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rjal
New Contributor I
1,682 Views

how many pci express lanes does the haswell cpu have ? 16 or 20 ?

Hi,

I've been looking at the documentation of the DZ87KLT-75K board. In the block diagram it looks as if the haswell has a 16x set of pci-e and another set of 4x pci-e lanes. this makes a total of 20 lanes. However i cannot find any documentation on this anywhere. The datasheet defines only 16 lanes. Where did the other 4 lanes come from ?

The PCH has 8 lanes pci-e v2.0 and they are all in use.

The DH87MC has the same peculiarity. I also checked boards from other vendors, no one has more than 16 pci-e v3 lanes coming off the processor.

Is this a documentation flaw or is there something special going on here ?

thanks,

Robert

Tags (1)
5 Replies
Silvia_L_Intel1
Employee
210 Views

Robert, the Haswell platform involves a single port with 16 lanes for PCI-e, but it only takes care of PCI-e3.0 and the motherboard PCH takes care of the PCIe 2.0 (2x 4x 8x16x). The 8 lanes included in the PCH are dedicated for the PCI-e 2.0 in general (distributed among the different PCI-e slots).

rjal
New Contributor I
210 Views

Hi Sylvia,

did you have a look at the technical manuals of the boards that i mentioned ?

http://downloadmirror.intel.com/22677/eng/DZ87KLT-75K_TechProdSpec02.pdf http://downloadmirror.intel.com/22677/eng/DZ87KLT-75K_TechProdSpec02.pdf page 16.

there are 3 pci-e 16x connectors. one is 16x with the second disabled or two are 8x each. then there is a 3rd connector directly to the processor with is pci-e 4x. where did that one come from ?

Robert

SLogi1
Novice
210 Views

20 lanes, PEG1 3.0x16 and PEG2 2.0x4

Ivy is the same

Sandy has all 2.0 lanes as 3.0 wasn't available yet

The PCH provides x8 which is what most onboard chips connect to, you cannot use the full bandwidth of these simultaneously though, as it goes back to the CPU over the DMI link and this is also shared with any USB/SATA/etc traffic that is r/w to memory. Any slots connected to PCH are not recommended for high bandwidth expansion cards. (e.g. 10GbE NIC or SAS array controller etc)

This is buried somewhere deep in one of their tech pdfs, I ran across it before. (sorry, no link)

I can actually confirm this as functional on my P8C WS: 3.0x16 to a videocard, 2.0x4 to an infiniband card, PCHx4 to a quad GbE, and finally a pair of PCHx1 to onboard 82574L nics.

If you do the math you'll notice the PCH couldn't possibly be supplying enough lanes, and hwinfo64 shows this nicely (and the gpu IS using all 16 lanes):

This board definitely has no switch chips, they are easy to notice if you have them. (typically PLX)

Not every motherboard seems to use the "extra" 4 from the cpu though, a lot just use the 'PEG1' x16 and anything else comes from the 8 lanes the series 6/7/8 PCH provides.

Some versions of these (Q and C2xx I know for sure) also provide an intel controller for a PCI 32bit bus instead of the flaky bridge chip a lot of consumer boards use.

rjal
New Contributor I
210 Views

ok, thanks. I have not been able to find this info yet and I've just checked my own ivybridge. No extra PEG here. I'll dig a little deeper.

now for the second peculiarity. If the digital ports of the graphics have been moved from the PCH to the CPU in haswell with only VGA remaining on the PCH, why is the thunderbolt interface connected to the PCH ? The DP should be coming from the CPU not the PCH ? right ?

Robert

rjal
New Contributor I
210 Views

i got the board and figured out what is going on here. The block diagram is wrong.

basically the board has a 8x/4x/4x option when 3 16x pci-e cards are used. there is no extra 4x pci-e port. Not sure how things are routed but I'm assuming the is a second set of pci-e switches to make the 2 pci-e@4x slots possible. (in which case there is extra latency I think.)