Question MSI MEG Z590 ACE PCIe understanding

[jasonseeb]

I thought I had a good understanding of the motherboard and PCIe lanes. I looked at my motherboard's manual and was slightly confused. The following is the description of the PCIe lanes.



What confused me was the "Support x16/x0/x4, x8/x8/x4, x8/x4+x4/x4." It seems to be talking about the 3x PCIe x16 slots, but the PCI_E5 slot is not from the CPU. I thought this 20-sum splitting was for the CPU lanes only (PCI_E1, PCI_E3, M2_1).
In my head up until now:
PCI_E1/ PCI_E3 / M2_1
x16 x0 x4
x8 x8 x4

My first question is, is this correct thinking?
My second question is about the x8/x4+x4/x4 breakdown. How does this work? If my first thought is correct, then I don't see how this works.

I included the picture below in case it provides help. This really threw my mind yesterday when researching a future motherboard purchase. Sending package of Sour Patch Kids to the first one who fully answers my question if you want.

 

[Tech Junky]

Single slot bandwidth is 16 adding another card drops it to 8*8 the bottom slot is only 4 electrically and is off the DMI.

You only get 20 total lanes to use off the CPU and the rest are handled by the DMI which has a total of 8. DMI lanes are shared for everything else that's not tied to the CPU.

Now if you had a 690/790 it bumps the bandwidth to gen 4 speeds which is double that of gen 3.

 

[jasonseeb]

Single slot bandwidth is 16 adding another card drops it to 8*8 the bottom slot is only 4 electrically and is off the DMI.

You only get 20 total lanes to use off the CPU and the rest are handled by the DMI which has a total of 8. DMI lanes are shared for everything else that's not tied to the CPU.

Now if you had a 690/790 it bumps the bandwidth to gen 4 speeds which is double that of gen 3.

I am clear on the breakdown of the descriptions now, except for the last one. Thank you very much!

What does x8/x4+x4/x4 mean? The middle x4+x4 confuses me. Can 2 devices ride in that middle slot and use the x8 split into x4+x4?

 

[Tech Junky]

The x4 is slot 5. It's the most it can handle electrically odor bandwidth.

X8 is the electrical bandwidth for slot 3.

Slot 1 can do x16 electrically.

If you add more x4 nvme drives it will decrease the bandwidth of the slots.l because they require x4 and it's non-negotiable. Which is why it shows nvme or card for slot 5. And adding M2_3 takes away SATA ports to make up for the loss of bandwidth.

The other issue is whether you use a 10th gen CPU it doesn't allows for as many lanes and doesn't enable gen 4 m2_1. Basically you need a 11th CPU to unlock the full potential of all slots and sockets. The x4*4 is splitting bandwidth for 10th CPU due to having less lanes for use.

 

[Tech Junky]

Compare Products

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www.intel.com

So, looking at the CPU specs you see the deficiency of he 10th gen being 16 lanes total. This gives you the X8*8 and nothing more to use which is why M2_1 would be disabled and why you get the funky lane splits mentioned. Also being that the 10th gen only supports PCIE 3 where 11th gen supports Gen 4. 12th gen expands both to Gen5 + doubling the bandwidth of DMI to gen 4.

Direct Media Interface - Wikipedia

en.wikipedia.org

The DMI bump from 590 >> 690 is significant in the total # of lanes but also the bandwidth due to 690 going to Gen4.

 

[jasonseeb]

The x4 is slot 5. It's the most it can handle electrically odor bandwidth.

X8 is the electrical bandwidth for slot 3.

Slot 1 can do x16 electrically.

If you add more x4 nvme drives it will decrease the bandwidth of the slots.l because they require x4 and it's non-negotiable. Which is why it shows nvme or card for slot 5. And adding M2_3 takes away SATA ports to make up for the loss of bandwidth.

The other issue is whether you use a 10th gen CPU it doesn't allows for as many lanes and doesn't enable gen 4 m2_1. Basically you need a 11th CPU to unlock the full potential of all slots and sockets. The x4*4 is splitting bandwidth for 10th CPU due to having less lanes for use.

I'm still confused about the "Support x16/x0/x4, x8/x8/x4, x8/x4+x4/x4". They describe all 3 x16 mechanical slots (Slot 1, 3, 5)
I'm going to make a table, and please correct me if I'm wrong.

Slot 1 = x16 electrical	Slot 3 = x8 electrical	Slot 5 = x4 electrical
x16	x0	x4
x8	x8	x4
x8	x4+x4 <====== This confuses me	x4

How can slot 3 be x4+x4?

I'm sorry if you did address this already and I did not understand.

Edit: I understand it must be for bifurcation now.

 

[Tech Junky]

Slot 3 going to x4 would be on 10th gen CPU and using slot 5 at the same time.

x8 - slot 1
x4 - slot 3
x4 - slot 5
16 lanes

If you had a 11th gen CPU then you have 20 lanes. x8/x8/x4 = 20

 

[jasonseeb]

Slot 3 going to x4 would be on 10th gen CPU and using slot 5 at the same time.

x8 - slot 1
x4 - slot 3
x4 - slot 5
16 lanes

If you had a 11th gen CPU then you have 20 lanes. x8/x8/x4 = 20

But slot 5 has nothing to do with CPU lanes, correct? It is on the PCH lanes(chipset).
I do have 11th gen CPU.
I believe I have figured out everything now, thank you.

 

[Tech Junky]

How can slot 3 be x4+x4?

I would assume that's in reference to using the 10th gen CPU. It drops the lanes to 8+4+4 which gives you the max of 16 the CPU has if you use an NVME in M2_1. If you put the NVME into a different socket you'd retain 8/8 if you were run dual cards in slots 1/3 w/ 10th gen.

It's all magic math until you fill all the slots to figure out where the deficiency lies. Most users only use a couple of slots anyway and wouldn't be encumbered by the lane limitations. Since most of the slots use the DMI/PCH anyway they just use the aggregate lanes the DMI provides. The issue lies in the board manufacturer and how they allocate things.

11th gen allocation should be...
16 lanes shared for 1/3 slots + 4 lanes for M2_1 = 20 lanes
DMI will support M2_4 OR slot 5 x4
M2_3 will cause you to lose SATA 5&6 // SAtA 1/2/3/4 600mbps * 4 2400mbps (DMI x4 @ 3.0)
Leaving 4 lanes for the remaining DMI attached devices




My board makes it a bit simpler than doing math but, then again it's why I skipped 10/11th gen GPU in the first place and waited things out for ADL to be released.

Also, I specifically looked for slot diversity in terms of which gen they were. Figuring gen 5 bandwidth would be something to play with as more devices come to market to make use of the speed. OTOH though I considered another board that has a native Gen5 M2 socket which most boards with z690 don't even have.

It all depends on what you're using or planning on upgrading. I'm more storage centric than gaming so, my goals for lane use are going to be different. My planning for slot use is going to be different than most as well. I also look for a bit more value than splurge when it comes to the core items needed to build a system.