The index registers have a parallel instruction to , which is . This is often seen in stack-oriented languages like Forth which at the end of every Forth word (atomic subroutines comprising the language) must jump unconditionally back to their thread interpreter routines. Typically this jump instruction appears many hundreds of times in an application, and using rather than saves a byte and two t-states each occurrence. This naturally makes the index register unavailable for any other use, or else the need to constantly reload it would negate its efficiency.