My concern is whether the M.2 slot reroutes to "add" 4 additional PCIe4 CPU Lanes instead of going through the chipsets 24 lanes. I assume that becoming a PCIe4 enabled motherboard, the M2A_CPU lanes do in fact switch to CPU Lanes.
So you hope that metal traces magically migrate within the PCB when changing the CPU…
I'm sorry to pour cold water on your parade, but that's unlikely to happen.
Currently, if you look at page 7 of the Z490 Vision D manual the M2A_CPU slot is not actually part of the 16 PCIe CPU Lanes; it goes through Chipsets 24 Lanes.
So it runs at PCIe 3.0 speed from the chipset no matter which CPU is used.
If you look at the manual for the Z590 Vision D on page 7, you might see what I am getting at in comparison with the Z490.
CPU lanes, at PCIe 4.0 speed, when using 11th gen. CPU and not available at all when using 10th gen., as shown in the
specifications for the Z590 Vision D:
The
specifications for the Z490 Vision D do not promise any PCIe 4.0 at all.
I will write to Gigabyte to clarify. This curiosity all stemmed from reading this document on MSI’s website.
MSI Z490 motherboards offer complete PCIe 4.0 solutions. With 11th Gen Intel Core Processors, MSI Z490 motherboards offer great bandwidth and performance for PCIe 4.0 M.2 SSDs and graphics cards.
www.msi.com
To do this, MSI would need a double set of traces in the PCB, one coming from the CPU and one from the PCH, going into a PCI switch chip which then feeds the M.2 slot. I suppose that's possible, but it adds to the costs and the design would have been impossible to validate before PCIe 4.0-capable CPUs were released.
Taking MSI Z490 Unify as example, I found this block diagram at page 27 of the manual:
No switch is shown, but a plain M.2 slot from the PCH, sharing lanes with a PCIe x4 slot (in x16 physical slot).
The specifications make no mention of PCIe 4.0—and the table doesn't bother to distinguish between the physical PCIe slots, of which two are electrically x16/x0 or x8/x8 from the CPU and the third one is x4 from the PCH. Lazy listing, and sloppy drawing.
The specifications and manual may be wrong, and the blog post, right. But I rather suspect the opposite: The block diagram is right and the blog post is a pipe dream from a marketing guy.
