the fx-501p and fx-502p were a quantum leap
in the ability and storage of programmable calculators at the time, for casio at least. the only things around to compete with them were high-end
hp models which cost a lot more, like the hp41c.
the 501 is the same as the 502 except it has only 128 program steps
instead of 256. the extra 128 steps in the 502 were well worth it, because
256 steps of merged keystrokes allowed the creation of programs previously
not possible - and some longer tunes!
you could store ten programs p0 thru p9 with loops (isz, dsz),
conditional jumps, subroutines and labels and quite powerfully, indirect
addressing - the ability to reference a memory by the contents of another
(special) memory. this feature harked back to the fx-201p
in which it was almost totally useless because of limited storage.
the 501 had continuous memory and the programs could be saved and
loaded from tape using the optional fa-1 interface.
make your own keyboard overlays
there are two overlays, one for programming and one for music programming.
click on the images for hi-res versions