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The landing page for AMD’s Smart Access Memory feature deliberately notes that users will require a 500 Series motherboard and Ryzen 5000 Series processor. While some enthusiasts have claimed that this is some kind of marketing ploy to get more Zen 3 CPUs off the shelves, we are seeing reports on social media that suggest the restriction is legitimate and stems from hardware limitations of older architectures.

According to a post on the CapFrameX account (via TechPowerUp), Zen, Zen+, and Zen 2 processors may not support (or, at least, take full advantage) of AMD’s Smart Access Memory feature due to performance limitations with an integer instruction called _pdep_u32/64.

A chart published by AnandTech (which we’ve copied below) makes the differences between Zen 2 and Zen 3’s handling of this particular instruction quite clear, with the previous generation suffering from latencies as high as 300 cycles (a user on r/AMD has clarified that this discrepancy stems from Zen 2’s instructions being emulated in microcode).

What’s amusing is that Intel has reportedly supported the _pdep_u32/64 instruction since its Haswell generation of processors, which launched all the way back in 2013. It isn’t clear whether motherboard manufacturers will provide updates to enable Resizable BAR support for such old CPUs, however.

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6 Comments

  1. Well, disappointing as it may be – it’s not like you went out and bought that 1700X back in 2017 just for it to work with SAM.

    AMD hasn’t taken anything away from the older CPUs, nor have they failed to deliver anything they had promised — you got what you paid for.

  2. From what I have heard, the older AMD CPUs don’t have the native instruction for SAM – they can do Resizable BAR, but it’s via a compound series of instructions, and once you do that, you lose the speed benefit.

    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.

  3. From what I have heard, the older AMD CPUs don’t have the native instruction for SAM – they can do Resizable BAR, but it’s via a compound series of instructions, and once you do that, you lose the speed benefit.

    Thanks for that info, I have not seen this at all, time for some reading to see what I missed.

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