So what happens when all slots are filled on a z790 motherboard?

[hammy]

 

So I'm trying to understand how PCIe lanes work on a motherboard - as they don't make sense to me, unless not every lane can actually be populated.

 

Take for example the ASUS PRIME Z790-P WIFI D5: 

• Z790 motherboard.
• Paired with a 14700k, the 14700k has 20 total PCIe lanes.
• 1x PCIe 5.0 x16 PCIe Slot on the top slot
• 3x PCIe 4.0 x4 PCIe Slots underneath
• 1x PCIe 3.0 slot between the bottom 2 PCIe slots

 

If my math is correct, aren't you wildly over the PCIe lane limitation set by CPU if you populate each slot?

 

1st slot = x16

2nd slot = x4

3rd slot = x4

4th slot = x1

5th slot = x4

 

Doesn't that result in 27 possible lanes? And what about m.2 slots, surely some of those get eaten up?

 

The manual offers no clarification other than a "suggested GPU layout" for 2+ GPU's:

 

 

So, how does this work? CAN you use all slots are their full speeds? Or no?

[RONOTHAN##]

10 minutes ago, hammy said:

So, how does this work? CAN you use all slots are their full speeds? Or no?

Most of the slots are wired through the chipset. Without going into too great of detail, the chipset takes a certain number of PCIe lanes from the CPU (on Z790 it's 8 PCIe Gen 4 lanes, though for technical reasons they're called DMI links in this context) and turns them into USB ports, SATA ports, and a ton of PCIe lanes. The amount of them is listed here:

Spoiler

https://www.intel.com/content/www/us/en/products/docs/chipsets/desktop-chipsets/z790-chipset-brief.html

These lanes are effectively sharing bandwidth with each other, so if you are trying to use 4 Gen 4 SSDs at the same time through the chipset you'll be limited to effectively half speed on each, but since situations like that are pretty rare, especially in the consumer space, it's a good solution to balance cost and overall feature set. 

 

To be clear, it's very much possible for motherboards to also have slots be wired so that if you use one, a different slot turns off. It's pretty common for there to be an M.2 slot or a PCIe slot that if you stick something in it, two of the SATA ports will be disabled for instance, or for there to be an M.2 slot and a PCIe slot that only one can be occupied at a time (the X570 Taichi is the first board off the top of my head to do this). Which slot takes priority depends on the motherboard, and if you head to the server space this can even be controlled by different jumpers, but it is a thing. This would be labeled somewhere in the manual and/or spec sheet, and the Z790-P you listed does not do this. 

 

This is usually well demonstrated by motherboard block diagrams if one is provided. Here's one from Gigabyte for their Z790 Aorus Elite AX:

Spoiler

 

[jaslion]

It'll look very full.

 

Pcie 5.0 drops, lanes are shared amongst the chipset and cpu. The first pcie slot is direct wired and the first m.2 slot. So those get priority. The rest is basically chipset shared. So no full speed if all are on. Also a couple sata ports will drop out.