The other key reason is that even with the right processor and motherboard combination, resizable BAR isn’t enabled by default.
For once, it’s supported by a single processor series at the moment, using one of two chipset options. All that nonsense aside, there are good reasons why we don’t test with SAM enabled by default.
Radeon RX 6800 comparison, we noticed a small number of people complaining that we didn’t test with SAM enabled and therefore claimed the comparison was unfair, misleading, used poor testing methodology, and so on. We've also heard reports of SAM even working on select Intel Z490 motherboards and separately on Radeon RX 5700 GPUs on B450 motherboards, so that might be something worth exploring eventually. Officially AMD has yet to acknowledge SAM support for 300 and 400 series chipsets (as well as Ryzen 3000 and older processors), but motherboard makers have put in the work and some older motherboards now support the feature. Nvidia has yet to deliver on that promise although it's only been a few weeks since.
Nvidia has also come out to say they will be enabling SAM on GeForce GPUs via a driver update, while stating that they’ve got it working on both AMD and Intel platforms. That makes the feature less appealing and restrictive, even though with some extra work in theory this could be offered on all current AMD and Intel platforms. It’s worth noting that as of writing, AMD's only offering SAM support with the new RDNA2 GPUs when paired with a Ryzen 5000 processors on a 500 series motherboard. Notwithstanding, AMD took a different approach with RDNA2 by using a 256-bit wide memory bus in conjunction with their Infinity Cache, and they felt it was time to take advantage of this feature and squeeze some extra performance that would let them be more competitive against Nvidia's Ampere. However, for the most part there’s been no need to make any changes to this limit either, as high-end GPUs typically feature wide memory buses and therefore have significantly more memory bandwidth at their disposal when compared to system memory. There’s no real reason why the 256 MB limit exists, it was put in place back in the 32-bit era and hasn’t been altered since.
Typically the CPU can access up to 256 MB of mapped VRAM, but with the resizable BAR it can have full access to the graphics cards' VRAM buffer. While technically SAM is not an AMD-exclusive technology, they are the first to take advantage of the resizable Base Address Register or resizable BAR, a feature introduced with the PCIe 3.0 spec.įor those of you not yet up to speed, SAM or resizable BAR defines how much of your graphics cards VRAM is to be mapped for access by the CPU. All in all, we plan to benchmark 36 games at 1080p, 1440p and 4K. Today we’re taking a detailed look at how AMD’s Smart Access Memory (SAM) technology influences performance in a wide range of games.