Manufacturer says PCI Express can run at (x16, x8, x8) or x16, x16, x1, why not 16x, 4x, 4x?

Question

So it is my understanding that you can pretty much stick any x1, x2, 4x or 8x PCI-E Device into an x16 PCI slot, but some manufacturers do not list this within the support configuration, in this example ASUS P8P67M Pro:

Expansion Slots
2x PCIe 2.0 x16 (x16, x8, x8 or x16, x16, x1)
1x PCIe 2.0 x16 (x4 mode, black)
1x PCIe 2.0 x1 

As you can see the above it does not imply that you can use 16x. 4x, 4x, or unless I'm mistaken does this mean the total bus support is 33x?

The idea is to use 16x, 4px and 4px... I have a feeling I can but obviously, don't want to splash out my cash in case I'm wrong.

Question(s):

1. Does 2x PCIe 2.0 x16 (x16, x8, x8 or x16, x16, x1) mean I have a maxium of 33x total?
2. Would 16x, 4x, 4x work?

Answer 1

You get a x16 link from your LGA1155 CPU -- in Intel parlance, this is the PEG or PCI Express Graphics port. Despite the name, it can be used for any kind of PCIe device, but as you can imagine, the most common PCIe x16 device is usually a GPU.

Your other PCIe lanes come from your P67 PCH, of which it can offer up 8 PCIe lanes. These can be ganged together to form at most 2x x4 links. However, that board is using one of those links for the Realtek Ethernet controller (fun fact: even with the Intel PHY, one of the x1 links is then consumed as the MAC <-> PHY interface, so you lose a port regardless for LAN), and potentially others for on-board USB 3.0, etc.

So the Sandy Bridge platform, without any other ICs, offers up 24 total PCIe lanes (22-23 in most practical implementations).

You will not be able to do x16, x4, x4. The x16 link would come from the CPU PEG port, a single x4 would come from the PCH...and then there is nothing left for the second x4 link (it would have to be x1). You could do x8, x8, x4, however -- the PEG can bifurcate into 2 x8, and then you get the x4 from the PCH.

I don't know why ASUS typo'd their description -- there's no PCIe switch on that board that I can see, so you will never have 2 "real" x16 links.

Reference:

Answer 2

Your total number of lanes is 34. One is already used (by NIC or something else integrated).

Yes, it is possible to accomplish what you want:

2x PCIe 2.0 x16 (x16, x8, x8 or x16, x16, x1) --> Use 16x and one 4x form here

1x PCIe 2.0 x16 (x4 mode, black) --> use the other 4x from here.

1x PCIe 2.0 x1 --> blank

A PCIe x4 card will function in a PCIe x16 slot. The slot will simply use 4 of the 16 lanes available, and the card will work properly.