/  ATAN -- ARCTANGENT
/  FOR 23 BIT F. P. FORMAT
/  WRITTEN BY
/	PHILLIP SIEMENS
/	LAWRENCE LIVERMORE LABS
/	AUGUST 26, 1975
/
/ THE FOLLOWING IDENTITIES ARE USED:
/   FOR ABS(X)>1, ATAN(X)=PI/2 - ATAN(1/X)
/   FOR ABS(X)<.001, ATAN(X)=X
/
/ A 6 TERM POLYNOMIAL IS USED TO APPROXIMATE THE ATAN
/ THERE ARE APPROXIMATELY 120 OCTAL LOCATIONS AVAILABLE
/ IN THIS SUBROUTINE.

	ENTRY ATAN

OPDEF	TADI	1400
OPDEF	DCAI	3400
OPDEF	JMPI	5400


ABSYM	OPH	25
ABSYM	OPL	26
ABSYM	OPX	27

LAP

ATAN,	6140		/VERSION #
	0004		/EDIT #
	TAD ATAN	/GET CALLING FIELD
	DCA ATAN1
ATAN1,	 HLT		/SET TO CALLING FIELD
	TADI ATAN#	/GET CDF TO ARG
	DCA ATAN2
	CLA CMA
	INC ATAN#
	TADI ATAN#	/GET ADDRESS OF ARG
	DCA 10		/SET UP POINTER TO ARG
	INC ATAN#
	TAD (17
	DCA 11		/POINTER TO FAC
	CLA CLL CMA RTL	/GET -3
	DCA ATAN1	/COUNTER FOR 3 WORDS
ATAN2,	 HLT
	TADI 10		/GET ARG WORD
	6211
	DCAI 11		/PUT IN FAC
	ISZ ATAN1
	JMP ATAN2
	TADI (22	/GET EXPONENT
	SPA SNA		/IS IT MINUS OR ZERO
	JMP SIZOK	/YES
	TAD (-1
	SZA CLA		/EXP > 1
	JMP BIGGER	/YES
	TADI (20	/GET HI PART
	AND (1777
	SZA CLA		/ > 1?
	JMP BIGGER	/YES
	TADI (20	/GET SIGN
	SPA CLA
	CMA		/MINUS
	DCA ATAN1	/REMEMBER THE SIGN
	TAD PI2		/PUT PI/4 INTO FAC
	DCAI (20
	TAD PI2#
	DCAI (21
	DCAI (22
	ISZ ATAN1	/MINUS?
	JMP EXITO	/NO
	CALL 0, CHS
EXITO,	CLA CLL CML RTL	/2
	TAD ATAN
	DCA ATAN3
ATAN3,	 HLT
	JMPI ATAN#

PI2,	3110		/PI/2
	3755
	0001

BIGGER,	CLA CMA
	DCA ATAN1	/INDICATE X > 1
	TADI (20	/MOVE X TO OP
	DCA OPH
	TADI (21
	DCA OPL
	TADI (22
	DCA OPX
	CLA CLL CML RTR	/2000
	DCAI (20	/SET FAC = 1
	DCAI (21
	IAC
	DCAI (22
	CALL 1, FDV	/GET FAC = 1/X
	ARG OPH

SIZOK,	CLL CLA
	TAD ACL		/GET EXPONENT
	SNA		/IS IT ZERO?
	JMP SEVAL	/YES
	TAD (10
	SZL CLA		/IS X SO SMALL, THAT ATAN(X)=X?
	JMP SEVAL	/NO
SIZEA,	ISZ ATAN1	/WAS X > 1
	JMP EXITO	/NO
	CALL 0, CHS
	CALL 1, FAD
	ARG PI2
	JMP EXITO

PAGE

SEVAL,	TADI (20	/SAVE X
	DCA X
	TADI (21
	DCA X#
	TADI (22
	DCA EX
	CALL 1, FMP	/MAKE X*X
	ARG X
	TAD ACH		/SAVE X SQUARED
	DCA XX
	TAD ACM
	DCA XX#
	TAD ACL
	DCA EXX
	DCA ACH
	DCA ACM
	DCA ACL

	TAD (-6
	DCA ACNT
	TAD ACOF
ARG2,	DCA ARG1#
	CALL 1, FAD
ARG1,	ARG COF
	CALL 1, FMP
	ARG XX
	TAD (3
	TAD ARG1#
	ISZ ACNT
	JMP ARG2
	CALL 1, FAD
	ARG ONE
	CALL 1, FMP
	ARG X
	JMP SIZEA

AS^T=
1.iono{oow~_/w~}~~{o}}5{~~?:WoU{|d}~gz&	9AADTC 	
NDT
C A
OAR
F,GAC	2AAD1R 
#GA	
 LC 1LDF,A
A1R
A,G R	FCG	
OLC
1LAF, 
M A
P R	
XGT
X A	
(DT
3 A	GAD
1RI
# S	NAZ
TCM	
APJ57 
	7776
	5252		/-.333263742
	7472
	7777
ONE,	3777		/ .999999
	7767
	0000
X,	0
	0
EX,	0
XX,	0
	0
EXX,	0

END