Here is why AMD might never include Smart Access Memory support on Ryzen 3000 and older models, while 6-year-old Intel CPUs can still benefit from this feature
Despite being first to add support for the Resizable BAR (AKA Smart Access Memory) feature on the new Ryzen 5000 desktop CPUs and RX 6000 GPUs, AMD might never offer backwards compatibility with older Zen models like Ryzen 3000, 2000 and 1000. The Resizable BAR feature is actually part of the PCIe 3.x specs and should be supported by all motherboards and CPUs that are compatible with the PCIe 3.x features , but it looks like this is not really the case for all AMD Zen-based CPUs. Meanwhile, Intel’s CPUs all the way back to 2014 are eligible for a Resizable BAR BIOS update, if motherboard manufacturers are actually kind enough to bother with such upgrades, that is.
TechPowerUp notes that the new Ryzen 5000 CPUs are the only AMD series to integrate the PCIe physical layer feature called full-rate _pdep_u32/64, which allows processors to see the GPU’s entire video memory as a single addressable block instead of separate 256 MB apertures. It is true that Zen2 CPUs use the same IO die and PCIe controller as Zen3, but AMD SAM is actually controlled through a new ISA CPU instruction set. These specific instructions are only emulated in microcode using other similar instructions in the Zen2 and older cores, and this makes everything predating the new Ryzen 5000 essentially too slow to benefit from the performance uplift offered by SAM.
From what we have seen thus far in tests, the Resizable BAR feature can offer performance improvements of at most 20%, but not too many games really benefit from it. Some Intel 400-series motherboards already offer beta BIOS upgrades that add the feature. Performance uplift is even looking a bit better compared to AMD’s implementation and there may still be room for improvement for the final version scheduled to release in early 2021. TechPowerUp also reveals that the Resizable BAR feature can be implemented on all Intel CPUs dating all the way back to 2014. That is when Haswell introduced what we now know as Resizable BAR with its 20-lane PCI-Express gen 3.0 root-complex. Now, we are not sure if motherboard manufacturers will bother to release updates for CPUs that old, but we could see support for Skylake, Kaby Lake and Coffee Lake at least, as these families are still popular to this date. Additionally, Nvidia is planning to add support for this feature soon, as well.
In a recent HotHardware Q&A session, AMD’s Frank Azor stated regarding the AMD SAM compatibility with previous CPUs that Team Red will “look into it and see if it is going to be possible to do it with any performance uplift and any reliability, and please stay tuned.” So maybe AMD has a solution for Zen2 and older CPUs, yet it is not really a priority right now? Of course, they want us to buy the new models, then again, why are they introducing AMD SAM right now when the feature has been available for more than 4 years now? Makes us wonder what other hidden features CPU makers are keeping from us.