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Both of the chips in the Pro 266 chipset are substantially improved over their predecessors. Most prominent on the feature list, of course, is that "266"—as in 266MHz access to memory. Double Date Rate (DDR) SDRAM sends data on the rising and falling edges of the clock, effectively doubling peak bandwidth. The fastest common variant of DDR SDRAM, PC2100, runs at 133MHz, or 266MHz DDR. Hence the name. The Via VT8633 north bridge chip provides this memory interface.
However, with Socket 370 processors stuck on Intel's relatively slow GTL+ bus, the system isn't likely to be able to take great advantage of that extra memory bandwidth. We'll put that theory to the test below.
More interesting are some of the other enhancements Via has made to their north and south bridge chips—especially V-Link. Most north and south bridge chips are connected by the PCI bus, which they share in common. Starting with its 8xx series chipsets, Intel put a much faster interconnect between these chips. Intel dubbed their new arrangement the Accelerated Hub Architecture. Via now follows suit with V-Link.
Like Intel's Accelerated Hub, V-Link offers twice the bandwidth of PCI, or 266MB per second. And like Intel's solution, V-Link is interesting because it doesn't use a "wide" connection with high pin counts, like 64-bit or 128-bit interconnects, to provide all that bandwidth. Instead, it takes a more "serialized" approach, using an 8-bit, 66MHz bus that transfers data four times per clock cycle. V-Link doubles up on DDR, effectively offering a 266MHz transfer rate. (We've seen the same trick in Intel's Pentium 4 bus, which runs at 100MHz, but is "quad pumped" for an effective 400MHz transfer rate.)
With V-Link in place, the PCI bus moves onto the VT8233 south bridge chip. Via's Super South Bridge has long offered a whole slew of I/O options, and this latest revision offers more than ever. The chip now comes with ATA-100 support, a total of six AC'97 channels for AMR/ACR modems or audio, two PHY interfaces to support Ethernet on ACR, and an LPC interface.
The LPC interface replaces the PCI-to-ISA bridge you'd find in previous chipsets. The LPC (Low Pin Count) interface comes royalty free from Intel, who first implemented LPC in its 8xx-series chipsets. LPC handles legacy I/O functions like keyboards, floppy disk drives, and parallel ports. To software, it looks just like an ISA bus, but it runs at 33MHz off the PCI clock, instead of at 8MHz like ISA. This move explains the AV32's lack of ISA slots, and in the grand scheme, it's definitely a good thing. (For more on LPC, start here.)
All in all, the Pro 266 is the most advanced chipset available for Pentium III-based systems.
