Until Steve Ketchem sent me a screenshot showing a '631 running in a SC420 (I blurred details that named the machine, etc., or that I thought might otherwise leak information), it seemed very, very unlikely indeed: there is only one report online of a SC420 being upgraded with a '631 and the preponderance of evidence suggested otherwise. It appeared that the Intel® E7221 northbridge used in the SC420 simply doesn't support the Cedar Mill processors, making BIOS support a moot point, but Steve's email complicates the picture.
Why Would I Want To?
Simply: value for money and not needing a heatsink/fan assembly the size of Texas.
The LGA775-packaged Intel Pentium 4 family can be divided into those with the 65nm Cedar Mill core, which include all of the 6x1 (e.g. 631, 641, 651, and 661) sequence; those with 90nm Prescott core, including the 5xx sequence (e.g. 521, 522, 524, 531, 541, and 551); and those with the 90nm Prescott 2M core, which includes all of 6x0 sequence (e.g. 630, 640, 650, 660, and 670). The ancient Williamette and Northwood cores predate LGA775 and Intel's current 3-digit numbering scheme.
The problem is that the 90nm Prescott cores gobbled power and, infamously, ran as hot as the hob of hell. They were sold at a time when Intel were having their asses handed to them by AMD™ and the only thing Intel could do, in advance of releasing the 65nm dual-core processors, was ramp up the clock-speed on their aging NetBurst™ processor architecture and put giant cooling assemblies on top. The TDP (thermal design power) of Prescott cores is 115W compared to 86W for a comparable Cedar Mill core, representing at least one third more energy having to be dissipated as heat. If you think 29W doesn't sound like much, try gripping a 25W lightbulb.
Now that both Prescott cores are obsolete, the Cedar Mill processors are, cycle for cycle and cache for cache, much better value for money, use less energy, and need less cooling. Only a fool would use either Prescott when he could use Cedar Mill.
The Dell PowerEdge SC420 stands accused of failure to support Cedar Mill processors.
Both the Intel E7221 northbridge, and the Dell SC420 which uses it, were developed long before the advent of the 65nm Cedar Mill core. The E7221 definitely supports the Prescott and Prescott 2M 90nm cores, since they are listed on Intel's Supported Processors page for their SE7221BK1 server platform, which uses the E7221 northbridge.
Case for the Prosecution
The list of supported processors for the Intel SE7221BK1-E motherboard does not include any 65nm/6x1 series processors.
The online product information for the SuperMicro P8SC8 and its siblings, the P8SCi and P8SCT motherboards, also based on the same chipset, explicitly state: “Supports an Intel® Pentium® 4 processor 6x0 or 5x0 sequence on 90nm process ONLY...” (their capitals).
Although the PowerEdge forums contain conflicting and confusing suggestions and opinions, there is not one single report of a working 6x1 processor. On the contrary there are at least two posts which report failure: this post and this one in which the poster also says that multiple similar failures are reported on a Japanese SC420 site.
Case for the Defence
Dell says that you can upgrade the processor in the SC420 and stipulates the processor requirements as “Intel® Pentium® 4 processor with a minimum clock speed of at least 2.8 GHz, and front-side bus speed of at least 800 MHz and at least 1 MB of internal cache”. The Cedar Mill processors meet these requirements.
Dell released BIOS Update A02 for the SC420 on the 13th of February 2006, which “Added support for newer CPUs”, this was after the release of the Cedar Mill Pentium 4 on the 3rd of January.
The customer reviews of the 631 processor on newegg.com include one which states “Workes great in my Dell Poweredge sc420”.
The Prosecution Responds
The quotes from the Dell literature are accurate and were correct when it was written. They did not anticipate Intel developing and marketing a processor as a “Pentium 4” when it was not compatible with the E7221. How could they? You cannot expect that either Dell or Intel would plaster this incompatibility all over the place: their silence is deafening.
Yes, Dell did indeed produce that BIOS update and it was, indeed, after the release of Cedar Mill. However, it was also after the release of the Prescott 2M cores which are supported. They say “added support for newer CPUs”, not “added support for all of the latest 65nm CPUs”, now that would be convincing!
Now, let's examine that customer review again. It says “Workes (sic) great in my Dell Poweredge (sic) sc420 (sic)”. Could someone who made all those spelling and capitalisation errors not have typed 420 where he meant 430 ? Or, perhaps, posted a review to the 631 thread when he actually bought a 630, just going on the speed and other parts of the description?
In light of Steve's email, I might try a SL9KG Cedar Mill processor (Steve's screenshot shows a 631 with D0 stepping, which can only be the SL9KG) in one of my four SC420s, but it's still pretty clear that you can't sling in any Cedar Mill processor and expect it to work.
The best “definitely” SC420 compatible processor, as far as I can tell, is one from the a 6x0 sequence. The 6x0s have two advantages over the 5xx sequences: having the “enhanced Intel SpeedStep technology”, so the processor can be clocked down when under lighter load; and having 2MiB of L2 cache.
Share Your Experience
If you have used a Cedar Mill processor in an SC420 or any other board with the E7221 northbridge, please comment below.