64 core EPYC Rome (Zen2)Architecture Overview?

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Mar 11, 2004
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I was gonna say, wasn't there talk that that K12 and Zen were being co-developed with similar ideas behind them? Just one being x86 and the other ARM. Which, especially in hindsight, they were smart to focus development, but I do wonder if they'd be able to apply Zen ideas to ARM for benefits to get into that space. I feel like AMD should at least consider ARM, if for no other reason than to offer hybrid designs, where they could have a bank of ARM cores, and a bank of AMD x86 cores. I think that would help them out in laptop space, where on battery it relies more on the ARM stuff, but then docked, or for certain use cases like gaming it uses the x86 stuff. Really seemingly there's nothing that would stop them from integrating another company's ARM chip just like they do their own modules, so they could slap a Snapdragon chip in there and they can offer all the benefits it has but also x86 compatibility. Which I think that'd be the way to do the Surface Book design, have the ARM chip in the tablet part so it goes to that, and then have the x86 and GPU in the laptop base.

Which, by now the standard ARM cores are probably as good or better than what K12 would have been (maybe not though?), so ditch the custom ARM and just integrate standard ones for low power use. Plus it would let them do Chrome boxes/books, and they could even tout it as a developer feature (where say they could run the ARM stuff in VM so they could test out their Android code). I have a hunch it'd be beneficial for consoles too (especially if they want to win back Nintendo's favor).

Interesting about combining the good parts of Bulldozer and Jaguar. Which, the latter was probably where the CCX design came from, no? I know the console chips, or maybe its just the Xbox, had Jaguar cores in two groups of 4 cores. Not sure what was positive about Bulldozer, although there probably was some aspects, if nothing else some of the logic blocks could probably be ported over, and then they could improve them bits at a time (kinda like them with the 3rd party memory controller).
 

amd6502

Senior member
Apr 21, 2017
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I was gonna say, wasn't there talk that that K12 and Zen were being co-developed with similar ideas behind them? Just one being x86 and the other ARM. Which, especially in hindsight, they were smart to focus development, but I do wonder if they'd be able to apply Zen ideas to ARM for benefits to get into that space. I feel like AMD should at least consider ARM, if for no other reason than to offer hybrid designs, where they could have a bank of ARM cores, and a bank of AMD x86 cores. I think that would help them out in laptop space, where on battery it relies more on the ARM stuff, but then docked, or for certain use cases like gaming it uses the x86 stuff. Really seemingly there's nothing that would stop them from integrating another company's ARM chip just like they do their own modules, so they could slap a Snapdragon chip in there and they can offer all the benefits it has but also x86 compatibility. Which I think that'd be the way to do the Surface Book design, have the ARM chip in the tablet part so it goes to that, and then have the x86 and GPU in the laptop base.

Which, by now the standard ARM cores are probably as good or better than what K12 would have been (maybe not though?), so ditch the custom ARM and just integrate standard ones for low power use. Plus it would let them do Chrome boxes/books, and they could even tout it as a developer feature (where say they could run the ARM stuff in VM so they could test out their Android code). I have a hunch it'd be beneficial for consoles too (especially if they want to win back Nintendo's favor).

Interesting about combining the good parts of Bulldozer and Jaguar. Which, the latter was probably where the CCX design came from, no? I know the console chips, or maybe its just the Xbox, had Jaguar cores in two groups of 4 cores. Not sure what was positive about Bulldozer, although there probably was some aspects, if nothing else some of the logic blocks could probably be ported over, and then they could improve them bits at a time (kinda like them with the 3rd party memory controller).
I kind of hope they stay far away from ARM cores. There isn't much wrong with licensing available cores, and those have come along pretty well; so maybe at most, a shoestring budget project like an A1100 successor. For a sister core, it seems like one would have to pour a whole lot of resources and time into reinventing the wheel just to make it somewhat better. There is an interesting Register article on the history of AMD's A1100 acorn server soc "Seattle", see reddit thread and link: https://old.reddit.com/r/Amd/comments/a13vwc/the_register_amazons_homegrown_23ghz_64bit/

Back then when the leading core (A57) was so weak may have been the time when one would think, hey "let's do our own much better version". Yet in hindsight that may not have paid off, as along came the A73 (in less time than one could have finished something homegrown).
 
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moinmoin

Senior member
Jun 1, 2017
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Note that AMD does still use ARM as part of their SoCs, specifically for TrustZone, likely also for other parts (I'd suspect the SCF with all its sensors and all the fancy PR names for "SenseMI" technology is internally running on ARM). ARM just isn't used as a performance product itself like K12 should have been, and that's a good decision since ARM competition is less predictable while within the x86 ecosystem AMD will always be a valid second source in the worst case (something they are already exploiting well using their semi custom business).
 
Mar 11, 2004
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I kind of hope they stay far away from ARM cores. There isn't much wrong with licensing available cores, and those have come along pretty well; so maybe at most, a shoestring budget project like an A1100 successor. For a sister core, it seems like one would have to pour a whole lot of resources and time into reinventing the wheel just to make it somewhat better. There is an interesting Register article on the history of AMD's A1100 acorn server soc "Seattle", see reddit thread and link: https://old.reddit.com/r/Amd/comments/a13vwc/the_register_amazons_homegrown_23ghz_64bit/

Back then when the leading core (A57) was so weak may have been the time when one would think, hey "let's do our own much better version". Yet in hindsight that may not have paid off, as along came the A73 (in less time than one could have finished something homegrown).
Yeah, the standard ARM designs are good enough that they'd be good to go with. Actually would probably be the best route just for the software alone (no specialization needed like a custom core likely would).

My argument is that, instead of trying to build Zen to scale down (not that its horrible, and especially Zen 2 should be good in laptops, but it won't touch ARM in low power), just use ARM. Pair it with Zen (kinda like how they're approaching memory/storage as a tiered system, do the same with processors) to offer hybrid setups, where on battery or idle it uses ARM. It would let them get into markets they currently aren't or don't compete well in (Chromebooks, tablets, laptops in general), and its not just lower cost stuff, I think it could help them get into premium segments better as well (stuff like the Surface line, if I were AMD I'd have a Surface Book like 2-in-1/convertible, where the ARM chip is in the tablet part, and the CPU/GPU in the base). They could partner with other companies who would focus on the ARM stuff, saving AMD the resources of that.

I think getting in good with Google and Microsoft would be beneficial for software development as well, so its more than just the sales. Google is working on a hybrid OS that fuses ChromeOS and Android, which I think would be a great fit for the type of thing I'm talking about. And I think it'd be a good fit for Surface (the Pro as well, where on battery it could focus on the ARM, and then it could have a dock to enable higher performance kinda like the Switch does - which there was a Surface dock before), and Microsoft is doing a push for Windows on ARM (because I think Microsoft recognizes that ARM is now powerful enough to start expanding beyond phones and even tablets). I think it would be a good fit for Apple too (Macbook Pro, where it has an Apple SoC but also like an 8+core Zen 2 CPU with decent AMD GPU, I'd think that'd be a Apple developers dream - plenty of power for Mac development and editing/content creation, with the ability to test iOS code right on the chip).

I think working with ARM companies would help for their console/custom business too. And maybe they can license their GPU in ARM designs of other companies. So, its key they get their GPU designs to improve substantially (especially in efficiency). Oh and speaking of GPU, I think these types of designs would be well suited for external GPU boxes/docks, so they could push thin and light laptops that have usable battery life because of ARM, but can offer substantial power when docked.

ARM SoCs also would help them with networking stuff, as it would let them add cellular (and even just Wifi and the like) without having to do their own.

I think it will also be important for their future. I think its just a matter of time til ARM takes over most consumer devices. Its angling for laptops, but I think as consumer stuff becomes more simplified terminal relying on cloud backend to do the heavy lifting (which keep Zen development for that), that ARM will become the norm because that stuff will focus on small size, low power, and connectivity. Stuff like AR glasses, where the ARM chip is more just handling I/O, and communication.

Note that AMD does still use ARM as part of their SoCs, specifically for TrustZone, likely also for other parts (I'd suspect the SCF with all its sensors and all the fancy PR names for "SenseMI" technology is internally running on ARM). ARM just isn't used as a performance product itself like K12 should have been, and that's a good decision since ARM competition is less predictable while within the x86 ecosystem AMD will always be a valid second source in the worst case (something they are already exploiting well using their semi custom business).
Right, but that's very different from what I'm talking about.

Absolutely, I think they'd be smart to eschew customizing ARM and instead just use the standard design or partnering with other companies that are doing ARM stuff.
 
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jpiniero

Diamond Member
Oct 1, 2010
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big.LITTLE on different architectures would be weird. If they were going to do this, Jaguar would be the pick, especially if they can get Sony/MS to pay for the majority of the design costs of shrinking it to 7.
 

JohnTheHero

Junior Member
Nov 19, 2018
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big.LITTLE of two different architectures would be a nightmare for developers. That would mean two totally different intruction-sets, and therefore you actually need two different binaries. Doesn't ARM allow big.LITTLE only between selected compatible cores of same ISA variants?
I cannot even imagine how ISA heterogeneous SOCs would work, how to do migrate thread from big to little? If you cannot migrate task in runtime (application restart required), or if it requires to much work to do so (takes too much time, and energy) then it wouldn't make sense to implement it.
I guess ISA-variant compatibility problem also stands on the way to zen-cat mix.
 
Apr 27, 2000
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ISA heterogeneous SoCs already exist. AMD has had ARM cores on-die for awhile, with TrustZone etc. Now from a code perspective, it's more complicated since you rarely (if ever) address a chip like TrustZone from the userspace.

Note that big.LITTLE is (as of now) out-of-date. It's all DynamIQ now. Which is basically the same thing, except that the core configurations are now more flexible.

Furthermore, I see no reason for AMD to continue using cat cores. Development on those has mostly ended. Low-power Zen would outperform cat cores easily. Do not underestimate what Zen can do at low voltages; AMD has simply chosen not to target any market (yet) where that would be viable, since the margins are so low.
 


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