Question about understanding PCI-e specs

[polyesterpeanut]

I'm looking into buying my first motherboard, and i'm wondering why in the specs section the number PCI x16 slots is not a single number but instead lists two slots, one with 16 lanes and another with 4 (http://www.tomshardware.com/reviews/gigabyte-z170-hd3-lga-1151-intel-motherboard,4279.html). Is it trying to say that although I can fit a 16 lane expansion card in the slot it will only support the bandwidth equivalent to a x4 slot?

 

[giantbucket]

99% of the time the first slot is 16x electrically and physically, and the other slots can be 16x physically but only 8x or 4x electrically - this means you can fit in a second card, but it just won't be running at full speed. not the best, but better than nothing.

sometiems, the second slot is run from the chipset (Z87 chipset of A88X chipset) while the first slot is [almost] always run from the CPU directly.

 

[polyesterpeanut]

99% of the time the first slot is 16x electrically and physically, and the other slots can be 16x physically but only 8x or 4x electrically - this means you can fit in a second card, but it just won't be running at full speed. not the best, but better than nothing.

sometiems, the second slot is run from the chipset (Z87 chipset of A88X chipset) while the first slot is [almost] always run from the CPU directly.

Very interesting, is there a way to tell if the second slot is run from the chipset or not?

 

[giantbucket]

motherboard manuals sometimes have a block diagram in the first few pages, which show what controls what. this one doesn't, but based on the second slot being only 4x, it's 99% likely to be run from the chipset while the first slot is run from the CPU. the second slot is fine for a low-end additional graphics card that runs secondary monitors, or whatever other PCIe card you may need

 

[k1114]

For most maintsream chipsets, all of the cpus only have 16 lanes so the gpu gets those 16 and the other slots will be the mobo. If it's two 16x slots that run at 8/8 with both filled than those are the cpu. But with one gpu in the first slot, the chipset can provide the other 16x slot to not reduce the gpu to 8x. That's just general, you can look at the mobo specs even without a diagram, you might be able figure it out. Like for the one you mentioned there's only that one 16x slot from the cpu.