VIA Apollo vs. AGP 4x

Because there can never be enough Dualatins, I obtained a Supermicro P3TDDE board. This is one of the relatively few boards which support dual Socket 370 Pentium III processors (including Tualatins) and at the same time sport an AGP 4x slot. This is not the Ultimate Museum PC as it lacks ISA slots, but it’s a nice board.

Such boards were relatively rare because by the time the Tualatins were released (2001), Intel was busy pushing Pentium 4s onto every desktop. On the other hand, there were numerous Tualatin-based server boards because the Tualatin (especially the Pentium III-S variant) had significantly better performance per Watt. Server boards typically had onboard graphics (often a Rage XL chip) and no AGP slots.

The P3TDDE was designed for workstations, with an AGP Pro 4x slot, 5 PCI slots, ATA/100, onboard Intel 100Mbps Ethernet controller, and support for up to 4GB PC133 SDRAM. The board has good power management, additional onboard Promise ATA/100 RAID controller, the BIOS supports larger than 128GB ATA disks, and being a Supermicro it’s a generally well made product.

The board is based on the VIA Apollo Pro 266T northbridge with VT8233 southbridge. The chipset supports DDR SDRAM thought the board does not; it’s questionable how much the Pentium III would really benefit anyway.

Since the AGP 4x slot is the most attractive feature of this board (being AGP 1.0/2.0 compatible, it can support just about every AGP card), how well does it work? Unfortunately, not well… because of the VIA chipset. To be more precise, the system is unstable with AGP 4x. 3D intensive applications lock up the system, or in the best case “only” kill the display.

The same problem occurs with several AGP cards which are known to be working. A quick Google expedition revealed that these problems were far from unusual. Both ATI and nVIDIA cards are affected, though not all. A Radeon 9200 works stable at AGP 4x, but it’s considerably slower than a Radeon 9800 at AGP 2x. It’s likely that the Radeon 9800 uses more advanced AGP features which cause trouble (R300 vs. R200 GPU generation).

The BIOS has curious options like “AGP drive strength” which is essentially black magic that no one understands. Until now I’ve only used AGP boards with Intel chipset and have been blissfully unaware of such abominations.

Despite my best efforts, I have not been able to get AGP 4x stable with either of two different Radeon 9800s (Pro and XXL) with Windows XP. Updated VIA 4-in-1 drivers, various BIOS setting combinations, nothing helped. After a few minutes of heavy 3D activity, graphics fails in one way or another.

It is likely that AGP 4x only brings a minimal speed improvement over AGP 2x. But somehow it feels like a huge letdown that a board/chipset advertising AGP 4x support is in reality unable to use AGP 4x in a reliable fashion with faster cards.

This was clearly a common yet very poorly understood problem. The solutions suggested generally amount to little more than voodoo (along the lines of “reboot the system”). I could not find any official word from VIA, let alone any authoritative documentation. The problem also clearly persisted across several generation of chipsets. All in all, this does not inspire confidence in VIA chipsets…

At the moment the board is running with AGP 2x and I keep wondering how much of a difference AGP 4x would have made. Incidentally, the board is surprisingly good at running Windows 7, with dual 1.4GHz Pentium III-S processors of course.