'Super-secret' debugger discovered in AMD CPUs

'Super-secret' debugger discovered in AMD CPUs
'Super-secret' debugger discovered in AMD CPUs

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By Dan Goodin in San Francisco
The Register
15th November 2010

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.

=E2=80=9CAMD processors (Athlon XP and better) have included firmware-based 
debugging features that expand greatly over standard, architecturally 
defined capabilities of x86,=E2=80=9D the hacker wrote. =E2=80=9CFor some reason, 
though, AMD has been tightly secretive about these features; hint of 
their existence was gained by glancing at CBID's page.=E2=80=9D

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.


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