Little SiS's 20-gauge: The 648FX
The 648FX north bridge from SiS has many of the same virtues as VIA's PT800. Support for an 800MHz front-side bus, one channel of DDR400 memory, and Hyper-Threading technology top the list. The 648FX is a little older than the PT800, though. Motherboards based on the 648FX have been available for some time now, while PT800 boards have yet to arrive in volume. In fact, 648FX boards are available online for as little as 69 bucks. That's one hecukva price for a Pentium 4 motherboard with an 800MHz bus.

The 648FX is decidedly SiS's low-end solution. Unlike VIA, SiS has already designed and produced a north bridge with a dual-channel memory controller, the 655. Although it's not shipping yet, SiS has announced the 655FX, the 800MHz version of the 655 with dual channels of DDR400 memory.

The 648FX north bridge runs without a heatsink on SiS's reference board

The 648FX talks to one of several SiS south bridge chips over a 16-bit, 533MHz chip-to-chip interconnect. SiS calls its interconnect MuTIOL, for Multithreaded I/O Link, and this implementation of MuTIOL provides 1GB/s of bandwidth. Apparently unsatisfied with the MuTIOL marketing name, SiS has now attached the term "HyperStreaming" to its chip-to-chip interconnect. Whatever you call it, the technology is formidable, though not dissimilar to competing interconnects like V-Link and a HyperTransport. Below is a logical diagram of the SiS HyperStreaming architecture. As you can see, MuTIOL is a switched, packet-based transport with a load-balanced array of distinct upstream and downstream links. This interconnect can transport multiple concurrent data streams between main memory and the various device controllers on the south bridge. Sophisticated packet management algorithms ensure best use of the available bandwidth through techniques like split transactions and isochronous transfers using prioritized channels for jitter-sensitive data streams. In other words, when you're playing MP3s off a network share while copying a large video file from a USB 2.0 device to a local hard drive, the sound shouldn't skip. That's what HyperStreaming is all about.

SiS's HyperStreaming architecture exposed

Our 648FX reference board is equipped with SiS' 963L south bridge chip. The 963L is simply the familiar SiS 963 south bridge without Firewire support. Firewire (or IEEE1394, as those afraid of patent lawyers like to call it) has been losing some ground lately as USB 2.0 has flourished. Nearly all of SiS's competitors' south bridge chips lack Firewire support, as well.

The 963L south bridge lacks Firewire and Serial ATA

The 963 family of south bridges can't do Serial ATA or RAID, either. SiS attempted to remedy this situation by including its SiS 180 chip on the 648FX reference board. Like a number of other chips, the SiS 180 is a PCI-based Serial ATA controller with RAID capabilities. Because the SiS 180 is not a part of the south bridge, it will encounter a big, hairy bottleneck in the form of the PCI bus. Standard 32-bit/33MHz PCI peaks at 133MB/s of bandwidth, and that's shared. Each dedicated Serial ATA port peaks at 150MB/s. You do the math. SiS obviously realizes the need for south bridge-based SATA; the company's new 964 south bridge chip will include built-in SATA RAID capabilities. When the 964 arrives in force, I'd expect it to be paired up with the 648FX north bridge.

Nevertheless, the 648FX + 963 combo has much to recommend it, including the usual array of core-logic chipset features, as you can see in the diagram below and in the table on the next page.

A block diagram of the 648FX chipset — Source: SiS