AMD Motherboards - More PCIe Bifurcation Options

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More PCIe Bifurcation Options

Printed From: ASRock.com
Category: Technical Support
Forum Name: AMD Motherboards
Forum Description: Question about ASRock AMD motherboards
URL: https://forum.asrock.com/forum_posts.asp?TID=113421
Printed Date: 25 Feb 2026 at 8:41pm
Software Version: Web Wiz Forums 12.04 - http://www.webwizforums.com

Topic: More PCIe Bifurcation Options

Posted By: NDRE28
Subject: More PCIe Bifurcation Options
Date Posted: 31 Dec 2025 at 6:54am

Hi!

My motherboard is the ASRock X670E Taichi.

My video card (RTX 5070) is mounted in my PCIe x16 (top slot), it can't be mounted in the bottom slot.

In the PCIe x8 (bottom slot) I have a U.2 to PCIe 4.0 x4 adapter card, with an Intel Optane SSD in it.

In the future, I'd like to buy a second Optane SSD, and mount both Optane drives in a dual adapter card, as the Glotrends PU21, or the Delock 90091.

However, the available PCIe bifurcation options of my motherboard (when both PCIe slots are used) are:
1. x8x8 = bottom & top PCIe slots run in x8 mode.
2. x8x4x4 = bottom PCIe slot runs in x8 mode & top PCIe slot runs in x4x4 mode.
3. x4x4x4x4 = both bottom & top PCIe slots run in x4x4 mode.

I would need a 4th option:
x4x4x8 mode = bottom slot to run in x4x4 mode & top slot to run in x8 mode.

I've suggested it to the ASRock Support Team.
They've replied to me, saying that they will send this request to their BIOS team.

I'll wait & see.

Replies:

Posted By: Xaltar
Date Posted: 31 Dec 2025 at 8:59am

Good luck, let us know how it goes

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Posted By: M440
Date Posted: 30 Jan 2026 at 5:19am

NDRE28 wrote:

Hi!

My motherboard is the ASRock X670E Taichi.

My video card (RTX 5070) is mounted in my PCIe x16 (top slot), it can't be mounted in the bottom slot.

In the PCIe x8 (bottom slot) I have a U.2 to PCIe 4.0 x4 adapter card, with an Intel Optane SSD in it.

In the future, I'd like to buy a second Optane SSD, and mount both Optane drives in a dual adapter card, as the Glotrends PU21, or the Delock 90091.

However, the available PCIe bifurcation options of my motherboard (when both PCIe slots are used) are:
1. x8x8 = bottom & top PCIe slots run in x8 mode.
2. x8x4x4 = bottom PCIe slot runs in x8 mode & top PCIe slot runs in x4x4 mode.
3. x4x4x4x4 = both bottom & top PCIe slots run in x4x4 mode.

I would need a 4th option:
x4x4x8 mode = bottom slot to run in x4x4 mode & top slot to run in x8 mode.

I've suggested it to the ASRock Support Team.
They've replied to me, saying that they will send this request to their BIOS team.

I'll wait & see.

IMO x4x4 (bottom) + x8 (top) is not possible.

PCIe lane groups are allocated contiguously from the root complex:

Lanes 0-15
|
|- top slot (primary root port)
|- bottom slot (secondary root port)

Once you split the top slot into x4x4, you?�ve already consumed lanes 0-7 in two groups.

You cannot then recombine lanes 8-15 into a clean x8 for the top slot again, because:

lane grouping is fixed

root ports are statically defined

BIOS can only enable pre-wired configurations

In other words:

You can split ?�downward?? but you cannot split the lower slot independently while keeping the upper slot wide.

That would require:

either a PCIe switch (PLX / Broadcom)

or a different physical routing layout

-------------
asrock b650m-hdv/m.2, ryzen 7700x@85watt, arch/kde

Posted By: NDRE28
Date Posted: 30 Jan 2026 at 7:31pm

Hi!

This function exists on the MSI MPG X670E motherboards.

Since it's the same (X670E) chipset, it should also be doable on the ASRock X670E Taichi, unless ASRock did a terrible job with their motherboards!