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'Super-secret' debugger discovered in AMD CPUs

Password-protected feature goes beyond x86

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A hardware hacker has discovered a secret debugging feature hidden in all AMD chips made in the past decade.

The password-protected debugger came as a shock to reverse-engineers who have hungered for an on-chip mechanism for performing conditional and direct-hardware breakpoint operations. Although AMD has built the firmware-controlled feature into all chips since the Athlon XP, the company kept it a closely guarded secret that was only disclosed late last week by a hacker who goes by the name Czernobyl.

“AMD processors (Athlon XP and better) have included firmware-based debugging features that expand greatly over standard, architecturally defined capabilities of x86,” the hacker wrote. “For some reason, though, AMD has been tightly secretive about these features; hint of their existence was gained by glancing at CBID's page.”

To put a chip into developer mode, a user must first enter what amounts to a password — 9C5A203A — into the CPU's EDI register. Czernobyl was able to deduce the secret setting by brute forcing the key.

Presumably, the debugger is an internal AMD utility used during development and then turned off before shipping. Its discovery by world + dog means that everyday users may have powerful new tools to hack, debug, and reverse-engineer their hardware. Now that its existence and the instructions for turning it on are known, the real discoveries about exactly what can be done with it are sure to commence. ®

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