Apparently the chip that this board uses, the R65F11, is an "enhanced" version of the 6502; can have up to 3kB built in instructions and 192B of RAM, in the chip. I'm not terrible at stack based languages, but then again my main calculator (HP-28s) is of course RPN base so I know how to handle it pretty well. Would you happen to have a link to MOPS? I'd like to try it on the SE and see if its code is portable to my RSC box. The RSC is incredibly picky about errors and doing as much as shifting a key will send it tumbling down, requiring a terminal based reset.