How they stack up
We'll be comparing these three new DDR chipsets against the existing Pentium 4 platforms, including the 850 with RDRAM, the 845 with PC133 SDRAM, and VIA's original P4X266. Before we get down to testing, let's take a look at how these products compare to one another on paper.

In terms of most standard features, the various Pentium 4 platforms have achieved rough parity. They're all reasonably modern chipsets since the Pentium 4 itself is just over a year old. As the traffic cops of a modern PC, they're generally functionally equivalent. All have LPC interfaces rather than ISA support. All of these chipsets use north-to-south bridge interconnects that are faster than the old standard, PCI. All support AGP 4X, ATA/100, USB, AC'97 audio, and various sorts of modem and network interfaces.

There are, however, a couple of key areas where there are real differences: memory support and north-south bridge interconnects. Here's how the chipsets compare.

Intel 845 (PC133) Intel 845 (DDR) Intel 850 VIA P4X266 VIA P4X266A SiS 645 (266) SiS 645 (333)
Memory types PC133 SDRAM PC2100 (266) DDR SDRAM Dual-channel PC800 Rambus DRDRAM PC2100 (266) DDR SDRAM PC2100 (266) DDR SDRAM PC2100 (266) DDR SDRAM PC2700 (333) DDR SDRAM
Effective memory clock speed 133MHz 266MHz 800MHz 266MHz 266MHz 266MHz 333MHz
Memory bus width 64 bits 64 bits 2 x 16 bits 64 bits 64 bits 64 bits 64 bits
Peak theoretical memory bandwidth 1.06GB/s 2.1GB/s 3.2GB/s 2.1GB/s 2.1GB/s 2.1GB/s 2.66GB/s
Maximum addressable RAM 3GB 2GB 2GB 4GB 4GB 3GB 2GB
North/south bridge interconnect type Intel Hub Architecture Intel Hub Architecture Intel Hub Architecture VIA V-Link VIA V-Link MuTIOL MuTIOL
Interconnect clock speed 266MHz 266MHz 266MHz 266MHz 266MHz 266MHz 266MHz
Interconnect bus width 8 bits 8 bits 8 bits 8 bits 8 bits 16 bits 16 bits
Peak theoretical interconnect bandwidth 266MB/s 266MB/s 266MB/s 266MB/s 266MB/s 533MB/s 533MB/s

I should note that these chipsets also support several slower forms of memory. For instance, all of the DDR-capable chipsets will work fine with 200MHz PC1600 memory. Likewise, the 850 chipset will support 600MHz PC600 RDRAM. However, these cheaper, slower forms of memory aren't really cost effective these days, so we won't focus much attention on them.

That said, the 850/RDRAM combo has the obvious theoretical edge in memory bandwidth. However, though it's not on the chart above, the 850 also has a theoretical handicap in terms of latency thanks to RDRAM's "serialized" approach to memory access. We'll see how that plays out in terms of performance shortly.

The Intel chipsets have a decided handicap in their ability to address memory. 2GB of main RAM may seem like a lot, but for workstations and servers, it may soon be a bare minimum, especially if memory prices stay as low as they have been this past year.

Beyond that, the SiS 645 has an obvious advantage with support for 333MHz DDR memory and with its 533MB/s MuTIOL interconnect. We'll test the 645 with PC2700 memory, but it's very unlikely that any of our benchmarks will succeed in generating enough concurrent I/O to show any benefit from MuTIOL. That's not to say such situations don't happen in real-world use; it's just that they're rare and not easily duplicated in repetable, commonly-used benchmark scenarios.