Stack management

The Parameter stack (PS) is maintained by SP and the Return
stack (RS) is maintained by IX. This allows us to generally use
push and pop freely because PS is the most frequently used.
However, this causes a problem with routine calls: because in
Forth, the stack isn't balanced within each call, our return
offset, when placed by a CALL, messes everything up. This is
one of the reasons why we need stack management routines below.
IX always points to RS' Top Of Stack (TOS)

This return stack contain "Interpreter pointers", that is a
pointer to the address of a word, as seen in a compiled list of
words.

                                                        (cont.)