Button Mapping:
    UP:     cursor up
    DOWN:   cursor down
    LEFT:   cursor left
    RIGHT:  cursor right
    START:  Execute/Exit Program
    SELECT: Erase Char
Put instruction:
Hold A:
    UP:     +: Cell increase
    DOWN:   -: Cell decrease
    LEFT:   <: Cell address decrease
    RIGHT:  >: Cell address increase
Hold B:
    UP:     .: Output Cell
    DOWN:   ,: Input Char
    LEFT:   [: Bracket left
    RIGHT:  ]: Bracket right