The calling-standard macros use the concept of a single, current,
    active routine. A routine is a programming entity that is
    associated with a procedure descriptor that may be called, or
    is a main routine specified as the transfer address of a linked
    image.

    Only one routine can be active or current at any given time
    during assembly. If more than one routine is defined in a single
    assembler source file, all items associated with the current
    routine, that is, within the lexical scope of the routine, must
    be completed before making a different routine current. The
    lexical scope of one routine cannot overlap the lexical scope
    of another routine.

    A routine becomes current or comes into scope by invoking the
    $ROUTINE macro with the appropriate arguments. $ROUTINE marks the
    beginning of the lexical scope of a routine. The complementary
    macro, $END_ROUTINE, marks the end of the current routine's
    lexical scope.

1  –  Routines and Program Sections

    Routines have three types of associated program sections:

    o  Code section-Contains the executable instructions of the
       routine. This section is typically read-only and executable.

    o  Data section-Contains data accessed by a routine. Typically,
       this is where variable data is stored. This section is
       typically nonexecutable, readable, and writeable.

    o  Linkage section-Contains a routine's procedure descriptor and
       the necessary linkage information for calling other routines,
       and for linkage to data not in the linkage section, if any.
       Also, constant data may be placed here. Typically, this
       section is read-only and not executable.

       The linkage section is considered a type of data section with
       the following function:

       -  Provides linkage information for calls made from a routine
          associated with the linkage section.

       -  Provides linkage information for data outside of the
          linkage section.

       -  Defines the associated routine's procedure descriptor so
          that calls can be made to the routine.

       -  Defines constant or static data.