SiS655TX
A quick glance at the chipset block diagram tells us everything we need to know. We've got support for all P4 processors, including Prescott, something recently confirmed by ASUS. A flexible dual channel memory controller feeds the CPU with the data it craves (more on that later). A 16-bit, 533MHz link between the bridges means we have a 1066MB/sec link between the two, enough for the peripherals connected via the SiS964. The we have the usual raft of peripheral interconnect busses and ports used for connecting everything to the system; 8 possible USB2.0 ports, 2 SATA and 2 IDE channels, PCI for connecting board mounted hardware and driving the slots (up to 6), a networking interface and AC'97 audio interface are the highlights.
All in all it's got all the features that Canterwood and Springdale enjoy, bar RAID functions on the SiS964, something that ICH5/R provides. Let's start with the 655TX bridge itself.
SiS655TX
The 800MHz front side bus support for all P4 processors, including Prescott, is a given. 64-bits wide, that's 6.4GB/sec of CPU-to-bridge bandwidth. The ability to feed the processor with as much of that 6.4GB/sec as possible is key to most P4 performance. In that respect, 655TX is more than capable, arguably even more so than Canterwood or Springdale. The key to that statement lies in HyperStreaming, a PAT-like set of fast paths between CPU and memory controller, and the flexible memory controller. The PAT-like enhancements are easy to follow. On certain types of memory transfer or operation, the memory controller can access data a bit quicker than normal, reducing latency and getting the data to the processor a bit quicker.The flexible memory controller is where 655TX has the edge over memory controllers derived from Intel's Canterwood parent. With Intel's controllers, dual channel operation is possible, but only with matched DIMMs with the same memory density and SPD settings. With the requisite set of DIMMs installed, filling both memory controllers on the bridge, the bridge aggregates them, giving rise to a single 128-bit memory channel, created from the pair of 64-bit controllers your DIMMs have populated.
SiS takes that one step further, allowing mismatched DIMMs and odd DIMM counts to operate in dual channel mode, albeit slightly differently than an aggregated 128-bit channel. With mismatched DIMMs, say one 256MB and one 512MB, the memory controller on 655TX can create two 64-bit memory channels and operate them concurrently. Two sets of memory transfers can take place at the same time, just split over the DIMMs that are populating the two discrete controllers. So the memory controller can't do a straight 128-bit transfer from both controllers at the same time, the higher performing method of dual-channel operation, but it can still provide more data than a single channel.
It's slower than the preferred 'pure' mode, but it allows for some added flexibility and increased performance over single channel running, giving you more scope for gradually matching your DIMMs. For the market that 655TX-powered boards will be aimed at - the discerning enthusiast on a tight budget - that's a distinct selling point. Memory is often sold in matched pairs these days, but the option is there to upgrade without that pressure.
Of course we have AGP4X/8X compatibility, supporting 1.5V devices and the 2.1GB/sec that the specification allows for, allowing you to slot in any graphics card produced in the last few years without a problem. With the 1066MB/sec bridge link, the 655TX bridge seems like a decent home for fast P4.
SiS964
The southbridge, in this case SiS964, is often the focal point of current motherboards. High performance northbridges are taken for granted, what we're really interested in is features. Lots of USB ports, FireWire, SATA, RAID; we look for them all on current boards, their inclusion only restricted by price. So low-cost, high-ability southbridges are often key to a product selling well, especially in the lower end of the price spectrum.SiS964 gives you 8 possible USB2.0 ports, a pair of SATA150 channels for two device support, two ATA channels for older hard disks and optical devices, a floppy drive controller, networking interface for up to 100Mbit/sec with a suitable physical interface, AC'97 audio interface and a PCI bus for extra hardware not provided on the diminuitive silicon itself.
There's no semi-hardware RAID like ICH5/R and the 655TX doesn't have CSA for a dedicated connection to a Gigabit Ethernet ASIC, but they are the only downsides when compared to the current Intel bridges. With CSA being of limited use in the majority of scenarios and a variety of discrete, cheap, RAID controllers on the market for board makers to use, things aren't too bad.
All in, a full featured, apparently high performance chipset for P4, supporting Prescott and a variety of flexible memory configurations, along with a competent southbridge. What's ASUS's take on it?
