If both XP and MP CPUs were identical, then why aren't MP CPUs available at the same speeds, why do they cost considerably more, and why aren't they available for sale at the same time as the XP chips become available?
They may very well share many features in common, but I doubt that they differ only in the L3 bridge being closed, as that is one physical difference that can easily be made to XP chips, thus killing off any market for the higher priced MP chips. They may very well be 'pin compatible', but that doesn't mean that they are identical, and in fact they aren't, though they do have a lot in common.
Some additional features of the MP chip (disabled in XP chips?):
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Dual point-to-point, high-speed system buses.
Innovative bus snooping capability.
Optimized MOESI cache coherency protocol.
The front-side bus of the AMD Athlon MP processor also provides multiple-bit error detection and single-bit error
correction with 8-bit Error Correcting Code (ECC).
The bios has to know about the CPUs that it supports, so that it can automatically (most new mainboards are jumperless) set the correct voltages, multipliers, fsb, etc. (e.g. Abit KT7 had a bios update to support the Athlon 1400 when it became available, also Abit NF7-S has had a bios update for the new AMD Thornton CPUs for when these become available).
Here's an interesting article ...
Modding the Barton XP to a Barton MP
Quote ...
There you have it, the Barton XP can indeed be modded to run in a dual processor board. As we've mentioned, it's only possible with certain motherboards and BIOS revisions. Since we did not have the luxury to test every single AMD 760MPX motherboards for compatibility, we can only verify at this point of time that the MSI K7D Master is possibly the only verified candidate for this modding.
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Well, that's all there is to modding the Barton XP core to an MP-capable processor. Whether it's a Morgan, Palomino, Thoroughbred or Barton, there's no reason why you cannot mod them to operate in dual processor mode. However, do note that AMD maintains that this method will not give you the reliability offered by true Athlon MP processors. Therefore, users should be reminded that this is mainly for enthusiasts seeking an affordable dual processor setup and not meant to be used for mission critical systems. Users should be responsible for any damages and losses of critical data or systems arising from this modification.
Seeing as an MP mainboard will only work with *modded* dual XP CPUs if the bios supports it, then it would be asking for trouble to try and run an MP CPU in an XP mainboard that was never designed to work with an MP CPU. Besides, who would want to use an MP CPU in a single processor mainboard? Not anyone really, so how can you possibly expect it to work, even if you do mod your chip to make look like an XP model CPU.
