best_example.rasm

start:
.lda #a
.lda #idx
.call add
.jmp -1
.stp


add:
.add #10, #12
.sta d2 ; d1 is used by call and ret and d0 by some variations of add, div, ... . Also keep in mind that using call again will overwrite the return address.
.jze -2
.ret

100 a