A Quick Note on Architecture & Features

With pages upon pages of architectural documents still to get through in only a few hours, for today’s launch news I’m not going to have the time to go in depth on new features or the architecture. So I want to very briefly hit the high points on what the major features are, and also provide some answers to what are likely to be some common questions.

Starting with the architecture itself, one of the biggest changes for RDNA is the width of a wavefront, the fundamental group of work. GCN in all of its iterations was 64 threads wide, meaning 64 threads were bundled together into a single wavefront for execution. RDNA drops this to a native 32 threads wide. At the same time, AMD has expanded the width of their SIMDs from 16 slots to 32 (aka SIMD32), meaning the size of a wavefront now matches the SIMD size. This is one of AMD’s key architectural efficiency changes, as it helps them keep their SIMD slots occupied more often. It also means that a wavefront can be passed through the SIMDs in a single cycle, instead of over 4 cycles on GCN parts.

In terms of compute, there are not any notable feature changes here as far as gaming is concerned. How things work under the hood has changed dramatically at points, but from the perspective of a programmer, there aren’t really any new math operations here that are going to turn things on their head. RDNA of course supports Rapid Packed Math (Fast FP16), so programmers who make use of FP16 will get to enjoy those performance benefits.

With a single exception, there also aren’t any new graphics features. Navi does not include any hardware ray tracing support, nor does it support variable rate pixel shading. AMD is aware of the demands for these, and hardware support for ray tracing is in their roadmap for RDNA 2 (the architecture formally known as “Next Gen”). But none of that is present here.

The one exception to all of this is the primitive shader. Vega’s most infamous feature is back, and better still it’s enabled this time. The primitive shader is compiler controlled, and thanks to some hardware changes to make it more useful, it now makes sense for AMD to turn it on for gaming. Vega’s primitive shader, though fully hardware functional, was difficult to get a real-world performance boost from, and as a result AMD never exposed it on Vega.

Unique in consumer parts for the new 5700 series cards is support for PCI Express 4.0. Designed to go hand-in-hand with AMD’s Ryzen 3000 series CPUs, which are introducing support for the feature as well, PCIe 4.0 doubles the amount of bus bandwidth available to the card, rising from ~16GB/sec to ~32GB/sec. The real world performance implications of this are limited at this time, especially for a card in the 5700 series’ performance segment. But there are situations where it will be useful, particularly on the content creation side of matters.

Finally, AMD has partially updated their display controller. I say “partially” because while it’s technically an update, they aren’t bringing much new to the table. Notably, HDMI 2.1 support isn’t present – nor is more limited support for HDMI 2.1 Variable Rate Refresh. Instead, AMD’s display controller is a lot like Vega’s: DisplayPort 1.4 and HDMI 2.0b, including support for AMD’s proprietary Freesync-over-HDMI standard. So AMD does have variable rate capabilities for TVs, but it isn’t the HDMI standard’s own implementation.

The one notable change here is support for DisplayPort 1.4 Display Stream Compression. DSC, as implied by the name, compresses the image going out to the monitor to reduce the amount of bandwidth needed. This is important going forward for 4K@144Hz displays, as DP1.4 itself doesn’t provide enough bandwidth for them (leading to other workarounds such as NVIDIA’s 4:2:2 chroma subsampling on G-Sync HDR monitors). This is a feature we’ve talked off and on about for a while, and it’s taken some time for the tech to really get standardized and brought to a point where it’s viable in a consumer product.

AMD Announces Radeon RX 5700 XT & RX 5700 Addendum: AMD Slide Decks
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  • zodiacfml - Tuesday, June 11, 2019 - link

    not really but AMD created well calculated prices for these cards (as usual), making it a dilemma to choose between AMD or Nvidia. the 7nm process has no benefit to the user unless we undervolt or under clock these cards. the price differential can be used if RTX hardware in Nvidia adds value or not.
    AMD has created same old story with their GPUs unlike the Ryzen CPUs
  • eva02langley - Thursday, June 13, 2019 - link

    With RTX? Is that a joke? because performance wise, AMD is beating them at mid-range.
  • GreenReaper - Tuesday, June 11, 2019 - link

    Nothing on the new video codec blocks (Video Core Next++)? I hope they slot AV1 support in before the 2020 APUs. I want to buy something I can at least play streaming video without spinning up the fans for the next 10 years, and it looks like Netflix, YouTube et. al. want to move off patent-encumbered formats as soon as possible.
  • levizx - Wednesday, June 12, 2019 - link

    Then you should wait another 2 generations. AV1 hardware acceleration is pretty much essential if you want future-proof.
  • Krysto - Wednesday, June 12, 2019 - link

    It would be a HUGE failure on AMD's part of Navi+ doesn't bring at least AV1 decode acceleration to PS5/Xbox Scarlett. But at least some sort of encode acceleration should be in there, too, because game streamers will badly need it.

    Without AV1 support, PS5/Xbox Scarlett will not be future-proof basically. You can refuse to believe it all you want, but it's true.
  • Meteor2 - Sunday, June 30, 2019 - link

    Absolutely true.
  • hubick - Tuesday, June 11, 2019 - link

    LG has their 8K SM99 75" HDMI 2.1 TV imminent, which sounds like it might only make you choose between it and a motorcycle, not a house, for the price, so I'd really like to see an HDMI 2.1 card to drive it. AMD fail :-(
  • Oxford Guy - Tuesday, June 11, 2019 - link

    Keep in mind how small the die is for Radeon VII. People are duped by the inclusion of HBM II. You're not really getting a super-powerful prosumer chip with Radeon VII. You're getting Vega recycled with a small die.

    I also think it's lame for people to get too excited over the replacement for Polaris, a midrange product at best that's years old. I wasn't excited about Polaris to begin with. All it was was AMD blasting past the efficiency curve for its LPP on a small die. Big deal.
  • Oxford Guy - Tuesday, June 11, 2019 - link

    Anandtech should really highlight die size in these reviews like it used to to give people better perspective about what they're really getting and how it compares with the past.
  • webdoctors - Tuesday, June 11, 2019 - link

    They already have transistor count in the table above, that's sufficient.

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