A 2001 Athlon Thunderbird returns an undocumented CPUID bit (EDX bit 18, leaf 80000001h) that AMD docs list as reserved on all processors.
Key Takeaways
The physical CPU returns C1C7_FBFFh in EDX; AMD’s own CPUID docs for Athlon Model 4 (Thunderbird) specify C1C3_FBFFh – one bit off, bit 18.
After publication, sandpile.org updated its records to identify bit 18 as an ECC capability indicator for AMD K7 processors.
Athlon Model 4 datasheet Revision G (Oct 2000) documents ECC and SCHECK pins; Revision K (Nov 2001) removes all mention with no explicit changelog entry.
AMD appears to have dropped non-MP ECC support for Socket 462 silently, folding ECC+MP into bit 19 for Athlon MP while pretending Socket 462 ECC never existed outside the MP line.
Non-AMD Socket 462 chipsets (VIA, ALi, nVidia) had no ECC support, giving AMD cover to quietly deprecate the feature for mainstream Athlons.
