SLI & AGP

With the Voodoo 5 3dfx have re-introduced us to SLI (Scan Line Interleave), in many ways it is a more advanced form & in some it is a step backwards, e.g. With the original Voodoo 2 all you needed to do was purchase another Voodoo 2 to gain SLI capability, with the VSA-100 chip you cannot perform such a feat, e.g. put 2 Voodoo 4’s in the same machine for SLI, or 2 Voodoo 5’s together for that purpose (Before anyone emails me with “But there’s a special motherboard that allows it”, that’s for Quantum3D’s Aalchemy – not a retail level item).

Rather than the old approach to SLI with the Voodoo 2 (One card renders odd, the other even lines) the Voodoo 5 (&4) uses a programmable band approach. The display can be spilt from 1 to 128 lines or certain lines may assigned to a particular VSA-100 chip, e.g. If a display is split into 64 lines then one VSA-100 chip would render the 1-32 band, while the other would render the 33-64 band & so on.

In total 32 VSA-100 chips can be used in SLI, although 3dfx are only going to be retailing a maximum 4 chip card, the Voodoo 5 6000. Quantum3D are offering VSA-100 based hardware using up to 32 VSA-100 chips.

Probably the worst, ummmm, feature (failing) of SLI is that texture data needs to be replicated on each VSA-100 chip. That said, only texture’s are replicated nothing more, for all intents & purposes the Voodoo 5 5500 has 64MB memory on-board, not dual-32MB – The video memory is unified. Besides, texture compression helps reduce the amount of texture memory needed.

As regards AGP support the Voodoo 5 5500 AGP supports AGP4 & earlier, but it only supports the AGP 1x data transfer rate. Basically it means you can install the Voodoo 5 5500 in any AGP slot, new or old, but data transfer rate will remain constant (AGP 1x) no matter what the slot supports.

The latest 3dfx drivers include an option for AGP Command FIFO to be set to AGP 1x or AGP 2x though. The Voodoo 5 (& 4) do not support execute mode. This is of little consequence though as AGP texturing is relatively slow & no good 3d card uses AGP texturing anyway (Generally because they have more than enough video memory available).

Overclocking

The Voodoo 5 5500 AGP was quite good at overclocking, but only as long as the AGP bus was at 66Mhz. When it was at this (standard) speed I could reach 186Mhz core/memory speed with no lockup’s. Unfortunately, when I overclocked my system itself to a 124Mhz FSB (83Mhz AGP bus) I couldn’t get it past 170Mhz core/memory without locking up in games after a few minutes. The default (166Mhz) is perfectly stable at this speed though & has been for quite some weeks now still.

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