A secret debugging mode, built into all AMD processors since the Athlon XP range, has been uncovered by a hacker - opening the doors for advanced program debugging beyond that specified in the x86 architecture.
Discovered by a hacker known as Czernobyl, the debug mode appears to be an internal AMD utility coded in to the processors by design but disabled when they are shipped. While the capability is still there, it is only accessible by setting certain values in the processor's registers in order to unlock it - values that AMD is keeping a closely guarded secret, akin to a password.
Czernobyl, however, has brute forced his way into the debug mode, and discovered a wealth of potential that could make AMD processors the chips of choice for hackers, coders, and especially reverse engineers, including data-aware conditional breakpoints and page-guard breakpoints that operate directly in the hardware.
The hacker describes his work in documenting the activation and usage of the hardware debugging toolkit as "a work in progress, but usable - hence released as is," but it's not for the faint hearted. A failure to insert the correct key into the registers will result in a general protection fault and a crash of the system - which isn't much use while you're reverse engineering some tricky code.
Czernobyl's full analysis, plus his guide to activating the hidden debug features, is available on the Woodmann site - although the excess traffic caused by hordes of Slashdotters descending upon the server has left it inaccessible at the time of writing.