AMD Radeon RX 6900 XT review: Smart Access Memory

What you need to access the feature - and what you get in return.

One of the most intriguing new additions to Radeon 6000 series cards is Smart Access Memory (SAM), which is AMD's branded implementation of the PCI Express Resizeable BAR feature baked into recent builds of Windows 10 and Linux. SAM allows supported processors to directly access all 16GB of video memory on the Radeon 6800 or 6800 XT, bypassing the usual 256MB I/O buffer limit and thereby increasing performance in supported games. AMD showed frame-rate gains in the region of two to 10 per cent in their Radeon 6000 announcement event, depending on the game and resolution, but in our prior 6800/6800 XT review, we found that games either showed improvements or they didn't, suggesting a driver-level awareness for each title. On top of that, performance gains from SAM increased the lower down the resolution chain you go. This isn't particularly helpful for these 4K-class cards, but more interesting for the RX 6800 and its 1440p focus, plus the arrival of lower performance 'medium Navi' cards coming soon to replace the RX 5700 series.

Right now, Smart Access Memory requires a particular setup: a 500-series motherboard (that's B550 or X570 at present) and a Ryzen 5000 processor (5600X, 5800X, 5900X or 5950X). AMD told us that there's no technical reason preventing SAM from working on other motherboards in the future - and in actual fact, we've now seen PCIe 'ReBar' now working on Intel Z490 boards - including the Asus Maximus 12 Extreme in our test suite, so we decided to give it a shot and found the same benefits from SAM on our system.

So, if your board - Intel or AMD - is supported, you'll need to update your BIOS, first of all. Once complete, reboot your system, enter the BIOS, then find the ReBar support. From here, each board is different, but on our X570 and Z490 Asus boards we needed to visit the 'Advanced' tab, enter 'PCI Subsystem Settings', then enable 'Above 4G Decoding'. This unlocks a new option, 'Re-Size BAR Support', which when set to Auto with a compatible graphics card activates SAM. Yes, the same options on both Asus Ryzen and Intel boards produced the same resuls.

Interestingly, there doesn't appear to be a way to verify the setting is enabled in Windows - perhaps an indicator could be added to the Radeon control panel in a future update.

For each game we've included tests at multiple resolutions with SAM enabled and disabled, plus we've also included an RTX 3090 target score for the RX 6900 XT to challenge.

Borderlands 3

We do log a small improvement in 4K performance with SAM enabled, and while clocking in at just 2.5 per cent, it is enough for the RX 6900 XT to push ahead of the RTX 3090. At lower resolutions, the wafer-thin lead enjoyed by the RX 6900 XT translates into much more substantial improvements over the Nvidia competition. At both 1440p and 1080p, SAM offers up circa 7.5 per cent of 'free' performance. It does not seem to impact power consumption either, making this more potent and useful than overclocking.

Borderlands 3: Bad Ass, DX12, TAA

  • RTX 3090
  • RX 6900 XT
  • RX 6900 XT SAM
  • RTX 3090
  • RX 6900 XT
  • RX 6900 XT SAM
  • RTX 3090
  • RX 6900 XT
  • RX 6900 XT SAM

Control

Nvidia is super-dominant in this title, as we have already seen. SAM is supported in Control, but the improvements for the RX 6900 XT clock in at 2.5 per cent, 4.0 percent and 6.1 per cent across 2160p, 1440p and 1080p resolutions. It demonstrates SAM's effectiveness at lower resolutions once again, even though the outlook isn't particularly compelling for a $999 graphics card that works best at 4K resolution.

Control: High, DX12, TAA

  • RTX 3090
  • RX 6900 XT
  • RX 6900 XT SAM
  • RTX 3090
  • RX 6900 XT
  • RX 6900 XT SAM
  • RTX 3090
  • RX 6900 XT
  • RX 6900 XT SAM

Assassin's Creed Odyssey

In our conference call with AMD about the RX 6900 XT, we came away with the impression that SAM support is integrated at some level in the driver, with Team Red talking about ensuring that the feature doesn't actually degrade performance on some titles - a real possibility for an emerging technology. Driver integration suggests titles compatibility may not be included for all titles and that may explain why SAM does not help this and many other titles. There may be performance degradation on this one, but it's more likely that Odyssey's variable cloud cover is responsible for the margin of error results.

*The benchmark annoyingly includes variable weather, with clouds sometimes appearing on the horizon and tanking performance. We also see a decent amount of variance in lowest one per cent scores from run to run, especially at 1080p, so it's best to look at the averages here and not read too much into the other metrics.

AC Odyssey: Ultra High, DX11, TAA

  • RTX 3090
  • RX 6900 XT
  • RX 6900 XT SAM
  • RTX 3090
  • RX 6900 XT
  • RX 6900 XT SAM
  • RTX 3090
  • RX 6900 XT
  • RX 6900 XT SAM

Metro Exodus

With 0.1, 0.3 and 0.5 per cent deltas between the RX 6900 XT results with SAM enabled and disabled, we'd be tempted to suggest that Metro Exodus probably isn't supported at the driver level, or else the memory access technology simply doesn't benefit the 4A engine at all.

Metro Exodus: Ultra, DX12, TAA

  • RTX 3090
  • RX 6900 XT
  • RX 6900 XT SAM
  • RTX 3090
  • RX 6900 XT
  • RX 6900 XT SAM
  • RTX 3090
  • RX 6900 XT
  • RX 6900 XT SAM

Our final range of tests looks at ray tracing, where our existing RX 6800/ 6800 XT review suggests that AMD will struggle. To give Team Red more of a fighting chance, we decided to add SAM testing to the RT benchmarks too. Click like the wind to see those results.

AMD Radeon RX 6900 XT Analysis

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About the author

Richard Leadbetter

Richard Leadbetter

Technology Editor, Digital Foundry  |  digitalfoundry

Rich has been a games journalist since the days of 16-bit and specialises in technical analysis. He's commonly known around Eurogamer as the Blacksmith of the Future.

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