So I have a GDB stub written in C / libopencm3 that can be used to bootstrap into a staapl protocol. What is the next step? The stub can call C functions. Make a small staapl ARM program that behaves as a C function. E.g. add1(). Now, going back to the existing code: an Staapl serial protocol handler is running in qemu. Host this inside the libopencm3 GDB stub.