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I am asking for help to understand the details of PCI Express 2.0 x 16

  • Asus
  • PCI Express
  • Motherboards
Last response: in Motherboards
December 4, 2009 9:57:05 PM

I am in the planning phases of building my next gaming machine and I want it to be a power house.

My question is regarding the purchase of a motherboard with at least two or three slots for the PCI-E x 16 because I am buying one ATI video care with the Radeon HD 5970 GPU, 2 GB. At a later time, I may buy another identical card and run both cards in crossfire.

What I don’t under stand, are the descriptions and here are 3 examples from my search on Newegg.

• 3 x PCIe 2.0 x16 (at x16/x8/x8 or x16/x16/x1 mode)

• 2 x PCIe 2.0 x16 (single at x16 or dual at x8 / x8 mode)

• 2 x PCI Express x16 slots, running at x16 (PCIEX16_1/PCIEX16_2)
2 x PCI Express x16 slot, running at x8 (PCIEX8_1/PCIEX8_2)
Note: The PCIEX8_1 and PCIEX8_2 slots share bandwidth with the PCIEX16_1 and PCIEX16_2 slots respectively. When PCIEX8_1 is populated with an expansion card, the PCIEX16_1 slot will operate at up to x8 mode; when PCIEX8_2 is populated with an expansion card, the PC IEX16_2 slot will operate at up to x8 mode.

I am not clear what to look for in PCI Express x 16 slots so that I have the best performance with the high-graphics card that I mentioned above.

Can two slots run at x16/x16? Is this the optimal configuration for future upgrades with a 2nd video card in crossfire?


More about : understand details pci express

December 4, 2009 10:28:10 PM

choice 2 and 3 are the same and the first option is for am2+/am3 and core i7(lga 1366). Pick lga 1366 because you get better pci-e bandwith and stronger cpu.
December 4, 2009 11:04:59 PM

ok the first one, that one>>>"• 3 x PCIe 2.0 x16 (at x16/x8/x8 or x16/x16/x1 mode) " means that on the mobo there are 3 PCI-E 2.0 expansion bays, the first 2 PCI-E expansion slots support up to full x16 speed, the last slot only supports up to x8. I assume the last 2 are shareing the same bus (think of lane on the road), so like the first PCI-E has its own lanes, where as the last 2 are sharing lanes. so if you have just 1 graphics card then you can just run it on the first PCI-E at full x16 speed, if you have 2 and want to run in x-fire or SLI, then you can also run the 2nd card at full x16 speed, but if you do then the last PCI-E bus can only run at x1 speed for what ever you run on it. And if you want to do tri SLI or tri crossfire (i donno if thats what you call them) but a 3 card setup anyways then you'll need to run all 3 cards at x8 speed.

with the 2nd one, this one >>> •" 2 x PCIe 2.0 x16 (single at x16 or dual at x8 / x8 mode) " that means theres 2 PCI-E expansion slots which both share the same bus, (think lanes again), where as the setup above (the first one) 1 PCI-E had its own bus (or lane if your thinking cars) and the 2nd and 3rd PCI-E was sharing 1, so this 2nd mobo has 2 PCI-E sharing 1 bus which the max speed is x16 so if you want to run 2 cards then the bus will have to split the speed making both bus running at x8. So meaning if you just have 1 card (in either of the PCI-E slots) then it'll run at full speed, when you plug in a 2nd card they'll both run at x8.

and the 3rd one, that one >>> "• 2 x PCI Express x16 slots, running at x16 (PCIEX16_1/PCIEX16_2)... " I assume it has 4 PCI-E slots 2 that can runn at x16(max speed) and 2 that can run at x8 (max speed) mode, where all 4 are sharing 2 bus, the first x16 is sharing with the first x8 and the 2nd x16 is sharing with the 2nd x8, the x8's can only ever use x8 speed even though its sharing with the x16. Meaning if you had 2 graphics card you'd put them into both the PCI-Ex16 and they'll run at full speed, (the mobo manual will tell u which ones are wich), but if you introduce a 3rd card then then 1 of the PCI-E will slow down to x8 and if your running tri SLI or x-fire then they'll all slow down to x8, but say you have independent cards (just for say extra graphics processing, but don't need SLI or x-fire) then you can have 1 at x16 and 2 at x8. Now if your running 4 cards then all 4 will be running at x8 since now all 4 PCI-E will be sharing its lanes.

I donno how well I explained that cause I felt like i typed way too much for something that I don't think really needs that much explaining lol but I hope you get it.

December 5, 2009 11:51:52 AM


You didn’t type too much; this sincerely helps me understand what I need to buy for a motherboard. Certainly, I don’t want that bottle neck if I go to 2 cards. I’ve noticed that mobo’s with 3 PCIe 2.0 slots usually will have the first two running at 16 and the 3rd at 8. So as I search for a mobo, I’ll make sure that at least two slots will have the capability of running at 16 with 2 video cards installed.

I appreciate the time you took to type this out; I have certainly been educated.

Thanks to all.