I wrote a raster graphics editor for my Casio programmable graphing calculator. I also wrote it exclusively on the calculator. Did it to one-up my class mate, who had done one previously--mine had better features (no dot in the center of circles, could also do ellipses, had a spray function), and it was half the LOC.

Casio BASIC had subroutines in IIRC 36 registers ([A-Z0-9], conditional jumps and labels.

I wish I had that source code. I can't see how I would ever do something like it again.