      SUBROUTINE DROTMG (DD1,DD2,DX1,DY1,DPARAM)
C
C     CONSTRUCT THE MODIFIED GIVENS TRANSFORMATION MATRIX H WHICH ZEROS
C     THE SECOND COMPONENT OF THE 2-VECTOR  (DSQRT(DD1)*DX1,DSQRT(DD2)*
C     DY2)**T.
C     WITH DPARAM(1)=DFLAG, H HAS ONE OF THE FOLLOWING FORMS..
C
C     DFLAG=-1.D0     DFLAG=0.D0        DFLAG=1.D0     DFLAG=-2.D0
C
C       (DH11  DH12)    (1.D0  DH12)    (DH11  1.D0)    (1.D0  0.D0)
C     H=(          )    (          )    (          )    (          )
C       (DH21  DH22),   (DH21  1.D0),   (-1.D0 DH22),   (0.D0  1.D0).
C     LOCATIONS 2-4 OF DPARAM CONTAIN DH11, DH21, DH12, AND DH22
C     RESPECTIVELY. (VALUES OF 1.D0, -1.D0, OR 0.D0 IMPLIED BY THE
C     VALUE OF DPARAM(1) ARE NOT STORED IN DPARAM.)
C
C     THE VALUES OF GAMSQ AND RGAMSQ SET IN THE DATA STATEMENT MAY BE
C     INEXACT.  THIS IS OK AS THEY ARE ONLY USED FOR TESTING THE SIZE
C     OF DD1 AND DD2.  ALL ACTUAL SCALING OF DATA IS DONE USING GAM.
C
      DOUBLE PRECISION GAM,ONE,RGAMSQ,DD2,DH11,DH21,DPARAM,DP2,
     1 DQ2,DU,DY1,ZERO,GAMSQ,DD1,DFLAG,DH12,DH22,DP1,DQ1,
     2 DTEMP,DX1,TWO
      DIMENSION DPARAM(5)
C
      DATA ZERO,ONE,TWO /0.D0,1.D0,2.D0/
      DATA GAM,GAMSQ,RGAMSQ/4096.D0,16777216.D0,5.9604645D-8/
      IF(.NOT. DD1 .LT. ZERO) GO TO 10
C       GO ZERO-H-D-AND-DX1..
          GO TO 60
   10 CONTINUE
C     CASE-DD1-NONNEGATIVE
      DP2=DD2*DY1
      IF(.NOT. DP2 .EQ. ZERO) GO TO 20
          DFLAG=-TWO
          GO TO 260
C     REGULAR-CASE..
   20 CONTINUE
      DP1=DD1*DX1
      DQ2=DP2*DY1
      DQ1=DP1*DX1
C
      IF(.NOT. DABS(DQ1) .GT. DABS(DQ2)) GO TO 40
          DH21=-DY1/DX1
          DH12=DP2/DP1
C
          DU=ONE-DH12*DH21
C
          IF(.NOT. DU .LE. ZERO) GO TO 30
C         GO ZERO-H-D-AND-DX1..
               GO TO 60
   30     CONTINUE
               DFLAG=ZERO
               DD1=DD1/DU
               DD2=DD2/DU
               DX1=DX1*DU
C         GO SCALE-CHECK..
               GO TO 100
   40 CONTINUE
          IF(.NOT. DQ2 .LT. ZERO) GO TO 50
C         GO ZERO-H-D-AND-DX1..
               GO TO 60
   50     CONTINUE
               DFLAG=ONE
               DH11=DP1/DP2
               DH22=DX1/DY1
               DU=ONE+DH11*DH22
               DTEMP=DD2/DU
               DD2=DD1/DU
               DD1=DTEMP
               DX1=DY1*DU
C         GO SCALE-CHECK
               GO TO 100
C     PROCEDURE..ZERO-H-D-AND-DX1..
   60 CONTINUE
          DFLAG=-ONE
          DH11=ZERO
          DH12=ZERO
          DH21=ZERO
          DH22=ZERO
C
          DD1=ZERO
          DD2=ZERO
          DX1=ZERO
C         RETURN..
          GO TO 220
C     PROCEDURE..FIX-H..
   70 CONTINUE
      IF(.NOT. DFLAG .GE. ZERO) GO TO 90
C
          IF(.NOT. DFLAG .EQ. ZERO) GO TO 80
          DH11=ONE
          DH22=ONE
          DFLAG=-ONE
          GO TO 90
   80     CONTINUE
          DH21=-ONE
          DH12=ONE
          DFLAG=-ONE
   90 CONTINUE
      GO TO IGO,(120,150,180,210)
C     PROCEDURE..SCALE-CHECK
  100 CONTINUE
  110     CONTINUE
          IF(.NOT. DD1 .LE. RGAMSQ) GO TO 130
               IF(DD1 .EQ. ZERO) GO TO 160
               ASSIGN 120 TO IGO
C              FIX-H..
               GO TO 70
  120          CONTINUE
               DD1=DD1*GAM**2
               DX1=DX1/GAM
               DH11=DH11/GAM
               DH12=DH12/GAM
          GO TO 110
  130 CONTINUE
  140     CONTINUE
          IF(.NOT. DD1 .GE. GAMSQ) GO TO 160
               ASSIGN 150 TO IGO
C              FIX-H..
               GO TO 70
  150          CONTINUE
               DD1=DD1/GAM**2
               DX1=DX1*GAM
               DH11=DH11*GAM
               DH12=DH12*GAM
          GO TO 140
  160 CONTINUE
  170     CONTINUE
          IF(.NOT. DABS(DD2) .LE. RGAMSQ) GO TO 190
               IF(DD2 .EQ. ZERO) GO TO 220
               ASSIGN 180 TO IGO
C              FIX-H..
               GO TO 70
  180          CONTINUE
               DD2=DD2*GAM**2
               DH21=DH21/GAM
               DH22=DH22/GAM
          GO TO 170
  190 CONTINUE
  200     CONTINUE
          IF(.NOT. DABS(DD2) .GE. GAMSQ) GO TO 220
               ASSIGN 210 TO IGO
C              FIX-H..
               GO TO 70
  210          CONTINUE
               DD2=DD2/GAM**2
               DH21=DH21*GAM
               DH22=DH22*GAM
          GO TO 200
  220 CONTINUE
          IF(DFLAG)250,230,240
  230     CONTINUE
               DPARAM(3)=DH21
               DPARAM(4)=DH12
               GO TO 260
  240     CONTINUE
               DPARAM(2)=DH11
               DPARAM(5)=DH22
               GO TO 260
  250     CONTINUE
               DPARAM(2)=DH11
               DPARAM(3)=DH21
               DPARAM(4)=DH12
               DPARAM(5)=DH22
  260 CONTINUE
          DPARAM(1)=DFLAG
          RETURN
      END
