AMD Radeon RX 6900 XT review: Blisteringly fast, but very niche

Smart Access Memory: Big help from Ryzen, sometimes

Spoiler alert: We’re going to copy most of the explanatory text about Smart Access Memory from our Radeon RX 6800-series review, but we managed to secure the hardware needed to test the intriguing new technology with the Radeon RX 6900 XT.

With Ryzen and Radeon finally both firing on all cylinders, AMD decided to unlock capabilities that help the Radeon RX 6000-series achieve even higher frame rates when it’s paired with a Ryzen 5000-series processor in an X570 motherboard.

AMD

Smart Access Memory gives the CPU full access to the GPU’s memory, rather than limiting it to the usual 256MB chunks. That unrestricted access to the data channel can improve gaming performance.

Smart Access Memory can’t be used by everyone, though. You need a Ryzen 5000-series processor and a compatible X570 motherboard that has had its BIOS updated to support AMD’s AGESA 1.1.0.0 firmware, along with a current version of Radeon Software (which you need to run the Radeon RX 6000-series anyway) and a 64-bit operating system. For now, X570 motherboards won’t enable Smart Access Memory by default; you need to dive into your BIOS’s advanced settings and activate both “Above 4G Decoding” and “Re-size BAR support” to coax it into working. AMD expects AM4 motherboard makers to turn on Smart Access Memory by default going forward.

AMD sent us a Ryzen 5900X and an MSI Godlike X570 motherboard to test Smart Access Memory. We paired it with AMD’s Wraith Max cooler, Geil’s Evo X 16GB DDR4 kit clocked at 3,200MHz, and a 1TB SK Hynix Gold S31 SSD, the best SSD for most people. We benchmarked the Radeon RX 6900 XT with Smart Access Memory both on and off across our entire testing suite on this system, and the GeForce RTX 3090 for comparison.

Note that this system has not been optimized for pure performance and was instead quickly cobbled together with what we had available. Ideally, we’d use faster memory with Ryzen 5000, and the cooling around the core components isn’t as comprehensive as what’s in our primary test rig, though we had a fan pointed at the build. Crucially, the results from this section can’t be compared directly against the benchmarks from our standard test rig in the previous section—it’s a completely different system in very different circumstances. The numbers below are apples-to-apples, though. AMD’s public SAM benchmarks tend to be run with the company’s new Rage mode performance tuning enabled, but we test in stock configuration with Rage mode off.

Brad Chacos/IDG

As you can see, Smart Access Memory provides extra performance pretty much across the board, but the significance varies wildly. Some games see little extra oomph; others respond much more favorably. SAM’s extra boost is enough to propel the 6900 XT past the RTX 3090 in some scenarios where it otherwise loses by a hair (such as Borderlands at 4K and Shadow of the Tomb Raider at 1440p). SAM isn’t enough to close the gap in games that heavily favor Nvidia’s GPU architecture, though.

AMD says SAM’s benefits differ not just game-to-game, but with your resolution and the graphics card installed. Don’t assume that the uplift we saw with the Radeon RX 6900 XT scales similarly with other Radeon RX 6000-series GPUs.

That means other games might respond more beneficially as well, however. Smart Access Memory seems to work especially well in games built using modern DirectX 12 and Vulkan APIs. AMD’s reviewer guide claims that Assassin’s Creed: Valhalla, Call of Duty Black Ops Cold War, Forza Horizon 4, Godfall, Resident Evil 3, and Red Dead Redemption 2 see significant boosts with SAM enabled, though the visual options you select also changes things. AMD says Forza Horizon 4 gets much more benefit from SAM when you’re using 8x MSAA anti-aliasing versus 2x MSAA, for example, and Godfall’s uplift is bigger with the High visual preset than Ultra.

Gordon Mah Ung

All in all, Smart Access Memory is a potent weapon in AMD’s arsenal, and one that can make a tangible difference—though it may not remain an AMD exclusive for long. SAM is based on the resizable BAR functionality that’s part of the PCIe spec, and Nvidia has already said it plans to unlock support for its GeForce GPUs at some point. We’re already seeing some manufacturers roll out BIOS updates for modern Intel motherboards that add the feature, though AMD says driver optimizations are a key part of Smart Access Memory as well.

Don’t let that diminish what AMD accomplished with its “Ultimate Gaming Platform” here, though. Yes, resizeable BAR has been a part of PCIe for a while, but it hadn’t been used before. Enabling the technology requires the processor, motherboard, and graphics card to all join in on the party—something AMD is uniquely positioned to do as a supplier of both CPUs and GPUs. Smart Access Memory should just work when you put a Radeon RX 6800 into a Ryzen 5000 system with a new X570 motherboard after you enable a few options, no messy BIOS updates required. (Older X570 motherboards require a BIOS upgrade to get SAM running.) Intel and Nvidia can’t offer that right now. While updating your firmware isn’t as scary as it used to be, it’s still far more complicated than simply downloading a new driver.

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