Asus Strix X670E-E Bifurcation question - where are the missing x4 PCIE lanes?

M

MichaelSee

n00b
Joined
Jan 7, 2024
Messages
2
Hi, I have a Asus Strix X670E-E motherboard recently and intend to install few cards that uses all the PCIE slots. I understand the PCIE slots can be configured as either x16/-/x4 (total 20 lanes and PCIEX16_2 disabled), x8/x4/x4 (total 16 lanes and all slots available) or x4/x4/x4/x4 for PCIEX16_1 and x4 for PCIEX16_3 (total 20 lanes and PCIEX16_2 will be disabled). My question is why are the 4 PCIE lanes missing when configuring as x8/x4/x4 and where did they go or reserved for?

Asus launched the RTX4060ti with m.2 slot recently, wouldn't it make sense to configure the BIOS bifurcation to be x8/x4 for PCIEX16_1 and still leave x4 for PCIEX16_2 and x4 for PCIEX16_3 (which is always available due to wiring directly to chipset)? I tried the latest BIOS 1807 that supports the "GPU with m.2" option for PCIEX16_1 but noticed the PCIEX16_2 is still disabled.

P.S. I am using 7950X and 64GB DDR5.
 
I suspect they needed to differentiate there line up drew somewhere to differentiate the Strix and Rampage.
So many x670e boards have configurations that don’t make sense. (x1 and x2 PCIe slots)

For x670e, the max PCIe slot configuration implemented by vendors is 3 slots with x8,x8,x4
 
evilmedic said:
So many x670e boards have configurations that don’t make sense. (x1 and x2 PCIe slots)
Click to expand...
Totally agree. I was disappointed in the loss of PCIe slots when I upgraded from an x570 Strix board. Rampage was way too much $$$.
 
Last edited:
Asus doesn't provide a block diagram in the manual like some vendors which could explain this (but might not necessarily.) It just is what it is. You can understand in detail how a motherboard assigns lanes to PCIe slots, M.2 slots, SATA ports, etc... depending on how you populate things by consulting the manual even without it. But the why is hard to figure out without this. This still might be unexplainable and just a poor choice from Asus.

Some of these AM5 boards turn me off on how they handle lane assignment, especially on X670E where they have a good bit to play with. X670E Aorus Master is the closest board I like in terms of layout (block diagram available on page 5 of the manual.) featuring no mysteriously wasted lanes and basically just a switch for a PCIe x2 slot or 2 SATA ports.
 
Dopamin3 said:
But the why is hard to figure out without this. This still might be unexplainable and just a poor choice from Asus.
Click to expand...
Surprising, and almost out of character for ASUS.

Dopamin3 said:
Some of these AM5 boards turn me off on how they handle lane assignment, especially on X670E where they have a good bit to play with. X670E Aorus Master is the closest board I like in terms of layout (block diagram available on page 5 of the manual.) featuring no mysteriously wasted lanes and basically just a switch for a PCIe x2 slot or 2 SATA ports.
Click to expand...
Can I assume that this MSI board is a direct competitor, in terms of design, to the ASUS Strix boards?
 
AM5 cpus have 28 lanes. x4 for the first chipset, 4x each for the first three m.2, 8x for the main slot and 4x for the second slot, and there's all your lanes. The second chipset is fed by the first chipset, and one of them feeds the third slot and the fourth m.2.

Looks like you get x16 in the main slot, and nothing in slot 2 or m.2-3 or x8 in the main slot and x4 in slot 2 and x4 in m.2-3 (use it or not). Having three options is too hard.
 
philb2 said:
Surprising, and almost out of character for ASUS.


Can I assume that this MSI board is a direct competitor, in terms of design, to the ASUS Strix boards?
Click to expand...
The Gigabyte Aorus master PCIe slot layout is ever more odd than the Strix 670e-e which does x8,x4,x4 The x670e aorus master only does x16,x4,x2 (and the x2 is pcie 3.0!)
 
x670e-e.png
 
MichaelSee said:
Thanks, but you can clearly see in configuration #2, there are total of 12 PCIE lanes, 4 lesser than the configuration #1 of 16 lanes. Hence the missing 4 lanes...
Click to expand...

They likely only wired PCIEX16_2 with 4 lanes and the other 4 lanes went to M.2_3. You can use one or the other or both but they each only have a max of 4 lanes.
 
Sumanji said:
Based on this, can you populate three M.2 drives in this mobo without losing any PCI-E lanes? I.e., slots 1&2 from the CPU, and slot 4 from the chipset?

Also the diagram makes it look like the board has two X670 chipsets... is that right?
Click to expand...

b650 platform is 1 asmedia promontory 21 chip hanging off a Zen4 CPU.
a620 platform is part of a asmedia promontory 21 chip hanging off a Zen4 CPU
x670 platform is 1 asmedia promontory 21 chip hanging off a Zen4 CPU + a second promontory 21 daisy chianed off the first one.
 
Last edited:
Sumanji said:
Based on this, can you populate three M.2 drives in this mobo without losing any PCI-E lanes? I.e., slots 1&2 from the CPU, and slot 4 from the chipset?

Also the diagram makes it look like the board has two X670 chipsets... is that right?
Click to expand...
Yeah just stay away from the m.2_3. I run 3 drives.
 
That's great, I'm on an X470 and you can only really have one drive at max speed (guess that's partially a limitation of older CPUs).

Look forward to whatever the next chipset will be though (X770?)... using double southbridge chips seems a little jank, and surely causes some bottleneck issues.
 
Sumanji said:
That's great, I'm on an X470 and you can only really have one drive at max speed (guess that's partially a limitation of older CPUs).

Look forward to whatever the next chipset will be though (X770?)... using double southbridge chips seems a little jank, and surely causes some bottleneck issues.
Click to expand...

There's only a 4 x 4.0 PCIe link off of the AM5 cpu, AMD links a downstream X670 off of the upstream X670 using another 4 x 4.0 PCIe connections almost doubling the amount of lanes it can provide. If you don't need that many lanes for things, go with a B650E chipset which is essentially the same thing without the 2nd downstream one.

https://www.anandtech.com/show/1758...-ryzen-5-7600x-review-retaking-the-high-end/4
 
I guess it's all still bottle necked by the 4 lanes back to the CPU though right? (I mean in extremis, if you are free saturating the bus). Probably not an issue in normal usage?

I would assume with the next gen they will shift the link to PCI-E Gen 5, and use a single southbridge chip... If only for cost reasons 😂
 
You must log in or register to reply here.
Back
Top