PCIe Gen5 8x = Gen4 x16?

Discussion in 'Processors and motherboards AMD' started by Belfaborac, Nov 2, 2022.

  1. Belfaborac

    Belfaborac Active Member

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    I'm using an MSI MPG X670E Carbon WiFi, with 2 PCIe slots which can be run as 16x/- or 8x/8x (actually there's an x4 slot too, but let's not bother with that). In the specs it is stated:

    PCI_E1 PCIe 5.0 supports up to x16 (From CPU)
    PCI_E2 PCIe 5.0 supports up to x8 (From CPU)


    So far, so good. What I'm unclear about is whether the second slot, PCI_E2, does provide Gen4 x16 or is also limited to x8? It is wired for x16, but I don't know if that means it will actually provide x16 @ Gen4.

    The reason I ask is that I've got a Gen4 x16 M.2 adapter card lying around, which needs x4 per drive. I.e. - with a slot providing x16 I will be able to populate it with 4 drives, whereas with x8 only 2 drives.

    Anyone know for sure which is the case?

     
  2. ruiner13

    ruiner13 Member Guru

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    I mean, if you've got the adapter just lying around why not just test it?
     
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  3. Belfaborac

    Belfaborac Active Member

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    I've got the adapter, but not any spare M.2s. Installing it empty probably wouldn't make me any wiser and I'm not about to pull the system apart just to get to the installed M.2s.
     
  4. cucaulay malkin

    cucaulay malkin Ancient Guru

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    8x can't do 16 lanes.
     

  5. Astyanax

    Astyanax Ancient Guru

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    Gen 5 8x has the bandwidth of Gen 4 16x, i believe is the question.

    Yes.
     
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  6. cucaulay malkin

    cucaulay malkin Ancient Guru

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    sorry, I thought this is about lanes.
    you need a pcie5 device to match it.
     
  7. Astyanax

    Astyanax Ancient Guru

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    oh, no you are right.

    let me check the manual, im sure its only 8x8x4x in this case, unless the chipset got a lane boost?

    Edit

    28 lanes total


    16 or 2x8 for GPU's, 16x_8 is electrically 8x only.
    4 are connected to Promonitory 21_1,
    4 host a gen 5 nvme.
    the last 4 are implementation defined by the motherboard vendor, they can be used for thunderbolt, peripheral, etc.

    Promonitory 21 1 and 2 are 16 lanes each
    P21_1 uses 4 of these towards the cpu, and 4 of these towards P21_2, leaving 8 to be desigated to SATA, USB, etc.
    P21_2 has 12 available designated to additional PCIE slots.
     
    Last edited: Nov 5, 2022
  8. Belfaborac

    Belfaborac Active Member

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    I couldn't really pull a firm conclusion out from the above, as far as a yea or nay as to whether or not I'll get 16x Gen4 from an 8x Gen5 slot.....
     
  9. GoldenTiger

    GoldenTiger Master Guru

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    No. You will get x8 gen 4 from an x8 gen 5 slot.
     
  10. Calenhad

    Calenhad Member

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    It will be a 1 to 1 connection, x8 to x8 no matter the pcie version.



    Kind of off topic: it is possible to use a fancier adapter with a multiplexer, that can convert the pcie 4.0 x16 slot to a pcie 5.0 x8 signal. But that would be an expensive piece of kit. And I doubt such a product even exists, but it is theoretically possible.
     

  11. Belfaborac

    Belfaborac Active Member

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    Excellent, thanks a lot! Or, well, not actually excellent, but at least I have an answer. Guess I'll just make do with another two drives, rather than 4. First World problems and all that...
     

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