Pretty easy. Intel only makes 875 cores. They put them on the 875 style pin grid, or the 865 style pin grid, at their own discretion. The 865 pin grid doesn't have the connections needed for ECC. Also, Intel disables a memory shortcut called "PAT" on the 865.
875: supports PAT natively, and ECC/Non ECC memory
865: doesn't support PAT natively, and Non ECC memory only.
some companies have used BIOS workarounds to re-enable PAT on the 865.
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