The RX microcontrollers use ID codes for boot mode ID code protection and for code protection in the OCD emulator, and/or option-setting memory, a set of processor registers for selecting the byte order, the state of the microcontroller after a reset, and setting similar options.

Which symbols are provided as a means to change the default values of the ID codes and processor registers varies between the RX architectures and device families. The header of the file defaults.s, in the directory rx\src\lib\rx\, lists the symbols and which device families they are available for.