AMD: Secret Undocumented Debugger In Chips

[grarpamp]

https://www.theregister.co.uk/2010/11/15/amd_secret_debugger/
http://www.silicon.co.uk/workspace/amd-denies-cpu-debugger-was-a-secret-13759
http://cbid.softnology.biz/html/undocmsrs.html
http://www.woodmann.com/collaborative/knowledge/index.php/Super-secret_debug_capabilities_of_AMD_processors_!
http://web.archive.org/web/20101125052628/http://www.czerno.tk/
https://www.reddit.com/r/technology/comments/e6tsk/knowledge_about_supersecret_debug_capabilities_of/
mov edi 0x9C5A203A


'Super-secret' debugger discovered in AMD CPUs
15 Nov 2010 at 19:10, Dan Goodin
Password-protected feature goes beyond x86
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.