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What's the difference between PCI express 2.0 x16/2.0 x8/2.0 x4?

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September 20, 2011 6:36:54 PM

On newegg the ASUS M4A89TD PRO/USB3 AM3 AMD 890FX SATA 6Gb/s USB 3.0 ATX AMD Motherboard says it has 2 PCI express 2.0 x16 slots.
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The MSI 870-G45 AM3 AMD 770 ATX AMD Motherboard says it has 2 PCI express 2.0 x16 slots (x16, x4)
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The ASRock 970 EXTREME4 AM3+ AMD 970 SATA 6Gb/s USB 3.0 ATX AMD Motherboard with UEFI BIOS says it has 3 PCI express 2.0 x16 slots (x16/0 or x8/x8, x4)
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I've been looking on google, and there seems to be a slight difference in performance between PCI express 2.0 x4, PCI express 2.0 x8 and PCI express 2.0 x16, but in the pictures all of the PCI 2.0 express slots look identical. I don't understand why there would be any difference at all between them if they look identical. How would I even know which one is a PCI express 2.0 x4 or PCI express 2.0 x16?
a c 120 V Motherboard
September 20, 2011 6:50:27 PM

Ok, the PCIe xXX is the amount of lanes the slot has. so x16 has 16 lanes, x8 has 8 lanes and so on.

The physical slot on x4, x8, and x16 are all the same.

If it says 2 PCIe x16 slots it means that there are 2 slots on the board that are capable of 16 lanes.

If it says 2 PCIe X16 (and then in brackets says x16, x4 or x8, x8) it means when you have 1 slot being used it uses all 16 lanes for that 1 slot. But if you plug multiple things into the PCIe slots (most common 2 GPU's) it will divide the lanes between the cards. So that (x16, x4) you will have the first GPU running with 16 lanes, and the second GPU running with 4 lanes.

The manual for any board is going to say what the slot is at natively (x16, x8, x4) and how it reacts if in SLI (multiple GPU's)

For GPU's most of the time you are not going to notice a difference between 16x and 8x because GPU's can not fully saturate the bandwidth of the slot, therefore it is hardware limited, not bus limited.

2 GPU's in xfire / SLI on 2 8x slots will not fully saturate the bus.
September 20, 2011 7:12:27 PM

Oh, ok that makes a lot more sense now. One small thing though, what do you mean by "hardware limited, not bus limited". I'm still very new to this stuff. Also, would it be a good idea to get a mobo that has 2 x16 slots because I'm trying to build a gaming/music pc for about $500, but I want to have options in the future. Will having the x16 slots make a difference in the next few years?
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a c 120 V Motherboard
September 20, 2011 8:28:55 PM

Hardware limited means that the amount of data passed is limited by how fast / how much data the hardware can put out, not the pipeline you are trying to put it through.

A GTX 590 (dual GPU) card, which is the most powerful card available on the market barely fully saturates a PCIe 2.0 x8 bus.

That means that even though the card is maxing out it's potential, the bus (assuming x16) still has >7 lanes still available. Which means that the hardware is only using ~ 50% available bandwidth.

So when I say hardware limited instead of bus limited I am merely stating that the bus (in this case PCIe 2.0) is nowhere close to being a limiting factor for any hardware currently in production.
July 26, 2012 3:59:04 PM

basically, what you are trying to say is that we cannot decrease the GPU preformanse by putting them into x8/x8 - Crossfire/SLI mode, because the strongest one uses only x8 bandwith. ok, I completely understand this. But it says that Asrock Extreme3 in a multi gpu mode gives x16/x4 bandwith. is that enough for e.g. two HD 7950?
August 2, 2012 5:38:52 PM

I have the same concern. My board now does X16 and X16. Many of theses new boards (and the one that someone is going to give me) only does X16 and X4. So even hough I can get a better newer processor, I am afraid that my two 1GB Radeon 6990's will actually runs slower in this new board because of the limitation placed on the second card.
August 2, 2012 7:24:52 PM

Thanks. I found this article right after posting. It states pretty similar results using older technology. I think thos individuals that move video cards over to faster CPU's from older systems with X16 to new ones with X8 probably see slight increases in frame rates only because the processing upgrade slightly outweighs the loss from the slower pipelines. Regardless, I am going to stick with my two X16 lanes for now. I think that moving to a sandy bridge i5 2500k from my i7 950 will only slow my gaming down unless I dish out big money for another board that does 2 X 16X. And if I am going to spend that kind of money I might as well get an i7. Boy how things start adding up. I'll just wait a while yet.
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