The key is MOV EDI, 0×9C5A203A

The key is MOV EDI, 0×9C5A203A

That’s the assembly instruction you need to unlock a secret ‘debug mode’ on AMD processors since the Athelon.  While you need to be in ring 0 to execute it; it did bring up some interesting possibilities of using the special debugging mode for reverse-engineering operation of the chip, accessing possible new features, or presenting a chink in the security armor.  So far, the security problems don’t seem to be probable, but if they cause undocumented resets/etc – they might be.

Anyway – interesting article.  Original posting here.

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