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The Pentium 4 gets SDRAM: Two new chipsets

Intel's 845 and VIA's P4X266 chipsets
— 3:00 AM on September 10, 2001

INTEL's PENTIUM 4 HAS SPENT its life until now mated exclusively to Rambus DRAM on Intel's 850 chipset. That fact hasn't particularly harmed its performance; we recently crowned the 2GHz version of the P4 the new king of x86 performance. The P4's use of RDRAM, however, has almost certainly harmed its market acceptance.

As a company, Rambus hasn't exactly endeared itself to the PC industry in the past few years. Legal patent and licensing wrangling, high RDRAM prices, and technical glitches have made Rambus infamous. Finally, after sticking with its memory partner for quite a while, Intel saw fit to begin distancing itself from Rambus late last year. The two companies still work together, but Intel has slowly and deliberately begun pursuing an alternate course by making SDRAM chipsets available for the Pentium 4. Since then, much of the market has been waiting for the Pentium 4 to transition to a new, 478-pin socket—and, inevitably, away from RDRAM.

The 845 chipset, released today, is a first step. This flavor of the 845 supports good ol' PC133 SDRAM. It's not the fastest memory around, but it's dirt cheap, which is important if the Pentium 4 is to replace the Pentium III in the high-volume corporate market. A version of the 845 chipset capable of using faster DDR SDRAM is in the works, but Intel says it won't arrive until after the first of the year.

Meanwhile, feisty chipset manufacturer VIA has been selling Pentium III and Athlon chipsets with DDR support for some time now. Soon, Pentium 4 motherboards based on VIA's Pentium 4 chipset will start hitting the shelves—complete with support for DDR SDRAM. That fact alone might be enough to annoy Intel just a little bit, but what's really got Intel up in arms something else: VIA has refused to pay for a license for the Pentium 4 bus, instead insisting that a cross-licensing deal between Intel and a company VIA acquired is sufficient. Last week, Intel filed suit against VIA for patent infringement.

Who says chipsets aren't exciting?

I don't pretend to know who's right and who's wrong in the VIA-Intel patent dispute, but the two companies' SDRAM chipsets for the Pentium 4 are definitely worth a look. The 845 is Intel's platform for pushing the Pentium 4 into the mainstream market. If the 845 serves its purpose, the transition away from the Pentium III will happen in earnest, and soon. (In fact, Intel will soon stop taking new orders for the PIII.) For VIA, the P4X266 represents an opportunity to capture a big chunk of the high-end and mainstream Pentium 4 chipset markets.

Let's see how they compare to Intel's RDRAM-driven 850 chipset.

Intel's 845
The 845 chipset, code-named Brookdale, begins life supporting only PC133 SDRAM. Intel claims DDR isn't quite ready for volume production yet, and they've proposed addenda to the JEDEC DDR200 and DDR266 specifications. The major concern, according to Intel, is AC overshoot/undershoot issues.

Now, whether these updates to the DDR spec are driven primarily by politics or by technical concerns, I can't tell you. I'm sure it's quite likely Intel's suggestions will improve things. I also imagine Intel would prefer not to be perceived as following AMD here. You can decide that question for yourself. Honestly, I don't know enough to make that call. I do know this: on the Athlon platform and with VIA's P4X266, DDR SDRAM works quite well.

Anyhow, for now, the 845 chipset supports only PC133 SDRAM, which offers a peak of 1066MB/s of bandwidth. That's quite a bit less than the 3.2GB/s peak throughput of the 850 chipset's dual RDRAM channels. The 845 compares better when the total amount of memory counts; it supports up to 3GB of SDRAM, while the 850 supports up to 2GB of RDRAM. Other highlights of the 845's memory controller include:

  • A write cache to prevent conflicts with critical memory reads
  • An IOQ depth of 12
  • 24 open memory pages
  • Memory refreshes are completed during low-priority memory activity to maximize bandwidth
Beyond the memory controller, however, the 845 is extremely similar to the 850. The two "north bridge" or memory controller hub (MCH) chips share a lot of the same logic: the AGP interface, front-side bus, and Intel Hub Architecture interconnect. The "south bridge" chip—or, in Intel's nomenclature, the I/O controller hub (ICH)—is the same between the 845 and 850 chipsets: Intel's 82801BA ICH2 chip.

A block diagram of the 845 chipset

The 845 MCH chip is covered by a beefy heat sink

The 845's I/O controller hub chip