AMD launched the Radeon RX 6800 XT, its latest mainstream flagship graphics card, late last year. The company was proud to show off what, at the time, it called its newest “proprietary” feature: Resizable BAR (Base Address Register), albeit under AMD’s own moniker of Smart Access Memory (SAM). We put the word “proprietary” in quotes because, as it turns out, there’s nothing wholly new or proprietary about Resizable BAR (or alter ego SAM). And now, it’s coming to an Nvidia GPU (and Intel CPU) near you.
That means you’ll be hearing these two terms bandied about plenty in 2021. But what is Resizable BAR, anyway, and how can it help you squeeze the most performance out of your desktop gaming PC? Let’s dig into the details of this old, yet also new, technology to find out.
Three Graphics Cards Walk Into a BAR…
Resizable BAR was actually first introduced, if not widely implemented, as a part of the move to the PCI Express 3.0 spec in desktop motherboards back in 2010. (It requires specific support at the CPU and GPU level, as well.) The feature, set via the system BIOS, determines how much of the graphics memory, or VRAM, on your video card is made available to be mapped for access by the CPU. Generally, this is limited to just 256MB of the card’s onboard VRAM—which is to say, not much of it. A motherboard with Resizable BAR activated, however, can boost the limit to the full capacity of the VRAM buffer.
So what does that actually mean in the real world? At the system level, for starters, rather than chopping up the data stream and sending smaller requests from the CPU to the GPU, the GPU can now ask for complete chunks of data—think, larger files containing elements like shaders and textures—and get them sent by the CPU all at once.
The result is a more efficient communication pipeline between your CPU and GPU. And, depending on the game title and the components in your system, it can provide noticeable gains in performance for AAA games that know how to make use of the tactic.
But what kind of real-world boost can you expect in frame rates with the feature turned on?
SAM, I Am: A Preliminary Testing Impression
At the time of the launch of the AMD Radeon RX 6800 XT, we got the chance to test the feature for ourselves (in its AMD SAM guise). However, in our runs of current AAA titles, as well as multiplayer games and older/legacy AAA titles, we found SAM only really affected results in the most demanding benchmark tests. In our test suite, those are represented by the AAA games Red Dead Redemption 2 and Assassin’s Creed: Odyssey. With SAM activated, both saw gains of roughly 5% compared with benchmark runs with the SAM feature turned off.
That said, we also ran into a range of driver-related issues with the card samples we were given to test by AMD at the time of the Radeon RX 6800 launch, which could mean that our results are not the most current representation of what SAM/Resizable BAR can do at its best. Other outlets that have since tested the feature after the launch-driver stumbles smoothed out have seen some substantial upticks (again, very much dependent on the title) that could exceed 15% gains in frame rates, in select cases.
We say “select,” because, as of now, the benefit of SAM is wildly dependent on the game you’re playing. While Assassin’s Creed: Odyssey might see significant uplifts (totaling 20% more frames per second at certain resolutions, according to tests from around the web), other games actually might see their performance decrease with the feature activated. Whether or not you’ll get a gain or a loss varies quite literally from game to game. And, seeing as the feature can be turned on or off only through the motherboard BIOS, the overall pragmatism of toying with SAM/Resizable BAR to see what works well and what doesn’t has yet to be seen.
We’ll be testing the feature more thoroughly in the coming months as it’s added to both Intel motherboards and Nvidia graphics cards, as well as re-running our old tests on the Radeon RX 6800 XT with the updated driver set. Stay tuned to PCMag as we dig into Resizable BAR, and see what it can really do when Nvidia and AMD do battle on even ground.
BAR or SAM: What You’ll Need to Make It Work
As it is a recently introduced feature, for now only very late-model components are capable of supporting Resizable BAR.
As of this writing (March 2021), Resizable BAR, under the SAM badge for AMD, works with the AMD Radeon RX 6800 and Radeon RX 6800 XT cards (as well as the upcoming Radeon RX 6700 XT), but only in concert with Ryzen 5000 or Ryzen 3000 Series CPUs based on Zen 2 or later architectures. Furthermore, only motherboards based on the AMD 400 and 500 series chipsets will support SAM. In addition, not all of them: It’s up to each motherboard manufacturer to patch the feature in through a BIOS update, if the board doesn’t ship with SAM support in place.
However, that hardware support list is about to get a lot bigger, as both Nvidia and Intel have announced plans to bring Resizable BAR support to their video cards and motherboards in the coming months.
Unlike AMD, though, which lets you enable the feature carte blanche to potentially increase the performance of your favorite game, Nvidia dictates which games can use the feature based on its Game Ready Driver releases. At this early March 2021 writing, the list of games that support Resizable BAR on “Ampere”-based Nvidia GeForce RTX 30 Series desktop video cards and gaming laptops with RTX 30 Series mobile GPUs includes just the following:
- Assassin’s Creed Valhalla
- Battlefield V
- Borderlands 3
- Forza Horizon 4
- Gears 5
- Metro Exodus
- Red Dead Redemption 2
- Watch Dogs: Legion
That’s a pretty darn small subset of the full gaming library that exists on PCs. (Flashbacks to DLSS, anyone?) However, it’s likely that Nvidia is being careful to roll out the feature selectively, to games it knows will benefit from Resizable BAR. Because some games can actually take a performance hit with the feature turned on, Nvidia’s driver-level implementation prevents users from accidentally hurting their performance by trying to use it in unsupported titles. This could be the better approach in the long run. (Who will want to keep score on which games benefit, and which games don’t, after all?) But it will be interesting to see how AMD handles its response as cards like the AMD Radeon RX 6700 XT near their launch date.
Furthermore, actually getting Resizable BAR activated on your Nvidia card will be a task unto itself. Thus far, the company has only launched its GeForce RTX 3060 with the feature already preinstalled in the VBIOS (that is, the card’s own BIOS). Any other model of RTX 30 Series desktop card will need to have its VBIOS flashed—a daunting prospect for any graphics card owner!—in order to get it to work. VBIOS versions with Resizable BAR enabled will be provided both by Nvidia (for Founders Edition cards) and partner card manufacturers as time goes on. As for GeForce RTX 30 Series laptops, you’ll want to check with the laptop vendor if Resizable BAR is supported.
In a recent conversation with Tom’s Hardware, Intel also confirmed it has activated the feature for its 10th Generation “Comet Lake” line of CPUs on select motherboards and mobile systems, and says that 11th Generation (“Rocket Lake”) desktop CPUs will carry support when they launch later this month:
