Xabre Rattling - SiS's New Graphics Chip

The Technology

On paper, the Xabre is far superior to its direct competitors, ATi's Radeon 7500 and NVIDIA's GeForce4 MX. In addition to the (by today's standards) conventional T&L unit, SiS has also outfitted this chip with a pixel shader unit compliant with version v1.3. The Xabre does not feature a vertex shader, though. Considering that a vertex shader's functions can be emulated by the CPU, this isn't much of an issue - especially considering the card's projected price. Like NVIDIA's GeForce4 MX, SiS's Xabre will have an optimized software vertex shader implemented in its drivers.

Xabre boards are the first graphics accelerators to be sold that can communicate with motherboards using the accelerated AGP 8x interface as specified in rev. 3.0 - if the motherboard supports it. How big the advantage of the faster transfers between chipset and video chip really is remains to be seen. The last jump from the slow AGP 2x (533 MB/s) to AGP 4x (1066 MB/s) failed to make much of a difference in practice, due in great part to the ever increasing size of graphics cards' on-board memory. Therefore, we shouldn't expect this latest increase in available bandwidth to 2132 MB/s to have a noticeable impact in real-world applications (e.g. games). Owing to some difficulty with various VIA-based motherboards, the Xabre card would revert to AGP 2x mode, which didn't turn out to be a great disadvantage in practice. Again, thanks to the large amount of memory available to the video chips, the AGP bottleneck would become pronounced only in extreme high-polygon scenarios, which today's chips would hardly be capable of processing at acceptable framerates anyway.

In a later test we will take a closer look at how much great a performance penalty a Xabre card incurs when operating in AGP2x mode. Nonetheless, since VIA-based motherboards can be found in the majority of today's computers, we decided to conduct all tests on this platform.

The features at a glance:

• 8x8 Full Driving GPU
• Xabre 200 and 400 utilize an AGP 8x interface as specified in rev. 3.0 of the AGP specification and can perform the following DirectX 8.1 functions in hardware: volume textures, BRDF, shadow mapping, pixel shader (v1.3), point sprite, etc.
• Pixelizer Engine
• Xabre incorporates a PS 1.3 compliant pixel shader engine
• Frictionless Memory Control (FMC)
• An optimized memory management based on memory interleaving (2x64)
• Jitter-Free Anti Aliasing
• Xabre supports 3 different anti-aliasing modes: 2x, Jitter and 4x.
• MotionFixing Video processor
• Video acceleration features like 4 fields per-pixel motion detection de-interlace function, video sources from MPEG decoder and Video capture and AVI interfaces.
• 3rd Generation T&L Engine
• A classical T&L engine, sans vertex shader. SiS will implement an optimized software emulation in the driver, though.
• CubicLook Utility
• 3D stereo shutter glasses and overclocking tool.
• DoubleScene Technology
• Dual view features supported through the on-board SiS301 encoder chip