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UPDATE: Adding details regarding difference between Intel processor packaging. Adding some minor clarifications, slight differences to text layout.

A few days ago some sources reported that according to ASUSTeK Computer, the new 65nm desktop processors from Intel, such as Presler and Cedar Mill, will only be supported by platforms based on Intel’s most recent chipsets. Nevertheless, some sources close to other mainboard makers indicated that single-core Cedar Mill processors may work on mainboards based on older chipsets from Intel Corp. as well, however, whether they work or not fully depends on the particular system board.

The situation with dual-core Intel Pentium D “Presler” processors was pretty clear, as they require chipsets that support Intel Pentium D processors. Formally, only Intel 945P, 945G, 955X, 975X and their modifications meet this requirement in the Intel chipset product range. But many users who have mainboards based on the older chipset models hoped that they would be at least compatible with the single-core Cedar Mill central processing units (CPUs).

X-bit labs attempted to undertake its own investigation on the matter, especially since most mainboard makers are working very hard on getting ready for the upcoming 65nm CPUs announcement. You can already come across lists of mainboards that will be actually or virtually qualified as “Cedar Mill Ready”.

A few sources indicated that a significant amount mainboards based on Intel 915-series chipsets will be able to support Cedar Mill after the corresponding BIOS update. Even the budget solutions based on Intel 915PL will get this opportunity, at least for entry-level processors. However, it does not mean that absolutely all Intel 915-series based mainboards will work just fine with the new Cedar Mill processors. Some of them guarantee Cedar Mill support only for the newest print-circuit board (PCB) revisions.

A problem with compatibility between Intel Pentium D “Smithfield” processors and Intel 915- and 925-series core-logic sets-based mainboards was a slightly different pin-out of the Pentium D compared to the Pentium 4 processors. Apart from logical differences, such as two GTLREF signals required for Intel’s dual-core implementation, the packaging of Intel dual-core chips sports some new features, such as FORCEPR# - used by the platform to force cores to enable thermal control circuit, three COMP signals (which should be terminated on the system board using precision registers) instead of two, VCCPLL signal required for compatibility with “future processors” and so on. By contrast, the Pentium 4 processors in 775-pin form-factor have a lot of the so-called FC signals, which are “available for compatibility with other processors”. Moreover, the E24 pin which is unused in case of the Pentium 4 became FC10 pin on the Pentium D.

Since Presler and Smithfield platforms are claimed to be compatible, it means that packaging of the former is at least very similar to that of the latter. Given that Presler processor consists of two separate Cedar Mill cores, the latter may have similarities when it comes to platform requirements, e.g., support for VCCPLL, FORCEPR# signals or anything else. So, in case a maker of mainboards designed its products in a forward-looking way and all the FC signals were implemented properly along with flexible processor power supply circuitry, the mainboard should be compatible with the new single-core chips by Intel. The boards which design does not sport new requirements will have to undergo changes, which is why a new PCB revision will be require for some.


Intel Pentium D processor pin-out

It is clear that even Intel i865- and 848P-based mainboards with Socket 775 will be able to work with Cedar Mill CPUs. Although here the revision number will matter even more, as the older mainboard modifications will not qualify for that.

Third-party chipsets, such as SiS 661FX/GX or VIA P4M800 also will support Cedar Mill CPUs after the corresponding BIOS update. NVIDIA nForce 4 SLI Intel Edition chipset known for its eternal conflict with the youngest Intel Pentium D modle 820 will be fully compatible with single-core Cedar Mill CPUs: all you will need is just a BIOS update. All contemporary ATI chipsets for Intel platform are compatible with dual-core CPUs, so there shouldn’t be any compatibility issues when Cedar Mill comes out.

It seems the mainboards manufactured after February 2005 have much bigger chances to acquire Cedar Mill support, as this is the time when Pentium 4 600-series processors on the 90nm Prescott 2M core and featuring 2MB L2 cache emerged along with guidelines how to make dual-core compatible mainboards. The Pentium 4 600-series are the closest relatives of the upcoming Cedar Mill processors, but the latter is made using 65nm manufacturing technology and is optimised for operation as a part of Presler product. Until recently most our concerns were connected with the fact that Cedar Mill is none other but half of Presler core, and this relation to the dual-core CPU could potentially impose certain limitations onto its compatibility with the existing mainboards.

However, there are a lot of mainboards dating back to mid 2004 that will also support Cedar Mill CPUs. These are the solutions produced right after the LGA775 platform and Intel 915/925 chipsets were announced.

Those, who want a mainboard to support the new Cedar Mill CPUs, will need to update its BIOS, so keep it in mind before shopping for a new processor.

In regards of Presler compatibility with contemporary mainboards, a separate story to this matter will be posted later on.

Anton Shilov of X-bit labs has contributed to the news-story.

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