SLI to the next level

3dfx was the first company to pioneer the linking of multiple chipsets for the consumer market with the Voodoo2, using a technique known as Scan Line Interleaving, or SLI for short. On the Voodoo2, two boards could be linked together, theoretically doubling the performance. Each card rendered half the image by rendering alternate scan lines of the image, hence the SLI name. One company, Quantum 3D, went as far as to produce a single board Voodoo2 SLI configuration. In fact, for high-end development systems and arcade machines, they offered a setup with four of these boards linked together for an 8-way Voodoo2 SLI configuration. However, since the Voodoo2, 3dfx has not released another product that supported SLI, until now…

The VSA-100 architecture allows for the long awaited return of SLI to 3dfx products. There is support for up to 32-way SLI, which Quantum 3D will likely produce for extremely high-end applications. Each chip has its own 128-bit wide interface to memory, meaning that, unlike NVIDIA’s GeForce, the VSA-100 will not be limited by memory bandwidth. Although the memory is setup to be completely shared, texture data has to be repeated for each VSA-100 chip. This is typical of all current multi-chip solutions, such as 3dfx’s own Voodoo2 and ATI’s Rage Fury MAXX. However, the VSA-100 does have the advantage of FXT1/DXTC texture compression in hardware, which will minimize the amount of wasted space.

The SLI on the VSA-100 is actually a little bit different from the setup used on the Voodoo2 and is in fact quite a bit improved. The first change is that the number of scan lines that each chip renders can be adjusted from 1 to 128 lines depending on a number of factors, including number of chips, resolution, fillrate required, etc. This is something 3dfx will have to tweak for optimal performance. They have yet to determine whether this will be something fixed in the driver or if it will dynamically adjust depending on the load or the application involved. Since it is definitely software programmable, chances are that it will be a variable that end users will be able to tweak through the registry or any number of tweaking utilities.

The other big change is the SLI interface. On the Voodoo2, the interface was a multi-board, analog solution that required very strict timing control and identical boards. Furthermore, the analog signal was prone to degradation as it passed over a short cable inside the PC. VSA-100 features a 30-bit digital interface between the chips, which are all on one board this time. There are 28-bit data bits, a data valid bit, and a clock.

One advantage of the Voodoo2’s multi board SLI setup is that you could buy one Voodoo2 board to get started and add a second one at a later date to nearly double performance. Of course, this time that’s not possible as all chips must be on one board.

Index Products - Voodoo4 & 5

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