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Does the CPU Control Expansion Slots

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December 17, 2012 8:20:23 PM

Ok, I see that the 3700 says "1x16, 2x8, 1x8 & 2x4" But I see some Z77 motherboards that have more x16 slots and I dont see any Z77 CPU's that support more than 1 X16. So, whats going on?
a c 887 à CPUs
a c 1152 V Motherboard
December 17, 2012 8:35:41 PM

That is what the CPU can support the chipset has additional lanes. Dual graphics are done on that platform at X8 X8 speed.
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December 17, 2012 9:56:22 PM

What do you mean by support? How does CPU support differ from motherboard support?
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a c 887 à CPUs
a c 1152 V Motherboard
December 17, 2012 10:01:46 PM

Some of the lanes are built into the CPU some into the chip set on the board that is what I am saying.
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a c 132 à CPUs
a c 81 V Motherboard
December 17, 2012 10:07:17 PM

The physical slots may be x16 but may be wired for x4 or x8.

Boards that support x16x16, x8x8x8x8 or other combinations totaling more than 16 PCIe 3.0 lanes with LGA1155 CPUs are using PLX or similar PCIe switches to distribute the CPU lanes' bandwidth across more devices. These chips add $40-80 apiece to the board's cost.
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December 18, 2012 12:03:46 AM

So, if I have 3770 then using more lanes than x16 reduces total bandwidth regardless of putting it in seperate x16?
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a c 132 à CPUs
a c 81 V Motherboard
December 18, 2012 3:33:03 AM

coolitic said:
So, if I have 3770 then using more lanes than x16 reduces total bandwidth regardless of putting it in seperate x16?

LGA1155 CPUs have only 16 PCIe lanes available and if you use multiple x16 slots, some of the lanes will be redirected to those other slots. The total bandwidth is still the same but the bandwidth available to individual slots will vary depending on how lanes have been re-arranged. On most boards, at least one of the x16 slots will be wired to the chipset's x4 PCIe 2.0 port which is less than ideal for multi-GPU.

It is technically possible to make the full x16 bandwidth available to multiple x16 slots by using something like PLX's PEX8780 PCIe switch but this is expensive and there isn't much benefit connecting GPUs at x16 rather than x8. Once you settle on x8, you can simply get an LGA2011 CPU which has 40x PCIe 3.0 lanes which allows you to run your GPUs at x16x8x8x8 without glue-logic.

BTW, when I say PLX's chips are expensive, you would need the PEX8748 to do x8x8x8x8 in a way that is similarly efficient to LGA2011 CPUs and that smaller chip already already costs around $170. So an LGA1155 board with it would end up costing in the neighborhood of $400.
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December 18, 2012 3:43:03 AM

Bandwidth is shared between all the slots on the board, so if you run one card, it can run at x16 but if you run 2 it has to split into x8/x8, 3 cards:x8/x4/x4.

Some higher end motherboards have different/multiple PCie switches/controllers that provide more bandwidth than a stock, but they tend to be very expensive(a well known example, the ASRock Extreme11, is no less than 599.99 off Newegg and performs near the bottom compared to other boards in benchmarks). However all LGA1155 will be able to run 1 GPU at x16, as the processor provides that bandwidth.

If you really want 4-way SLI consider either an LGA2011 build or even a dual C602/C606 build with an SR-X/Z9PE, they both provide more bandwidth than a "consumer" level LGA1155.

Good luck!
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December 18, 2012 6:40:09 PM

darksparten said:
Bandwidth is shared between all the slots on the board, so if you run one card, it can run at x16 but if you run 2 it has to split into x8/x8, 3 cards:x8/x4/x4.

Some higher end motherboards have different/multiple PCie switches/controllers that provide more bandwidth than a stock, but they tend to be very expensive(a well known example, the ASRock Extreme11, is no less than 599.99 off Newegg and performs near the bottom compared to other boards in benchmarks). However all LGA1155 will be able to run 1 GPU at x16, as the processor provides that bandwidth.

If you really want 4-way SLI consider either an LGA2011 build or even a dual C602/C606 build with an SR-X/Z9PE, they both provide more bandwidth than a "consumer" level LGA1155.

Good luck!


I wasnt going to use SLI (maybe later though). Just a creative sound card because Realtek does not like punkbuster.
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December 24, 2012 11:44:07 PM

I found out that LGA1155 boards with multiple x16 do share bandwidth, but not all do it the way darksparten said, some (price ranging 200+) just share the bandwidth but not by x16,x8,x4.... increments.
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