Comment by DiabloD3
2 days ago
To further expand on this, "southbridge" is what we now call a chipset expander (or 50 other company or product line specific names).
Its a switch that has a bunch of unified PHYs that can do many different tasks (non-primary PCI-E lanes, SATA ports, USB ports, etc), leveraging shared hardware to reduce silicon footprint while increasing utility, and connects to PCI-E lanes on the CPU.
Don’t EPYC CPUs avoid using a chipset altogether? I think in that case, it would be NB+SB.
Yes.
The "northbridge" in modern Zen systems is the IO die, and in Zen 1/+, its the tiny fractional IO die that was colocated on each chip (which means a Zen 1/+ Epyc had the equivalent of 4 tiny northbridges).
However, they just embed the equivalent design of the chipsets into the IO Die SoC on Epycs.
Fun fact: For desktop, since Zen 1 (and AM4-compatible non-Zen CPUs) they included a micro-southbridge into the IO die. It gave you 2 SATA ports and 4 USB ports, usually the only "good" ones on the board. On Epyc, they just put the full sized one here instead of pairing it with an external one.
This also means, for example, if you have 4 USB3 10gbit ports, and its not handled by a third party add-on chip? Those are wired directly into the CPU, and aren't competing for the x4 that feeds the southbridge.
Also fun fact: The X, B, and A chips are all sibling designs, under the name of Promontory, made jointly with ASMedia. They're essentially all identical, only updated for PCI-E and USB versions as time went on, as well as adding more ports and shrinking die size.
The exception is the X570, its an AMD-produced variant of the Promontory that also contains the Zen 2/3 IO Die, as they're actually the same chip in this case. The chips that failed to become IO Dies had all their Promontory features enabled instead, and became chipset chips. The Zen 2/3 Epycs shipped their IO die, at least partly, as two X570s welded together, with even more DDR PHY thrown in, as some sort of cost saving.
I don't think that panned out, because the X/B/A 600 and 800 variants (Zen 4 and 5) went back to being straight Promontory again.
Wikipedia has some good charts for this: https://en.wikipedia.org/wiki/List_of_AMD_chipsets