PCIe slots - Questions

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PCIe slots - Questions

Postposted on Tue Feb 19, 2013 10:37 am

Motherboard spec states:
PCIe 2.0 x16: 3(x16/0 or x8/x8, x4)
PCIe x1 : 2

Questions:

1. What does (x16/0 or x8/x8, x4) means? No two x16 cards?
2. Can the two x1 slots be usable regardless of how the three x16 slots are used?
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Re: PCIe slots - Questions

Postposted on Tue Feb 19, 2013 10:49 am

You sorta have the first question answered. For the 3 presumably full length slots, one has only 4 lanes, and two share 16 between them. If you use both of those two, each will only have 8 available to them. If you only use one, that one will have the full 16. This is a very common arrangement. Note that effectively PCIe 2.0 x16 is equivalent to PCIe 3.0 x8. When PCIe 3.0 came out, it was shown the performance impact of having x8 vs x16 was very small - I don't know of studies done on newer graphics cards. Also, a full length card plugged into a full length slot will generally always work, regardless of how many lanes the card is capable of using or are available, BUT there may be a performance impact. You will definitely notice if you plug a graphics card into the x4 slot, for example.

The two x1 slots are probably connected to the south bridge and share interconnect bandwidth with it (sorry if I got the terminology wrong gerbils), they will both work in any configuration.
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Re: PCIe slots - Questions

Postposted on Tue Feb 19, 2013 11:07 am

churin wrote:2. Can the two x1 slots be usable regardless of how the three x16 slots are used?


It depends on where the x1 slots are located. If they are too close to the x16 slot and you put a high end video card in there it will physically block the x1. You need to look at a picture of the MB to determine this.
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Re: PCIe slots - Questions

Postposted on Tue Feb 19, 2013 12:50 pm

EJ257 wrote:
churin wrote:2. Can the two x1 slots be usable regardless of how the three x16 slots are used?

It depends on where the x1 slots are located. If they are too close to the x16 slot and you put a high end video card in there it will physically block the x1. You need to look at a picture of the MB to determine this.

OK, the two x1 slots are always available as long as there is no such physical constraints.
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Re: PCIe slots - Questions

Postposted on Tue Feb 19, 2013 1:04 pm

Last review I read on the subject (I can't remember where, but it was probably either Techpowerup or [H]) averaged out the scores.

Testing a GTX570 with PCIe 2.0, an 8x slot was about a 2% performance penalty compared to a 16x slot, with the penalty growing to a 5% less than a 16x slot when using a 4x slot.

In other words, using a 4x slot isn't a complete deal-breaker, but there will be a small measurable difference. As for 16x vs 8x, I very much doubt you'd notice without repeating monotonous benchmark loops to average out the variance between each test run.
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Re: PCIe slots - Questions

Postposted on Tue Feb 19, 2013 1:06 pm

TwistedKestrel wrote:You sorta have the first question answered. For the 3 presumably full length slots, one has only 4 lanes, and two share 16 between them. If you use both of those two, each will only have 8 available to them. If you only use one, that one will have the full 16.

Does (x16/0 or x8/x8, x4) mean (one video card + one x4 card) or (two video cards + one x4 card)? If this is so, then in either case, does it mean only 20 lanes are used by these cards?
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Re: PCIe slots - Questions

Postposted on Tue Feb 19, 2013 1:55 pm

churin wrote:OK, the two x1 slots are always available as long as there is no such physical constraints.


Yes.

churin wrote:Does (x16/0 or x8/x8, x4) mean (one video card + one x4 card) or (two video cards + one x4 card)? If this is so, then in either case, does it mean only 20 lanes are used by these cards?


Yes.

Basically there are only 16 lanes of PCIe 2.0 coming from the CPU socket. These go to the first and second x16 slots. If you only use one of them you get the full 16 lanes (x16) for that card. If you plug in 2 cards you only get to use 8 lanes each (x8/x8). You are limited by the number of lanes from the CPU.

The third x16 slot goes to the P67 chipset on the board but its only connected to 4 lanes of PCIe 2.0 (x4). The x1 slots also go through the P67. The P67 supports up to 8 lanes of PCIe 2.0. Your board is using 6 out of the 8 lanes from the P67. Where are the other 2? You probably need a bigger board because the one your looking at may not have the real estate for those 2 extra x1 slots.
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Re: PCIe slots - Questions

Postposted on Tue Feb 19, 2013 3:43 pm

EJ257 wrote: The P67 supports up to 8 lanes of PCIe 2.0. Your board is using 6 out of the 8 lanes from the P67. Where are the other 2? You probably need a bigger board because the one your looking at may not have the real estate for those 2 extra x1 slots.
On a P67 those other two lanes are typically used for on-board peripherals: ethernet (since most boards use a discrete networking chip rather than adding a PHY to the gigE built into the chipset), USB 3.0 and additional SATA, etc. If we had the make and model of this particular board we could say exactly.
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Re: PCIe slots - Questions

Postposted on Tue Feb 19, 2013 7:52 pm

The motherboard is ASRock 979Extreme4.
What is P67 referred to on posts by EJ257 and UberGerbil?
Last edited by churin on Tue Feb 19, 2013 8:17 pm, edited 1 time in total.
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Re: PCIe slots - Questions

Postposted on Tue Feb 19, 2013 8:03 pm

P67 refers to Intel chipset: http://ark.intel.com/products/52810/Intel-BD82P67-PCH

Edit: I'm assuming they were using the P67 as an example in explaining the PCIe lane arrangement.
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Re: PCIe slots - Questions

Postposted on Tue Feb 19, 2013 8:22 pm

Are the two x1 slots supported by AMD SB950 instead?
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Re: PCIe slots - Questions

Postposted on Tue Feb 19, 2013 8:36 pm

I do not believe the AMD SB950 southbridge provides any additional PCIe connectivity. So on an AMD motherboard with a 9xx chipset the PCIe lanes come out of the northbridge.
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Re: PCIe slots - Questions

Postposted on Tue Feb 19, 2013 9:38 pm

How about GA-790XTA-UD4?
This board has two full size slots. Presently they are populated by one video card and one x1 card. How does the x1 card affect the video card performance?
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Re: PCIe slots - Questions

Postposted on Tue Feb 19, 2013 10:37 pm

Video card will run at x8 when both full-length slots are in use. Move the x1 device to one of the x1 slots if you can, & make sure video card is in the slot that is capable of full x16 operation (not sure which one that is, check the manual).

Even if you can't move the x1 device, running the video card in an x8 slot will only have a small impact on performance.
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Re: PCIe slots - Questions

Postposted on Wed Feb 20, 2013 9:09 am

The video card is on the one where it will function as x16 card if no card is on another full size slot. One of the two x1 slots is accessible but it was found that a connector could not be plugged on the card because it was located adjacent to the one end of the chassis opening for the expansion slots. So, I thought why not using the full size slot instead which is located in the middle of the chassis opening.

It is true I did not notice any change in video performance, but I wonder why 50% reduction in numbers of lanes available have only " a small impact of performance". Could anyone explain why?
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Re: PCIe slots - Questions

Postposted on Wed Feb 20, 2013 9:29 am

churin wrote:It is true I did not notice any change in video performance, but I wonder why 50% reduction in numbers of lanes available have only " a small impact of performance". Could anyone explain why?

Because 16 lanes is overkill. In most cases 8 lanes are more than enough to keep the GPU fed, since you end up being limited by the CPU and/or GPU, not the interconnect between the two.
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Re: PCIe slots - Questions

Postposted on Wed Feb 20, 2013 10:20 am

just brew it! wrote:I do not believe the AMD SB950 southbridge provides any additional PCIe connectivity. So on an AMD motherboard with a 9xx chipset the PCIe lanes come out of the northbridge.


I will beg to differ on this one.
My giga 990XA, bottom slot is 16x physical, and 4x wired through the southbridge
http://static.techspot.com/articles-inf ... agramT.png

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Re: PCIe slots - Questions

Postposted on Wed Feb 20, 2013 10:51 am

anotherengineer wrote:
just brew it! wrote:I do not believe the AMD SB950 southbridge provides any additional PCIe connectivity. So on an AMD motherboard with a 9xx chipset the PCIe lanes come out of the northbridge.


I will beg to differ on this one.
My giga 990XA, bottom slot is 16x physical, and 4x wired through the southbridge
http://static.techspot.com/articles-inf ... agramT.png

yes wth indeed

I stand corrected (and learned something today).

The bizarre design of the 990XA is apparently necessitated by the fact that this board has 3 physical 16x slots. They basically ran out of lanes on the northbridge...
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Re: PCIe slots - Questions

Postposted on Wed Feb 20, 2013 11:13 am

just brew it! wrote:
churin wrote:It is true I did not notice any change in video performance, but I wonder why 50% reduction in numbers of lanes available have only " a small impact of performance". Could anyone explain why?

Because 16 lanes is overkill. In most cases 8 lanes are more than enough to keep the GPU fed, since you end up being limited by the CPU and/or GPU, not the interconnect between the two.

Thank you very much for enlightening me on this.
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Re: PCIe slots - Questions

Postposted on Wed Feb 20, 2013 11:31 am

I do not see any x16 PCIe coming out of the southbridge on the diagram of 990XA. How should I interpret the diagram?
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Re: PCIe slots - Questions

Postposted on Wed Feb 20, 2013 12:54 pm

churin wrote:I do not see any x16 PCIe coming out of the southbridge on the diagram of 990XA. How should I interpret the diagram?

anotherengineer's point was that his 3rd x16 slot is actually only x4, since it is hanging off of the southbridge. Upper left corner of the southbridge in the diagram.
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Re: PCIe slots - Questions

Postposted on Wed Feb 20, 2013 4:36 pm

just brew it! wrote:
churin wrote:I do not see any x16 PCIe coming out of the southbridge on the diagram of 990XA. How should I interpret the diagram?

anotherengineer's point was that his 3rd x16 slot is actually only x4, since it is hanging off of the southbridge. Upper left corner of the southbridge in the diagram.

I got it. I wonder why the PCIe connector is full size even though only x4 card can be used on it. Maybe there are only two different physical sizes for PCIe cards. Is this correct?
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Re: PCIe slots - Questions

Postposted on Wed Feb 20, 2013 5:10 pm

You're not limited to using only x4 cards in it. But a x16 card in that slot will run at x4 speed.

The full-length slot is so you have the retention clip at the end of the slot for the video card.
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Re: PCIe slots - Questions

Postposted on Wed Feb 20, 2013 5:36 pm

I have never used more than one video card. So it is hard for me to imagine a situation where three video cards are installed.
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Re: PCIe slots - Questions

Postposted on Thu Feb 21, 2013 10:10 am

UberGerbil wrote:On a P67 those other two lanes are typically used for on-board peripherals: ethernet (since most boards use a discrete networking chip rather than adding a PHY to the gigE built into the chipset), USB 3.0 and additional SATA, etc. If we had the make and model of this particular board we could say exactly.


Thank you. I forgot they could also do this.

churin wrote:I wonder why the PCIe connector is full size even though only x4 card can be used on it. Maybe there are only two different physical sizes for PCIe cards. Is this correct?


They used a full sized slot to be flexible. I think every size but x2 exist. x8 slots you could find on server boards but I have never seen them on consumer hardware. The cool thing about PCIe is that the power, signaling, and clock are always present on those pins left of the notch. The data lanes are all right of the notch starting with x1 and you can use as many or as little as you need. In your example you can plug a x1 card into that full size slot with only x4. It would only use the first set of lanes and it would work like an x1 slot. Since it is a full sized slot, if you plug a x16 card into it the card will detect only 4 lanes and will only allow the first 4 lanes on the card to be used.
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Re: PCIe slots - Questions

Postposted on Thu Feb 21, 2013 11:55 am

EJ257 wrote:
UberGerbil wrote:
churin wrote:I wonder why the PCIe connector is full size even though only x4 card can be used on it. Maybe there are only two different physical sizes for PCIe cards. Is this correct?

They used a full sized slot to be flexible. I think every size but x2 exist. x8 slots you could find on server boards but I have never seen them on consumer hardware. The cool thing about PCIe is that the power, signaling, and clock are always present on those pins left of the notch. The data lanes are all right of the notch starting with x1 and you can use as many or as little as you need. In your example you can plug a x1 card into that full size slot with only x4. It would only use the first set of lanes and it would work like an x1 slot. Since it is a full sized slot, if you plug a x16 card into it the card will detect only 4 lanes and will only allow the first 4 lanes on the card to be used.

Thank you very much for your reply.
Please note that what I did was plugging a x1 card into the second full size slot on 790XTA-UD4 although the slot referred to in my post above is the the third full size slot on 979Extreme4. I was concerned about negative impact of x1 card on the second full size slot to the performance of the video card on the first full size slot.
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Re: PCIe slots - Questions

Postposted on Thu Feb 21, 2013 12:09 pm

EJ257 wrote:
UberGerbil wrote: . . . The cool thing about PCIe is that the power, signaling, and clock are always present on those pins left of the notch. The data lanes are all right of the notch starting with x1 and you can use as many or as little as you need. . . .

Aren't the above something useful only for PCIe card designers and not for users?
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Re: PCIe slots - Questions

Postposted on Thu Feb 21, 2013 4:47 pm

churin wrote:
EJ257 wrote:
UberGerbil wrote: . . . The cool thing about PCIe is that the power, signaling, and clock are always present on those pins left of the notch. The data lanes are all right of the notch starting with x1 and you can use as many or as little as you need. . . .

Aren't the above something useful only for PCIe card designers and not for users?

It means cards will work in slots that don't necessarily match the native width of the card. The card may not run at peak performance, or you may waste capabilities of the PCIe slot, but it will still function. That's a useful feature for end users.
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Re: PCIe slots - Questions

Postposted on Thu Feb 21, 2013 8:38 pm

just brew it! wrote:
churin wrote:
UberGerbil wrote: . . . The cool thing about PCIe is that the power, signaling, and clock are always present on those pins left of the notch. The data lanes are all right of the notch starting with x1 and you can use as many or as little as you need. . . .

Aren't the above something useful only for PCIe card designers and not for users?

It means cards will work in slots that don't necessarily match the native width of the card. The card may not run at peak performance, or you may waste capabilities of the PCIe slot, but it will still function. That's a useful feature for end users.

UberGerbil's statement as quoted is about the motherboard circuit design around the PCIe slot. I had difficulty understanding exactly what he meant and thought end users need not be concerned about.
I took a close look at the mainboard and now understand exactly what his statement means.
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Re: PCIe slots - Questions

Postposted on Thu Feb 21, 2013 9:30 pm

churin wrote:UberGerbil's statement as quoted is about the motherboard circuit design around the PCIe slot. I had difficulty understanding exactly what he meant and thought end users need not be concerned about.
I took a close look at the mainboard and now understand exactly what his statement means.
I didn't write that -- EJ257 did, and then a nested quote tag got mangled so that it got mis-attributed (see above). Nevertheless, it is technically correct. And I'd point out in the statement the mention of power in addition to signalling and clock, as this is often overlooked or misunderstood: an x1 (or x4 or x8) card has exactly as much power available to it as an x16 card. (Though this mostly doesn't matter, since video cards are the consumer PCIe component that consumes the most power -- and they tend to consume so much they need auxiliary power connectors anyway -- it removes one point of worry when you're thinking about mixing and matching slots and cards).
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