AMD APUs - how many PCIe lanes?

[asheenlevrai]

Hey

 

I'm reading that AMD 8700g will use PCIe4.0 (instead of PCIe5.0) which is not very surprising. Similarly, the 5700g used PCI3.0 while the 5000s desktop CPUs used PCIe4.0.

 

Now apparently people are disappointed because of the following (GPU lanes below): AMD 8700g has 20 PCIe4.0 lanes distributed this way:

 - 8 lanes for the GPU (instead of the max 16)

 - 4 lanes for NVMe storage

 - 4 lanes for another NVMe storage port

 - 4 lanes for the chipset.

 

Thus I was wondering how things were for the previous generation. I looked up specs of the 5700g but couldn't find any reliable information. They are all over the place. Some say it has 16 PCIe3.0 lanes, some say 20, some say 24. Tom's Hardware even mention the following:

"the chip has 20 lanes of PCIe 3.0 connectivity (16 for graphics, four for the chipset, and four for storage) compared to 24 lanes of PCIe 4.0 found on the Ryzen 5000 models for the desktop PC."

I don't understand how they get to 16+4+4=20 but I may miss something here. Please let me know if I do.

 

Is there any way to figure out exactly how many PCIe lanes a CPU/APU provides?

 

I'd like to know if the 5700g was already limited to x8 GPUs (when using the 1st M.2 slot for storage) or if it indeed could use x16 cards + x4 SSDs directly connected to the CPU.

 

Thank you very much in advance for your help.

 

Best,

-a-

[GuiltySpark_]

As far as I can tell, 5700G has 20 (16 for Graphics, 4 for storage) Gen3 lanes dedicated to devices and 4 lanes for the chipset. 

 

8700G, according to Anandtech, is doing things a little differently. 

 

Quote

The Ryzen 8000G series is different in that they omit PCIe Gen 5 lanes and instead opt for PCIe 4.0 lanes, with the Phoenix (8700G/8600G) coming with 20 x PCIe 4.0 lanes, of which x16 are usable between graphics and M.2 storage. This means the Ryzen 7 8700G has a PCIe 4.0 x8 link for a graphics card (not x16), with 8 x PCIe 4.0 lanes for two PCIe 4.0 x4 M.2 SSDs.

Part of me thinks this is down to how the motherboard arranges things but end of the day x8 lanes for graphics is plenty for a chip like that. 

[RONOTHAN##]

9 minutes ago, asheenlevrai said:

I'd like to know if the 5700g was already limited to x8 GPUs (when using the 1st M.2 slot for storage) or if it indeed could use x16 cards + x4 SSDs directly connected to the CPU.

The 5700G had the same number of PCIe lanes as the standard Ryzen 3000/5000 series CPUs did, 20+4 (20 general purpose lanes, 4 dedicated to the chipset). The reason you'll see a mix of people saying that AM4 has 24 lanes and 20 lanes is because of the chipset lanes and whether you want to actually count them as PCIe lanes. I'm personally more in the "they have 20 lanes" camp because there's no way to use those 4 lanes for anything other than the chipset and therefore it isn't really useful to talk about their chipset lanes, similar to how it would be kinda weird to say LGA 1700 CPUs have 28 lanes because of their 8 DMI links (these are basically PCIe lanes with a different lanes). 

 

15 minutes ago, asheenlevrai said:

Now apparently people are disappointed because of the following (GPU lanes below): AMD 8700g has 20 PCIe4.0 lanes distributed this way:

 - 8 lanes for the GPU (instead of the max 16)

 - 4 lanes for NVMe storage

 - 4 lanes for another NVMe storage port

 - 4 lanes for the chipset.

Small correction, those 4 lanes for a second NVMe slot aren't actually dedicated to a second NVMe slot. They're general purpose lanes just like the 8 for the GPU, so it's up to the motherboard for how they'll implement them. Boards like the B650 LiveMixer will implement them as an x4 PCIe slot, while some boards will do a NVMe slot like you described. 

 

 

Realistically, the debate around how many PCIe slots a CPU has is not directly relevant. A CPU could have 40+ lanes, yet if the motherboard doesn't wire them up fully, it doesn't really matter. The X99M Extreme4 for instance, the CPUs on that board support up to 40 PCIe lanes, yet in practice that motherboard only has 32 of those lanes wired up. You want to look in your motherboard manual as it will tell you exactly what lanes go where when any given CPU is installed, as this is realistically all that actually matters. Think of the CPU lanes as an effective maximum, and it's up to the motherboard to actually use as many as it can. 

[GuiltySpark_]

2 minutes ago, RONOTHAN## said:

You want to look in your motherboard manual as it will tell you exactly what lanes go where when any given CPU is installed

I cannot tell you how happy I am that proper block diagrams are beginning to be a thing, at least with Asrock and MSI on AM5. This needs to become a standard feature!