You feed an assembler program source over serial port and it just executes it in manner somewhat similar if you programmed it using machine code compiled from the source. Not really useful except for educational purposes. It wasn't a real '51 assembler though, with strongly stripped down instruction set. It was mostly limited to storage commands etc (no branching/jump, you just send MOV P0,#0FFh and all pins of P0 are set, you send SETB TR0 and Timer 0 starts, etc.) - all the conditionals etc were meant to be done on the PC side (so you don't send a fixed file, you generate the commands interactively) The project was abandonned about halfway before it got near something usable though... (my promotor had a very annoying habit of changing the design completely when it was between half and 99% ready)