 
      SUBROUTINE XDADD(X,IX,Y,IY,Z,IZ)
C***BEGIN PROLOGUE  XDADD
C***DATE WRITTEN   820712   (YYMMDD)
C***REVISION DATE  831027   (YYMMDD)
C***CATEGORY NO.  A3d
C***KEYWORDS  EXTENDED-RANGE DOUBLE-PRECISION ARITHMETIC
C***AUTHOR  LOZIER, DANIEL W. (NATIONAL BUREAU OF STANDARDS)
C           SMITH, JOHN M. (NBS AND GEORGE MASON UNIVERSITY)
C***PURPOSE  TO PROVIDE DOUBLE-PRECISION FLOATING-POINT ARITHMETIC
C            WITH AN EXTENDED EXPONENT RANGE
C***DESCRIPTION
C     DOUBLE PRECISION X, Y, Z
C     INTEGER IX, IY, IZ
C
C                  FORMS THE EXTENDED-RANGE SUM  (Z,IZ) =
C                  (X,IX) + (Y,IY).  (Z,IZ) IS ADJUSTED
C                  BEFORE RETURNING. THE INPUT OPERANDS
C                  NEED NOT BE IN ADJUSTED FORM, BUT THEIR
C                  PRINCIPAL PARTS MUST SATISFY
C                  RADIX**(-2L).LE.ABS(X).LE.RADIX**(2L),
C                  RADIX**(-2L).LE.ABS(Y).LE.RADIX**(2L).
C
C***REFERENCES  (PROGRAM LISTING FOR XDSET)
C***ROUTINES CALLED  XDADJ
C***COMMON BLOCKS    XDBLK2
C***END PROLOGUE  XDADD
 
 
