to me, that's for multi GPU setup or having SSDs in RAID using bifurcation.
But when x16 port shares bandwidth with M.2 slot (like in ASUS X870-A Gaming) and is split into x8/x4 when M2 slot is occupied, 4 lanes are wasted (unless there isn another M.2 slot):

 

[HBSound]

There are two M.2 slots on that X870-I Gaming board, one connected to CPU (PCIe Gen 5.0 x4) and second connected to chipset (PCIe 4.0 x4).
Another slot would require additional space on the board. On the other hand, there is no M.2 slot that would cripple PCIE x16 slot to half speed.
So, you have PCIe x16 + M.2 x4 + 2x USB 4.0 = there are your 24 CPU lanes. Another 4 lanes CPU uses to communicate with chipset (PCIe 4.0 x4). That's 28 lanes in total.

The fact about USB 4.0 is not written anywhere, it's AMD's thing ... they specified for X870(E) boards to support USB 4.0.
One USB 4.0 port should require two PCIe Gen 5.0 lanes, although I don't know this for sure.
Logically, 40 Gbps of USB 4.0 cannot be fully utilized using just one PCIe Gen 5.0 lane (~ 32 Gbps).

My personal opinion is that USB 4.0 is not worth it. It may be worth it on PCIe Gen 6.0 but not now.
Then such stupid board designs like ASUS X870-A Gaming are made ...
When two PCIe x16 ports share bandwitdh of 16 lanes (thus can be split into x8/x8), that's perfectly fine to me, that's for multi GPU setup or having SSDs in RAID using bifurcation.
But when x16 port shares bandwidth with M.2 slot (like in ASUS X870-A Gaming) and is split into x8/x4 when M2 slot is occupied, 4 lanes are wasted (unless there isn another M.2 slot):
View attachment 360685

I get it, and I see where you headed this. Access to the USB4.0 is tricky because you have access in the real world, but you never know if you will utilize the USB4.0. I have a 4090, and at 4.0X8 speed, from my understanding, the GPU is going at about 95% of its total capacity, so in the real world, I may never feel a difference. So, right now, I plan to occupy the M.2.1 slot with a 4TB 5.0X4 SSD and the M.2 slot with a large SSD (4.0X4) just for backup and storage use only.

At this point in my life and work, etc. I have used an external device or drive, mostly a USB, to insert the operations system. Outside of that, 100% of my work and access to what I do is driving via WIFI/Network Connector.

 

[HBSound]

This.

Take a look at this board:

ROG STRIX X870-A GAMING WIFI | Gaming motherboards｜ROG - Republic of Gamers｜ROG Global

The ROG Strix X870-A Gaming WiFi answers the call with rock-solid 16-stage power, hyperspeed DDR5 slots and next-gen PCIe® 5.0 connectivity. It features four M.2 SSD slots with heatsinks, USB4® suppor

rog.asus.com

M2_2 slot shares bandwith with PCIE16_1. When M2_2 is occupied, main GPU slot gets crippled to x8 so that the board can support USB 4.0.

I am trying to understand this a little more technically.

The M2_1 (CPU Controlled) - Gen5.0X4. (4TB Main Drive)
The M2_2 (Chipset Controlled) - Gen4.0X4. (8TB Storage Drive)
The PCI (CPU Controlled) - 4090 GPU Gen4.0X16

When the USB is occupied, who is crippled in this equation? The GPU is taking from Gen4.0X16 to Gen4.0X8. Right?

Are you suggesting the M2_2 shares the sandwich with the PCIIE16 if the M2_2 is controlled by the chipset and the PCI is controlled by the CPU? How are they sharing bandwidth?

 

[Sarajiel]

The M2_1 (CPU Controlled) - Gen5.0X4. (4TB Main Drive)
The M2_2 (Chipset Controlled) - Gen4.0X4. (8TB Storage Drive)
The PCI (CPU Controlled) - 4090 GPU Gen4.0X16

You are correct about M2_1.

However, you are wrong about M2_2 and the PCIe Gen5 x16. If you connect any drive via M2_2, the PCIe Gen x16 top-slot gets bifurcated into 8+4+0. That means your GPU will run as x8 and M2_2 gets its 4 PCIe lanes. The additional 4 PCIe lanes are basically wasted.

If you want to use the GPU as x16, your second M.2 has to go into either M2_3 or M2_4 which are connected via the chipset.

Additionally, if you populate M2_3, the PCIe Gen4 x4 electrical (x16 mechanical) bottom-slot from the chipset gets deactivated.

When the USB is occupied, who is crippled in this equation?

Using USB4 doesn't come with an additional penalty, it already uses the 4 "leftover" PCIe Gen5 lanes from the CPU. The ones you were missing in your original post. These are the ones AMD refers to as "general purpose lanes" and have typically been used for a 2nd M.2 directly connected to the CPU on the 600 series motherboards.


This board is pretty much just a noob trap.

 

[A Computer Guy]

You are correct about M2_1.

However, you are wrong about M2_2 and the PCIe Gen5 x16. If you connect any drive via M2_2, the PCIe Gen x16 top-slot gets bifurcated into 8+4+0. That means your GPU will run as x8 and M2_2 gets its 4 PCIe lanes. The additional 4 PCIe lanes are basically wasted.

If you want to use the GPU as x16, your second M.2 has to go into either M2_3 or M2_4 which are connected via the chipset.

Additionally, if you populate M2_3, the PCIe Gen4 x4 electrical (x16 mechanical) bottom-slot from the chipset gets deactivated.

What a mess. Is it this complicated on the Intel 12th/13th/14th gen side too?

 

[Sarajiel]

What a mess. Is it this complicated on the Intel 12th/13th/14th gen side too?

Not really, but a lot of Z790 boards bifurcate the PCIe Gen5 x16 slot into x8+x4, and connect a Gen5 M.2 to the x4 set of lanes. Since Intel doesn't allow 8+4+4 bifurcation on their consumer CPUs, the other 4 lanes can't be used for anything at all. Intel usually connects their Thunderbolt 4 controllers via the chipset, from what I remember. The Z790 chipset also offers more lanes, therefore most boards can easily connect two or three M.2s via the chipset without the need to disable PCIe slots.

Other X870(E) motherboards from ASUS and Gigabyte also bifurcate 8+4+4, but actually connect two M.2 slots to the bifurcated lanes. The ROG Strix X870-A is just dumb (and cheap) in that regard.
The Crosshair X870E Hero is even crazier and allows for either x8+x8 into two PCIe slots, or x8+x4+x4 into one PCIe slot and two M.2s.

edit:

It's a bit easier to understand for the ASUS ROG Strix X870-I Gaming WIFI, the motherboard the OP was originally looking for:
- 16 lanes for the PCIe Gen5 x16
- 4 lanes for the Gen5 M.2 (M2_1)
- 4 lanes for USB4
- 4 lanes into the X870 chipset that provides the Gen4 M.2 (M2_2)

 

[HBSound]

You are correct about M2_1.

However, you are wrong about M2_2 and the PCIe Gen5 x16. If you connect any drive via M2_2, the PCIe Gen x16 top-slot gets bifurcated into 8+4+0. That means your GPU will run as x8 and M2_2 gets its 4 PCIe lanes. The additional 4 PCIe lanes are basically wasted.

If you want to use the GPU as x16, your second M.2 has to go into either M2_3 or M2_4 which are connected via the chipset.

Additionally, if you populate M2_3, the PCIe Gen4 x4 electrical (x16 mechanical) bottom-slot from the chipset gets deactivated.


Using USB4 doesn't come with an additional penalty, it already uses the 4 "leftover" PCIe Gen5 lanes from the CPU. The ones you were missing in your original post. These are the ones AMD refers to as "general purpose lanes" and have typically been used for a 2nd M.2 directly connected to the CPU on the 600 series motherboards.


This board is pretty much just a noob trap.

Ok, where are M2_3 and M2_4, from my understanding, this unit can only hold - M2.1 (5.0X4) (2880) and M2.2 (4.0X4) (2880).

 

[Sarajiel]

Ok, where are M2_3 and M2_4, from my understanding, this unit can only hold - M2.1 (5.0X4) (2880) and M2.2 (4.0X4) (2880).

No, this board (ROG Strix X870-A) offers up to four M.2 slots, but this means that you lose other options in that use case.
You are playing Tetris with your hardware in that board. Once you use one piece, another gets blocked. If you want more than two M.2 slots, consider another board. If you are sure, you won't ever need more than two M.2s, then use M2_1 and M2_4 to avoid disabling other options. As I said before, this board is a noob trap that pretty much only exists to upsell more expensive motherboards.

 

[HBSound]

No, this board (ROG Strix X870-A) offers up to four M.2 slots, but this means that you lose other options in that use case.
You are playing Tetris with your hardware in that board. Once you use one piece, another gets blocked. If you want more than two M.2 slots, consider another board. If you are sure, you won't ever need more than two M.2s, then use M2_1 and M2_4 to avoid disabling other options. As I said before, this board is a noob trap that pretty much only exists to upsell more expensive motherboards.

I got you, but you and I are speaking about the different boards. I am awaiting the release of the ASUS ROG STRIX X870-I GAMING WIFI (w/AMD Ryzen 9 9950X) "Subjet above," and I am trying to see just how these PCI lanes all are planning out. For a second, I was like, where in the world are these drives? But the ITX version only has two M2 slots.

 

[Sarajiel]

but you and I are speaking about the different boards.

It's a bit easier to understand for the ASUS ROG Strix X870-I Gaming WIFI, the motherboard the OP was originally looking for:
- 16 lanes for the PCIe Gen5 x16
- 4 lanes for the Gen5 M.2 (M2_1)
- 4 lanes for USB4
- 4 lanes into the X870 chipset that provides the Gen4 M.2 (M2_2)

You probably reacted, before I was finished with my edit.

The X870-I looks like a decent board, and I'm tempted to get it myself, unless Gigabyte releases a new m-ITX board that supports three M.2s. It's similar to the ASUS ROG Strix X670E-I Gaming WIFI which offers USB4 via Intel's Maple Ridge (Thunderbolt 4) controller instead of the ASMedia one.

There are other m-ITX board that offer two M.2s connected via the CPU:
- ASUS ROG Strix B650E-I Gaming WIFI
- ASRock B650E PG-ITX WiFi
- GIGABYTE B650I AORUS Ultra (+ a 3rd M.2 via the chipset)
However, none of these offers USB4 support.

Forgive my rambling, but I find that new lane-splitting that is required for USB4 on AM5 absolutely hilarious.

 

[HBSound]

You probably reacted, before I was finished with my edit.

The X870-I looks like a decent board, and I'm tempted to get it myself, unless Gigabyte releases a new m-ITX board that supports three M.2s. It's similar to the ASUS ROG Strix X670E-I Gaming WIFI which offers USB4 via Intel's Maple Ridge (Thunderbolt 4) controller instead of the ASMedia one.

There are other m-ITX board that offer two M.2s connected via the CPU:
- ASUS ROG Strix B650E-I Gaming WIFI
- ASRock B650E PG-ITX WiFi
- GIGABYTE B650I AORUS Ultra (+ a 3rd M.2 via the chipset)
However, none of these offers USB4 support.

Forgive my rambling, but I find that new lane-splitting that is required for USB4 on AM5 absolutely hilarious.

Not a problem at all; you're good.

I just want to make sure that as I am buying and planning, I am not buying stuff that will not function due to the way the board is designed. Right now, I will use this in my ITX build, and for me, I use CAD, 3D, etc. I use a workstation, and I have no issues, but when it comes to speed, the more minor chips win the speed department, and the workstation wins the power ultimate power department. I want to see if I can build the intimate ITX workstation and deliver what I need it for.