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SUBROUTINE FBS (ZS,ZD)
C
C GIVEN A LOWER TRIANGULAR FACTOR WITH DIAGONAL SUPERIMPOSED, AND
C WRITTEN WITH TRAILING STRING DEFINITION WORDS, FBS WILL PERFORM
C THE FORWARD-BACKWARD SUBSTITUTION NECESSARY TO SOLVE A LINEAR
C SYSTEM OF EQUATIONS.
C
C THE ARE TWO METHODS AVAILABLE FOR THIS PROCESS.
C METHOD 1 - THIS METHOD READS AS MANY RIGHT HAND VECTORS INTO MEMORY
C AS POSSIBLE, AND THEN READS THE LOWER TRIANGULAR MATRIX
C USING GETSTR AND GETSTB TO SOLVE FOR THE SOLUTION VECTORS.
C MORE THAN ONE PASS MAY BE REQUIRED IF INSUFFICIENT MEMORY
C EXISTS FOR LOADING ALL RIGHT HAND VECTORS AT ONE TIME.
C THIS METHOD IS THE OLDER OF THE TWO METHODS.
C (SEE SUBROUTINES FBSF, FBSF1, FBSF2, FBSF3 AND FBSF4)
C METHOD 2 - THIS METHOD IS THE SAME AS METHOD 1 WITH THE EXCEPTION
C THAT MEMORY EXISTS FOR LOADING PART OR ALL OF THE LOWER
C TRIANGULAR MATRIX INTO OPEN CORE AFTER LOADING ALL OF
C THE RIGHT HAND VECTORS INTO OPEN CORE. THIS METHOD
C WILL ELIMINATE THE NEED TO READ THE LOWER TRIANGULAR
C MATRIX TWICE (ONCE FORWARD AND ONCE BACKWARD).
C (SEE SUBROUTINES FBSI, FBSI1, FBSI2, FBSI3 AND FBSI4)
C
C THE SELECTION OF METHOD 1 OR 2 IS DEPENDENT UPON WHETHER
C MEMORY EXISTS FOR READING THE LOWER TRIANGULAR MATRIX INTO MEMORY
C
C SEE SUBROUTINES FBSF AND FBSI FOR OPEN CORE LAYOUTS
C
INTEGER DBL ,DBB ,SYSBUF
&, WORDS ,RLCMPX ,RC
&, TYPEL ,TYPEB
REAL ZS(1)
DOUBLE PRECISION ZD(1)
COMMON /FBSX / DBL(7) ,DBU(7) ,DBB(7) ,DBX(7) ,LCORE
1, PREC ,SIGN ,SCRX
COMMON /SYSTEM/ SYSBUF ,NOUT
COMMON /TYPE / PRC(2) ,WORDS(4) ,RLCMPX(4)
C
C GENERAL INITIALIZATION
C
C DIAG 46 FORCES METHOD ONE
C
CALL SSWTCH ( 46, L46 )
IF ( L46 .NE. 0 ) GO TO 1000
NCOL = DBL( 2 )
TYPEL = DBL( 5 )
TYPEB = DBB( 5 )
RC = RLCMPX( TYPEB )
C
C NRHVWD = NUMBER OF WORDS REQUIRED FOR EACH RIGHT HAND VECTOR
C NRHV = NUMBER OF RIGHT HAND VECTORS
C
NRHVWD = WORDS(TYPEL) * NCOL
NRHV = DBB(2)
C
C CHECK FOR RIGHT HAND VECTORS BEING THE IDENTITY MATRIX
C
IF ( DBB( 4 ) .EQ. 8 ) NRHV = NCOL
C
C COMPUTE THE MEMORY TO READ ALL OF THE RIGHT HAND VECTORS INTO MEMORY
C
NEED = NRHV * NRHVWD
IREMAIN = LCORE - 2 * SYSBUF - NEED
C
C IF LESS THAN ONE COLUMN WORTH OF MEMORY AVAILABLE, USE METHOD ONE
C
IF ( IREMAIN .GE. NRHVWD ) GO TO 2000
C
C METHOD ONE - FIRST, CHECK FOR SUFFICIENT MEMORY FOR PROCESS
C
1000 MEMAVL = LCORE - 2*SYSBUF - NRHVWD
IF ( MEMAVL .LE. 0 ) CALL MESAGE ( -8, -MEMAVL, SUBNAM )
CALL FBSF ( ZS, ZD )
GO TO 7000
C
C METHOD TWO
C
2000 CALL FBSI ( ZS, ZD )
7000 CONTINUE
RETURN
END
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