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S3's MultiChrome dual-GPU technology

Shiny shiny

S3 GRAPHICS HAS BEEN making a slow, sustained push for respectability since the arrival of its first Chrome-series graphics processors in 2003. That push continued with the introduction of the updated Chrome S27 GPU last fall, and at that time, S3 hinted at a new wrinkle in its plans. Like ATI and NVIDIA, S3 would be introducing a multi-card graphics capability, dubbed MultiChrome. We got a brief preview of MultiChrome in action back in November, but S3 has been quietly preparing the tech for prime time since then.

The time has come for MultiChrome to make its formal debut, and we've spent a few days playing with it in Damage Labs. Has S3 succeeded in doubling up on Chrome S27 GPUs for nearly twice the performance? Let's find out.

The Chrome S27 GPU
Before we get to talking about two of 'em, we should probably talk briefly in the singular about the Chrome S27 graphics chip. The S27 is a DirectX 9-class GPU ultimately derived from S3's original DeltaChrome design but enhanced in important ways over the years. S3's latest Chrome silicon includes a native PCI Express interface, 24 bits of precision per color channel throughout the graphics pipeline, and additional registers to speed execution of shader programs. S3 has a lower end version of this GPU called the Chrome S25, but only the S27 has S3's blessing for use in a MultiChrome config.

The basic graphics architecture of the Chrome S27 is fairly conventional. The GPU has four vertex shader units and eight pixel shader processors, but only four texturing units and four render back ends capable of turning a fragment into a pixel. Like most low-end GPUs, the S27 does have its compromises. Among them is a relatively simple memory controller with no crossbar design, and antialiasing capabilities that are limited to 2X or 4X ordered-grid supersampling. These limitations might be problematic in a more expensive product, but they're not show-stoppers here.

Like earlier iterations of the Chrome series, the S27 supports DirectX 9's Shader Model 2.0 programming model. That puts it slightly behind the curve compared to the latest GPUs from ATI and NVIDIA; those support Shader Model 3.0. The difference between the two programming models is complicated, but it essentially boils down to a few key things. SM3.0 allows for dynamic flow control in pixel shader programs, so that longer programs can fork and potentially take different paths depending on conditional results. Shader Model 3.0 also has a higher threshold for numeric precision, requiring datatypes as precise as 32 bits of floating-point data per color component in a pixel. Those are probably the two most important innovations in Shader Model 3.0, and they are simply not likely to mean much to a low-end graphics card at present. These cards simply aren't fast enough to use long, branchy shader programs to render effects complex enough to push the boundaries of SM2.0's required 24 bits of precision per color channel. For most intents and purposes, the Chrome S27 ought to be able to crank out visuals equivalent to those produced by its SM3.0-ready competitorsĀ—just as the Radeon X800 was able to keep pace with the GeForce 6800 when ATI and NVIDIA split on this issue.

Interestingly, S3 has chosen to have its Chrome S20-class GPUs manufactured by Fujitsu on a 90nm process. Thanks to this relatively advanced manufacturing technique, the Chrome S27 can run at a clock speed of 700MHz while keeping power consumption low. S27 cards ship with either 128MB or 256MB of GDDR3 memory, also clocked at 700MHz.

Those specs should do nice things for the S27, competitively speaking. The 256MB version of the S27 lives in the price neighborhood of the Radeon X1300 Pro and GeForce 7300 GS. At 700MHz, the Chrome S27 can pump out a peak of 2.8 billion pixels per second, each of them with a texture applied. The 700MHz memory clock gives the S27 a peak of 22.4 GB/s of bandwidth on its memory interface. By comparison, the Radeon X1300 Pro tops out at 2.4 billion textured pixels per second and only 11.2 GB/s of memory bandwidth. Any qualms about S3's less sophisticated memory controller should be erased by S3's use of much faster memory. The only real wild card here is pixel shader power. The S27 has eight of S3's shader processors running at 700MHz, while the X1300 Pro has four of ATI's more capable pixel shader units running at 600MHz. Pixel shader performance resists boiling down to basic math more than most things in graphics, because different architectures can have markedly variant delivered performance per clock.

That's a basic summary of the S27's place in the universe. We have a review of this GPU in the works, in direct comparison to the competition from ATI and NVIDIA, but in a quirk of scheduling, this brief look at MultiChrome is ready before that review. Stay tuned for a fuller look at the S27 in single-card form.

When they say Chrome, they mean it