A hacker claims to have discovered a debugging mechanism within AMD CPUs which enables people to learn more about how exactly AMD's chips work, and reverse engineer them. [I'm gonna build one in my shed over the weekend. sub.ed.]
The hacker “Czerno” posted his findings here. He claims that AMD processors have included firmware-based debugging features that expand greatly over standard, architecturally defined capabilities of x86. “Czerno” does not know why, but claims AMD has been secretive about these features which we would have thought would have given them some advantage over Intel. He said that hint of their existence was gained by glancing at the list of undocumented machine specific registers (MSRs). He said that there are four new machine specific registers (MSR) that are involved in the expanded debug facilities.
All those MSRs are "password" protected against casual access: read/write access (RDMSR/WRMSR) to the registers is granted only if EDI holds the correct password value (9C5A203A), otherwise, a GPF exception occurs. All these registers default/reset values are zero. The registers may do something with built-in self test of microprocessors. However they could also be something to run the processor's microcode during development. However since “Czerno” has been a bit vague about it all it is not clear if any of it is true.
We wonder if our readers have views on it.