Simple receive/transmit works but the monitor itself doesn't want to do much. I assume there is not enough time inbetween bytes for the interpreter code to run. Let's measure. 13us.. That's quite a bit at 0.1us instruction cycle. I had put the period to 10us. Hmm.. can't redefine words as macros? Ok, that's not the problem. It was a missing definition. So the 1 command (push) works.