Not the author, nor an expert, but I'm guessing it could be setup so that the 'OS' throws and interrupt when it receives a transmission signal requesting the RE

Not the author, nor an expert, but I'm guessing it could be setup so that the 'OS' throws and interrupt when it receives a transmission signal requesting the REPL process [1]. So whatever normal processes that the device needs to run would get paused by the interrupt signal, and the REPL process would start up. [1] https://en.wikipedia.org/wiki/Operating_system#Interrupts https://en.wikipedia.org/wiki/Operating_system#Interrupts