backdoor

n. A way of circumventing a system s security, usually known only to the software developer(s). In less of a security context, backdoors called cheat codes are often included in video games. Once known to the user, these can be used to access secret play levels or obtain superpowers.

background debug mode

n. A resource contained in many modern processors that can be used to help isolate defects in a program. Abbreviated BDM. Many CPUs include quite a bit of extra logic just to give developers a debugging interface. A few pins are either multiplexed with other functionality or dedicated to debugging.

BDM resources include a high-speed synchronous communications link to a connected development computer, which issues commands to the BDM logic in the CPU. This interface lets the programmer see and alter all registers, memory, and I/O, as well as set breakpoints, run, stop, and single step. A few processors dedicate more pins to debugging and can echo limited real-time trace data out of the pins on command.

History: For many years, the in-circuit emulator was the most common debugging tool used in embedded systems development. With the advent of very-high-speed CPUs, however, creating a reliable electrical connection between the ICE and the target CPU became difficult and expensive. Caches and address-translation mechanisms (e.g., pipelines and MMUs) removed any correlation between the signals at the processor s pins and the program flow, further rendering ICEs problematic. Finally, very fine pitch surface mount and BGA packages created mechanical-connection difficulties that were...