 Words written in Forth are compiled into an executable form . The classical " indirect threaded " implementations compile lists of addresses of words to be executed in turn ; many modern systems generate actual machine code ( including calls to some external words and code for others expanded in place ) . Some systems have optimizing compilers . Generally speaking , a Forth program is saved as the memory image of the compiled program with a single command ( e.g. , RUN ) that is executed when the compiled version is loaded . 
