Last year, AMD announced a new feature it dubbed Smart Access Memory. This capability gave RDNA2 GPUs a bit of a kick when compared against Nvidia cards back when AMD launched the 6800 and 6800 XT, with performance improvements of 3-7 percent. Nvidia and Intel both promised their own versions of the capability, and Team Green’s has arrived, at least in part. The official name for the feature, as specified by PCI-SIG, is Resizable Base Address Register, or Resizable BAR for short. AMD markets Resizable BAR as “Smart Access Memory.” There are, as far as we know, no feature implementation differences between supporting ReBAR (I’m sorry, but the acronym was just sitting there) and supporting SAM.
The RTX 3060 is the first Nvidia Ampere GPU to officially support the Resizable BAR feature, which was originally defined in the PCI Express 2.0 specification. Support is apparently also enabled for all GeForce RTX 3000-equipped laptops, but desktop GPU support for the RTX 3060 Ti, 3070, 3080, and 3090 won’t appear until later in March. Nvidia did not identify a reason for the delay.
This is an AMD slide, but ignore the AMD branding. This is now a feature that’s going to be supported across Nvidia and AMD cards, on both Intel and AMD motherboards, once Ampere, RDNA2, 11th Gen Rocket Lake, and Ryzen 5000 are all in-market.
Resizable BAR support is available for AMD systems that combine either a 400-series or 500-series motherboard with a Ryzen 5000 CPU. On Intel, Resizable BAR is available on Z490, H470, B460, and H410 motherboards. It’ll also be enabled for all 11th Generation chipsets when those products are available. AMD has enabled Resizable BAR for all Radeon 6000 products and Nvidia has pledged to bring the feature to all desktop and laptop Ampere cards. Intel is additionally expected to debut the capability for its Xe graphics cards for those when they become available.
Resizable BAR is formally defined as:
[An] optional capability that allows hardware to communicate resource sizes, and system software, after determining the optimal size, to communicate this optimal size back to the hardware. Hardware communicates the resource sizes that are acceptable for operation via the Resizable BAR Capability register. Software determines, through a proprietary mechanism, what the optimal size is for the resource, and programs that size via the BAR Size field of the Resizable BAR Control register.
In this specific instance, AMD, Intel, and Nvidia are using Resizable BAR / Smart Access Memory to allow the CPU to access more than 256MB of VRAM at once. Nvidia is supporting this feature on Assassin’s Creed: Valhalla, Battlefield V, Borderlands 3, Forza Horizon 4, Gears 5, Metro Exodus, Red Dead Redemption 2, and Watch Dogs: Legion. Additional titles will be announced in March. Note that enabled Resizable Bar will require you to update both your driver and your VBIOS. Nvidia and other OEMs will be releasing updates and information on how to perform these steps.
After installing compatible UEFIs, VBIOS, and driver, you’ll be able to tell if you’ve gotten the feature enabled properly or not. Open the Nvidia Control Panel, hit the “System Information” text box on the bottom-left, and a window will open like the above. Check the text box marked in red — if Resizable BAR has been properly enabled, it’ll show up there.
It’s not clear if Resizable BAR always improves performance in every game, but so far there doesn’t seem to be a downside to enabling the capability. This capability was baked into the PCI Express 2.0 standard back in 2008 but only introduced to consumer systems in 2020. While it’s not going to blow your mind with dramatic improvements, no manufacturer is going to turn down a 3-7 percent gain these days.
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