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A hacker has discovered a special debugging mechanism within central processing units (CPUs) made by Advanced Micro Devices. The mode potentially allows to learn more about how exactly AMD's chips work, reverse engineer them and make further discoveries regarding their capabilities. It should be understood that that present all the assumptions regarding the mode are just theories.

According to a discovery made by hackes who names himself Czerno AMD processors (Athlon XP and better) have included firmware-based debugging features that expand greatly over standard, architecturally defined capabilities of x86. For some reason though, the manufacturer has been tightly secretive about these features; hint of their existence was gained by glancing at the list of undocumented machine specific registers (MSRs).

There are at least 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.

Potentially, those registers may do something with built-in self test of microprocessors. They may also be required to properly implement processor's microcode during development. Considering the fact that nobody knows what exactly has been found, consequences of the discovery are hard to imagine.

Tags: AMD, Athlon, Phenom, Sempron, duron

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