Your Radeon RX 9000 GPU May Not Run At Full Speed On Ryzen AI 400

This story probably won't come as a surprise to anyone who pays attention to desktop APUs, but the news is simple: the new Ryzen AI 400 desktop processors that AMD just launched don't come with a full complement of PCIe lanes. In fact, they actually have even fewer lanes for high-speed I/O than some of the last-generation APUs, which were already quite limited compared to standard desktop processors. Let's get into it.

AMD's "Raphael" and "Granite Ridge" CPUs, the Ryzen 7000 and Ryzen 9000 desktop processors, have 28 PCIe lanes. Four lanes are siphoned off to connect to the motherboard "chipset", which is really a single chip that breaks out those four PCIe lanes into functions like SATA, extra USB, and indeed, more PCIe lanes. Motherboards are designed primarily based on these parts, which comprise the majority of Socket AM5 CPU sales.

Meanwhile, the Ryzen 8000G CPUs have two variants. The Ryzen 5 8600G and Ryzen 7 8700G are based on the "Phoenix" die that has 20 PCIe lanes. This is eight fewer, which already cuts you down quite a bit, but they're also limited to PCI Express 4.0 instead of the faster 5.0 standard of the other chips. Still, this gives you 16 lanes, which is enough to run a graphics card at PCIe 4.0 x16. The smaller "Phoenix 2" die used in the Ryzen 5 8500G and Ryzen 3 8300G only has 14 PCIe lanes before the chipset siphons off its portion, leaving just ten lanes for expansion.


The new Ryzen AI 400 parts aren't quite that anemic; they come with 16 lanes in total. However, that's still fewer than even the Phoenix parts. After the chipset takes its tax, you're left with just twelve PCIe 4.0 lanes to connect other devices: graphics cards, SSDs, fast networking. Twelve lanes is enough for a GPU running at PCIe 4.0 x8 and a PCIe 4.0 x4 SSD, but that's exactly the rub; with even a budget-friendly Ryzen 5 7500F, you'd be able to run your GPU at PCIe 5.0 x16 and a PCIe 5.0 SSD alongside, and still have four lanes left over.

To be clear, PCIe 5.0 x16 is exactly four times the bandwidth of PCIe 4.0 x8, not to mention the small-but-significant latency advantage of PCIe 5.0 over 4.0. This becomes notable when we talk about current-generation gaming graphics cards such as the GeForce RTX 5070 Ti, or indeed AMD's own Radeon RX 9070 XT, both of which have PCI Express 5.0 x16 interfaces. On a system with a Ryzen AI 400 APU, these cards will be cut down to just a quarter of their potential PCIe speed, and while that won't have an outsized effect on average frame rates, it does hurt 1% lows and frametime stability in a major way.


So does that make the Ryzen AI 400 parts a bad deal? Well, no, because they're not really intended for use with fast discrete GPUs like this anyway. PCIe 4.0 x8 is the exact interface width of something like a Radeon RX 7600 or, indeed, an Intel Arc B-series GPU. Nevermind that the integrated graphics on these chips are relatively capable and they're really meant to be used that way, regardless. Besides, the other characteristics of these parts (like their heterogeneous combination of Zen 5 and Zen 5C CPU cores, and their slashed L3 cache) make them much less attractive than standard desktop CPUs if you're going to be using them for gaming.

In other words, if you end up in a situation where you need to use a Ryzen AI 7 450G with a Radeon RX 9070 XT, it's not the end of the world. Just understand that you're only getting a portion of the performance you would be getting with any standard Ryzen 7000 or Ryzen 9000 CPU, and even the Ryzen 8000G series parts are a better choice in most cases.