The $XABSUM_STORE macro moves user-specified values into fields
    of the specified XABSUM. The expanded $XABSUM_STORE code executes
    at run time on a previously initialized (allocated) XABSUM, in
    contrast to the $XABSUM macro, which initializes the XABSUM at
    assembly time. The $XABSUM_STORE macro must reside in a code
    program section.

    Format

      $XABSUM_STORE  XAB=xabsum-address,

                     NXT=next-xab-address