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The R420 GPU, developed by ATI Technologies, was the company's basis for its 3rd-generation DirectX 9.0/OpenGL 2.0-capable graphics cards. Used first on the Radeon X800, the R420 was produced on a 0.13 micrometer (130 nm) low-''K'' photolithography process and used GDDR-3 memory. The chip was designed for AGP graphics cards. Driver support of this core was discontinued as of Catalyst 9.4, and as a result there is no official Windows 7 support for any of the X700 - X850 products.〔http://support.amd.com/us/gpudownload/windows/Legacy/Pages/radeonaiw_vista64.aspx?type=2.4.1&product=2.4.1.3.7&lang=English〕 ==Development== In terms of supported DirectX features, R420 (codenamed Loki) was very similar to the R300. R420 basically takes a "wider is better" approach to the previous architecture, with some small tweaks thrown in to enhance it in various ways. The chip came equipped with over double the pixel and vertex pushing resources compared to the Radeon 9800 XT's R360 (a minor evolution of the R350), with 16 DirectX 9.0b pixel pipelines and 16 ROPs. One would not be far off seeing the X800 XT basically as a pair of Radeon 9800 cores connected together and also running with a ~30% higher clock speed. The R420 design was a 4 "quad" arrangement (4 pipelines per quad.) This organization internally allowed ATI to disable defective "quads" and sell chips with 12, 8 or even 4 pixel pipelines, an evolution of the technique used with Radeon 9500/9700 and 9800SE/9800. The separation into "quads" also allowed ATI to design a system to optimize the efficiency of the overall chip. Coined the "quad dispatch system", the screen is tiled and work is spread out evenly among the separate "quads" to optimize their throughput. This is how the R300-series chips performed their tasks as well, but R420 refined this by allowing programmable tile sizes in order to control work flow on a finer level of granularity. Apparently by reducing tile sizes, ATI was able to optimize for different triangle sizes. When ATI doubled the number of pixel pipelines, they also raised the number of vertex shader engines from 4 to 6. This changed the ratio of pixel/vertex shaders from 2:1 (on R300) to 8:3, showing that ATI believed the workload in games as of 2004 and onward to be more pixel shader and texturing oriented than geometry based. Normal and parallax mapping were replacing sheer geometric complexity for model detail, so undoubtedly that was part of the reasoning. Strangely, the X700 mainstream card (RV410) had 6 vertex shaders while only being equipped with 2 quads. As such, this chip was obviously designed for a heavier geometry load than texturing, perhaps being tailored for a role as a FireGL chip. RV410 also significantly outgunned NVIDIA's GeForce 6600GT (3 vertex shaders) on geometry throughput. With R420's and RV410's 6 vertex shaders combined with higher clock speeds than the previous generation, ATI was able to more than double the geometry processing capability of 9800XT. Although the R420-based chips are fundamentally similar to R300-based cores, ATI did tweak and enhance the pixel shader units for more flexibility. A new pixel shader version (PS2.b) allowed slightly greater shader program flexibility than plain PS2.0, but was still shy of full PS3.0 capabilities. This new revision to PS2.0 increased the maximum number of instructions and registers available to pixel shader programs.〔Baumann, Dave. (ATI Radeon X800 XT Platinum Edition / PRO Review ), Beyond3d.Com, May 4, 2004.〕 ATI revealed ''Temporal Anti-Aliasing'', a new anti-aliasing technology their chips were capable of. By taking advantage of the frame-to-eye effects of a framerate higher than 60 frame/s, the GPU is able to better smooth aliased edges by rotating the anti-aliasing sampling pattern between frames. A 2X software setting became perceptively equivalent to 4X. Unfortunately, it required the system to be able to maintain at least 60 frame/s or temporal anti-aliasing would cause a noticeable flickering, because the user would be able to see the alternating AA patterns. If the framerate could not be maintained, the driver will disable Temporal AA. However, in games which this performance level could be maintained, Temporal AA was a nice addition to ATI's excellent anti-aliasing options. Note, ATI's "Temporal AA" was actually a temporal dithering filter for spatial AA, not de facto temporal anti-aliasing (which must involve a controlled blending of the temporal subsamples from consecutive screens). Another notable addition to the core was a new kind of normal map compression, dubbed "3Dc". Similar to how texture compression had been part of the Direct3D specification for years and was used for compressing regular textures, normal map compression compacted this new type of surface detail layer. Because DirectX Texture Compression (DXTC) was block-based and not designed for a normal map's different data properties, a new compression method was needed to prevent loss of detail and other artifacting. 3Dc was based on a modified DXT5 mode, which in fact was a fallback option for hardware not supporting 3Dc. Software making heavy use of normal mapping could gain a significant speed boost from the savings in fillrate and bandwidth by using 3Dc. ATI showcased many of their chip's new features in the promotional real-time demo called, ''Ruby: The Doublecross''. Most of the rest of the GPU was extremely similar to R300. The memory controller and memory bandwidth optimization techniques (HyperZ) were identical. R420 was actually a secondary 4th generation project for ATI, with the original R400 plan being scrapped.〔(ATI R400 ), Endian.net, accessed July 6, 2006.〕 R400 would have been more feature-complete, likely with full PS3.0 support among other enhancements, but it is believed that ATI deemed R400 unnecessarily complex for the applications that would be available, and potentially risky to develop on the available semiconductor manufacturing processes of the time.〔(What was R400? (thread) ), Beyond3d.Com forum, November 11, 2004.〕 R400 technology was thus moved to the subsequent generation, renamed to "R500" (became "R520"), while the 4th generation was served with the R300-derived R420. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ATi Radeon R400 Series」の詳細全文を読む スポンサード リンク
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