Leak Indicates AMD’s Socket AM5 Won’t Support PCIe 5.0 at Launch

Back in 2019, AMD made PCI Express 4.0 the centerpiece of its Zen 2 launch. The X570 motherboard family paired well with the Zen 2 CPU family, and the increased I/O support gave AMD an additional argument for why some buyers might prefer its chips over Intel’s. Now, it looks as though Intel will leap ahead with Alder Lake, while AMD has no immediate plan to follow on desktop. AMD’s next Epyc refresh, Genoa, which will be built on Zen 4, is expected to offer PCIe 5.0, so AMD will apparently split its refresh plans between desktop and server next year.

This isn’t all that surprising, even if AMD has relied upon playing up its bandwidth advantages against Intel over the past few years. PCIe 4.0 drives are not as ubiquitous as PCIe 3.0 hardware was when AMD launched PCIe 4.0. We’re seeing such rapid advances in interconnect technology because the PCI-SIG ran into problems scaling bandwidth above PCIe 3.0. PCIe 2.0 arrived four years after PCIe 1.0 and PCIe 3.0 arrived four years after 2.0, but it took seven years for PCIe 4.0 to launch. Now, PCIe 5.0 will hit the market roughly two years after PCIe 4.0 did.

This information is part of the Gigabyte document trove that was hacked a few weeks ago. As such, it should be taken with a substantial grain of salt. According to this slide, AM5 supports USB4 in an external controller but the technology does not appear to be integrated in-chipset. USB 3.2 is supported natively at both its 10Gbps and 20Gbps transfer speeds.

One difference between the current X570 and the AM5 boards expected in 2022 is the addition of four more PCIe 4.0 lanes off the CPU. In the diagram above, four additional lanes of PCIe 4.0 have been dedicated to USB4 support. We suppose it’s possible that some vendors might ship without USB4 in order to offer a second PCIe x4 NVMe connection on the motherboard. There is no word on what DDR5 clock speeds AMD will support at launch. DDR5-4800 is the minimum spec, and it’s not impossible that we might see support for DDR5-5500 or even DDR5-6400 at launch.

As to how much this matters? That’s an interesting question. Intel may have some very early PCIe 5.0 drives for reviewers to test when Alder Lake drops, but manufacturers have reported that they plan to have PCIe 5.0 drive controllers in-market by early 2022 at the earliest. Apart from a major, Alder Lake-specific push, it’s likely that PCIe 5.0 drives will arrive sometime after platforms that can use them.

Latency is far more important than bandwidth when it comes to making consumer applications feel responsive, which is why an older SATA-based SSD drive still makes a system feel orders of magnitude faster than a hard drive, but doesn’t seem that much slower than an NVMe SSD in raw performance. It’s unlikely that anyone using an NVMe drive of any vintage would notice a huge improvement in day-to-day I/O tasks.

If you handle an enormous amount of file copying on a regular basis — if, for example, you do a lot of A/V editing — the calculus here may change somewhat. Keep in mind, however, that in many cases, a top-tier PCIe 3.0 drive might offer better performance than a midrange PCIe 4.0 drive. This also applies to PCIe 4.0 versus 5.0. In some cases, it might even apply to PCIe 3.0 versus 5.0.

Here’s why: Many SSDs these days are based on TLC or QLC NAND with an SLC cache. This is part of how manufacturers have kept increasing SSD space while simultaneously cutting prices, but the tradeoff here is performance consistency. A higher-end drive with either a larger SLC cache or a drive based on fast MLC or TLC combined with a high-end controller may not lose performance once it has written more than 75-150GB of data depending on the size of the SLC cache.

All of which is to say: The same buyers who might genuinely benefit from PCIe 5.0 in the near-term future are also the ones who move the most data on a regular basis. Because these customers are data-heavy to begin with, it’s important to consider the specifics of the SSD you want to buy. PCIe 5.0 drives, when they appear, should be high-performance products — but just because an SSD carries a faster number doesn’t automatically mean it’ll be better suited to every task.

Although the near-term benefits of adopting PCIe 5.0 over 4.0 are small, the limited group of buyers who can benefit from every I/O improvement it can find will have a new reason to choose Intel hardware over AMD, assuming Alder Lake delivers in other respects. Presumably, AMD will move to PCIe 5.0 with the second generation of AM5, provided this slide is accurate. As always, all leaks and rumors should be taken with an eye towards the fact that they are leaks and rumors, not official communication.

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