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	TITLE	RAND	RANDOM NUMBER GENERATOR
	SUBTTL B.E -21.OCTOBER 1970
	;DRAND.F4 IS A SAMPLE PROGRAM, WHEN USED IN CONJUNCTION
	;WITH RAND.MAC(LOADED AND SAVED AS DRAND.SAV) WILL
	;ILLUSTRATE THE USE OF RAND.MAC
 
	;MULTIPLICATIVE CONGRUENTIAL RANDOM NUMBER GENERATOR TO
	;BE CALLED AS A FUNCTION FROM A FORTRAN PROGRAM. THERE
	;ARE TWO ENTRY POINTS:
	;	1) IRAND RETURNS A RANDOM INTEGER BETWEEN
	;	   0 AND 2**35 - 1.
	;	2) RAND RETURNS A RANDOM FLOATING POINT NUM-
	;	   BER BETWEEN 0.0 AND 1.0.
	;BOTH CALLS REQUIRE A SINGLE INTEGER ARGUMENT WHICH
	;IS USED TO GENERATE A RANDOM INTEGER BY MEANS OF THE
	;FORMULA:
	;	R = (5**13)*ARG  (MOD 2**35)
	;THE RANDOM INTEGER THUS GENERATED IS STORED OVER
	;THE ARGUMENT TO BECOME THE ARGUMENT FOR THE NEXT CALL,
	;HENCE THE ARGUMENT MUST APPEAR IN THE CALLING SEQUENCE
	;AS A VARIABLE RATHER THAN A CONSTANT. IF THE CALL
	;WAS TO RAND, THE NEW RANDOM INTEGER IS CONVERTED TO A 
	;FLOATING POINT NUMBER BETWEEN 0.0 AND 1.0 BEFORE 
	;RETURNING.  VIDE GORENSTEIN, "TESTING A RANDOM NUMBER
	;GENERATOR", COMM ACM, FEB 67.
 
; Modified to PUSHJ/POPJ calling convention 11 Oct 1980
; by Paul T. Robinson, Wesleyan Univ. for DECUS conversion to DEC-20

;	EXTERNAL JOBAPR
	external .jbapr
	ENTRY	RAND,IRAND
	P=17
 
RAND:	SETOM	FPFLAG#		;SET FLOATING POINT SWITCH
	JRST	IRAND1
IRAND:	SETZM	FPFLAG		;SET INTEGER SWITCH
IRAND1:	JSR	IRAND2		;JSR IN ORDER TO PRESERVE FLAGS
	popj	p,		;return
IRAND2:	Z
	MOVE	0,@(16)		;GET ARG
	SKIPN	0		;ARG=0?
	MOVEI	0,1		;YES - RESTART
	MOVE	1,.jbapr		;FOUL FLAG-TRAPPING
	MOVEM	1,FLAGS#
	MOVEI	1,IRAND3
	MOVEM	1,.jbapr
	IMUL	0,CON		;GEERATE NEXT NUMBER
IRAND3:	MOVE	1,FLAGS
	MOVEM	1,.jbapr
	TLZ	0,400000	;MAKE SURE IT'S POSITIVE
	MOVEM	0,@(16)		;PUT IT AWAY FOR NEXT CALL
	SKIPN	FPFLAG		;INTEGER ANSWER REQUIRED?
	JRST	2,@IRAND2	;YES - RETURN WITH IT
	LSH	0,-10		;NO - CONVERT TO FLOATING POINT
	FSC	0,200		;AND DIVIDE BY 2**35
	JRST	2,@IRAND2	;RESTORE FLAGS & RETURN
CON:	5*5*5*5*5*5*5*5*5*5*5*5*5
	END