Tuesday, April 10th 2012

Third Long PCIe Slot On High-End Z77 Boards Wired to CPU, Works Only with Ivy Bridge

It turns out that the "third" long (physical x16, electrical x4) PCI-Express slot on most higher-end Z77 chipset-based motherboards, across vendors, are wired to the CPU, and not the Z77 PCH, as the media assumed. Early buyers of these motherboards were greeted by an informative sticker stuck to the third slot, which tells them that to use the third slot, a 3rd Generation Core "Ivy Bridge" processor must be installed, although the motherboard very much supports 2nd Generation Core "Sandy Bridge" processors.

This can be explained by taking a close look at the block diagram of Intel Z77 Express system. Z77, in combination with "Ivy Bridge" processors, allows the CPU root complex to drive three devices. Its single PCI-Express x16 link can be arranged in three ways: x16/NC/NC; x8/x8/NC; and x8/x4/x4. As you can see, the third long slot is taken into the configuration. Intel figured out that since PCI-Express 3.0 x4 offers bandwidth comparable to PCI-Express 1.0 x16 to gen 3-compliant graphics cards, it's wise if the third electrical x4 slot is also wired to the CPU's PCIe root complex. This renders most high-end LGA1155 motherboards with such CPU-driven third x16 (x4) slots 3-way SLI/CrossFireX-capable. Sweet.

Source: PC Watch Akiba
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26 Comments on Third Long PCIe Slot On High-End Z77 Boards Wired to CPU, Works Only with Ivy Bridge

#1
1c3d0g
That's a fair trade-off. If you can do triple-SLI, you can afford an Ivy Bridge CPU.
Posted on Reply
#2
slyfox2151
by: 1c3d0g
That's a fair trade-off. If you can do triple-SLI, you can afford an Ivy Bridge CPU.
lol fair point.
Posted on Reply
#3
Chaitanya
anyways most people buying these CPU's in first place wont be using more than 2 video cards.
Posted on Reply
#4
DonInKansas
by: Chaitanya
anyways most people buying these CPU's in first place wont be using more than 2 video cards.
Why?
Posted on Reply
#8
Grings
I wonder if any motherboard manufacturers will make the 3rd slot switchable between the chipsets pci-e2 lanes and the cpu's?

It would suck if putting a sound card or raid controller etc in that slot makes one of you're graphics cards drop to 4 lanes.
Posted on Reply
#9
eidairaman1
by: Vulpesveritas
LGA1155= Mainstream.
ya and soon to be EOL with 1150 coming up shortly
Posted on Reply
#10
Dj-ElectriC
Everything that intel makes is soon to be EOL :D
Posted on Reply
#11
eidairaman1
by: Dj-ElectriC
Everything that intel makes is soon to be EOL :D
aint that right heh:laugh:
Posted on Reply
#12
Steven B


however it isn't true on all board, for the UD3H its wired to PCh, for UD5H its CPU.
Posted on Reply
#13
RejZoR
What a rubbish crap. Most higher end boards in the long past had 2x full x16 PCIe. Like long forgotten X58
Posted on Reply
#15
Jurassic1024
by: The Von Matrices
An extra PCIe 2.0 x4 was present in the 1155 Sandy Bridge processors as well (connected to the processor, not the PCH); it just wasn't enabled except when paired with a server chipset. See the Asus P8B WS for example and a review of the server chipsets.
Except all those slots are wired for PCIe 2.0, and we're talking about using the 3rd slot for another PCIe 3.0 GPU in a slot wired for PCIe 3.0 x4. Am I wrong?
Posted on Reply
#16
Jurassic1024
by: Grings
I wonder if any motherboard manufacturers will make the 3rd slot switchable between the chipsets pci-e2 lanes and the cpu's?

It would suck if putting a sound card or raid controller etc in that slot makes one of you're graphics cards drop to 4 lanes.
I found it odd, if i put my x1 sound card in my second x16 slot (wired for x8), my GPU in the first x16 slot downgrades to x8.
Posted on Reply
#17
Aquinus
Resident Wat-man
by: Jurassic1024
Except all those slots are wired for PCIe 2.0, and we're talking about using the 3rd slot for another PCIe 3.0 GPU in a slot wired for PCIe 3.0 x4. Am I wrong?
It is wired for PCI-E 2.0 x4 on the 3rd PCI-E x16 slot. :mad:

Asus' Site and NewEgg both says this for Z77 Sabertooth:
Expansion Slots
2 x PCIe 3.0/2.0 x16 (x16 or dual x8) *2
1 x PCIe 2.0 x16 (x4 mode, black) *3
3 x PCIe 2.0 x1
...
*2: PCIe 3.0 speed is supported by Intel® 3rd generation Core™ processors.
*3: The PCIe 2.0 x16 slot shares bandwidth with PCIe 2.0 x1_1 slot, PCIe 2.0 x1_2 slot and PCIe 2.0 x1_3 slot. The PCIe 2.0 x16 slot default setting is in x1 mode.
Source

Same is true of the P8Z77-V DELUXE.

by: Jurassic1024
I found it odd, if i put my x1 sound card in my second x16 slot (wired for x8), my GPU in the first x16 slot downgrades to x8.
That is because that first 1x lane is actually the 9th lane on your first slot. They're switched.
Posted on Reply
#18
eidairaman1
in all honesty if you want to do Multi Card Configs go with skt 2011 and skip 1155 altogether
Posted on Reply
#19
Aquinus
Resident Wat-man
by: eidairaman1
in all honesty if you want to do Multi Card Configs go with skt 2011 and skip 1155 altogether
It has more PCI-E lanes for a reason. What is there not to like about x8/x8/x8/x8 @ PCI-E 3.0?
Posted on Reply
#20
Morgoth
i got 4 full 16x slots... anda bunch of 8x slots
Posted on Reply
#21
Dos101
by: eidairaman1
in all honesty if you want to do Multi Card Configs go with skt 2011 and skip 1155 altogether
^^ This. Although there are some 2011 motherboards that are only x8/x8.
Posted on Reply
#22
newtekie1
Semi-Retired Folder
by: Chaitanya
anyways most people buying these CPU's in first place wont be using more than 2 video cards.
Actually, I'd say most people buying these probably won't be using more than 1 video card.
Posted on Reply
#23
Steven B
why is everyone hating on Z77 lol
Posted on Reply
#24
Jurassic1024
by: Aquinus
It has more PCI-E lanes for a reason. What is there not to like about x8/x8/x8/x8 @ PCI-E 3.0?
Scaling sucks when you go past 2 cards and it has nothing to do with what PCIe version or number of PCIe lanes your CPU has, assuming you're matching the PCIe version your motherboard and/or CPU supports to the version supported by your GPU(s).

by: Steven B
why is everyone hating on Z77 lol
It offers very little. Quicksync and PCIe 3.0 benefit only a small few, and 3770K performance is minimal over the 2600K.

by: Aquinus
It is wired for PCI-E 2.0 x4 on the 3rd PCI-E x16 slot. :mad
So is the third x16 slot on my P8P67 EVO. It has to do with the limited lanes on the 2nd generation Core CPU's if those 4 lanes are coming from the PCH.

by: eidairaman1
in all honesty if you want to do Multi Card Configs go with skt 2011 and skip 1155 altogether
If you want poor scaling, go ahead.
Posted on Reply
#25
eidairaman1
by: Jurassic1024
If you want poor scaling, go ahead.
you know there is a multi-quote button, use it

and 2011 has more expansion and capability than 1155 does
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