North Bridge

Since launching the SiS735 chipset, SiS has been taking incremental steps on the Athlon platform. SiS745 mainly added FireWire and DDR333 memory support to the 735 chipset, while SiS746 added AGP 8X to the mix. When AMD introduced its 333MHz bus last year SiS had to go back to the drawing board again, hence, SiS746FX was born. SiS746FX also brought DDR400 memory support, but as we learned with nForce2 last year, the Athlon XP platform performs best when both the memory bus and system bus operate at the same clock frequency.

Memory/system bus

This brings us to the SiS748 chipset. As you’ve probably guessed by now, its primary addition is support for AMD’s latest front side bus speed, 400MHz. This boosts peak bandwidth from 2.7GB/sec to 3.2GB/sec, increasing system performance. Like SiS’ previous chipset offerings, the memory bus and system bus can operate asynchronously or synchronously. At 400MHz, memory ratios of 1:1 and 6:5 (for DDR333 memory support) are offered, while at 333MHz system bus, ratios of 1:1, 5:6 (DDR400), and 5:4 (DDR266) are available.

In fact, SiS also provides all the memory ratios for AMD’s older 200MHz and 266MHz processors. This gives end users lots of flexibility. If you want to run your brand new Athlon XP 3200+ with older DDR333 memory, the SiS748 chipset allows that. OEMs love this feature as well, they can use this capability to pair one of AMD’s newer “Barton” Athlon XP processors with older DDR266 memory. The processor will grab the prospective customer’s attention, while he overlooks the slower memory the system ships with.

Memory controller

The memory controller itself supports up to 3GB of memory, with up to 3 memory sockets supported in DDR200/266/333 mode. This brings total support to 1GB per DIMM. Due to the higher frequency of DDR400 however, the maximum number of DIMMs supported drops to two (for up to 2GB of memory). This falls short of KT600, which supports up to 4GB of memory and nForce2, which also supports up to 3GB of RAM, but with all three DIMM sockets available in DDR400 mode.

Rounding out the feature set of the SiS748 North Bridge is its AGP 8X interface. There isn’t much new to say here other than the fact that we have yet to find a single application that takes advantage of this new graphics interface.

No cooling on the North Bridge

Upper half of SiS748 reference board

One interesting aspect that we’ve found in all of SiS’ recent discrete chipsets is the lack of a heatsink on the North Bridge chip. This is largely accomplished due to the 0.18-micron manufacturing process it’s produced on.

With the higher bus speed this chipset supports you’d expect it would need some form of cooling, especially under load. However, SiS must not feel the same way. We conducted quick finger tests at random intervals during our testing and found that the SiS748 chip does indeed get fairly hot, unlike earlier chips, which barely got warm to the touch. The lack of a North Bridge heatsink means lower production cost to prospective motherboard manufacturers, but we’d rather see it implemented in final products.