--- /dev/null
+/ARCTANGENT ROUTINE OS8 FORTRAN II LIBRARY
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+/COPYRIGHT (C) 1974,1977 BY DIGITAL EQUIPMENT CORPORATION
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+/THE INFORMATION IN THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE
+/AND SHOULD NOT BE CONSTRUED AS A COMMITMENT BY DIGITAL EQUIPMENT
+/CORPORATION. DIGITAL EQUIPMENT CORPORATION ASSUMES NO RESPONSIBILITY
+/FOR ANY ERRORS THAT MAY APPEAR IN THIS MANUAL.
+/
+/THE SOFTWARE DESCRIBED IN THIS DOCUMENT IS FURNISHED TO THE PURCHASER
+/UNDER A LICENSE FOR USE ON A SINGLE COMPUTER SYSTEM AND CAN BE COPIED
+/(WITH INCLUSION OF DIGITAL'S COPYRIGHT NOTICE) ONLY FOR USE IN SUCH
+/SYSTEM, EXCEPT AS MAY OTHERWISE BE PROVIDED IN WRITING BY DIGITAL.
+/
+/DIGITAL EQUIPMENT CORPORATION ASSUMES NO RESPONSIBILITY FOR THE USE
+/OR RELIABILITY OF ITS SOFTWARE ON EQUIPMENT THAT IS NOT SUPPLIED BY
+/DIGITAL.
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+\f/ VERSION 11A
+/ VERSION NUMBER IS AVAILABLE AT ENTRY POINTS
+ ENTRY ATAN
+
+ATAN, BLOCK 1
+ 11
+ TAD ATAN
+ DCA L4
+ TAD ATAN#
+ DCA L4#
+ INC ATAN#
+ INC ATAN#
+ CALL 1,IFAD
+L4, ARG 0
+ CLL
+ TAD ACH
+ SNA
+ JMP EXIT
+ SPA
+ TAD (4000
+ DCA ACH /TAKE ABSVAL OF ARGUMENT
+ RAR
+ DCA SIGN /AND REMEMBER SIGN
+ TAD ACH
+ TAD (-2014
+ SPA CLA
+ JMP LSTN45 /IF ARG>1,
+ JMS INVRS /INVERT ARG - SUBTRACT RESULT FROM PI/2
+ CLA CMA
+LSTN45, DCA L4# /L4# IS THE "PI/2-RESULT" SWITCH
+ TAD ACH
+ TAD (-1774 /THIS IS AN APPROXIMATE TEST TO SEE
+ SPA CLA /IF THE NEW ARG IS <2-SQRT(3)
+ JMP LSTN15 /IF IT IS,
+ CALL 1,FAD /PERFORM A "DIFFERENCE OF TANGENTS"
+ ARG SQRT3 /TRANSFORMATION TO SUBTRACT PI/6
+ TAD (20
+ JMS INVRS /FROM THE RESULT. THE ARG IS TRANSFORMED
+ CALL 0,CHS /INTO SQRT(3)-4/(ARG+SQRT(3))
+ CALL 1,FAD
+SQT3, ARG SQRT3
+ CLA CMA
+LSTN15, DCA INVRS /USE INVRS AS A SWITCH TO INDICATE THIS
+ CALL 1,STO /TRANSFORMATION OCCURRED
+ ARG T
+ TAD (-4
+ DCA L4
+ TAD SQT3#
+ DCA L3#
+ATLOOP, INC L3# /NOW PERFORM A STANDARD TAYLOR SERIES
+ INC L3# /EXPANSION (WITH TRUNCATED COEFFICIENTS)
+ INC L3#
+ CALL 1,FAD /CONVERGENCE WILL BE GOOD SINCE WE
+L3, ARG SQRT3 /HAVE MADE THE ARGUMENT BE <.3
+ JMS FMPT
+ JMS FMPT
+ ISZ L4
+ JMP ATLOOP
+ JMS FMPT
+ CALL 1,FAD
+ ARG T /FINISH UP THE SERIES
+ ISZ INVRS
+ JMP NOPI6
+ CALL 1,FAD /ADD PI/6 IF NECESSARY
+ ARG PIOVR6
+NOPI6, ISZ L4#
+ JMP NOPI2
+ CALL 0,CHS /SUBTRACT FROM PI/2 IF NECESSARY
+ CALL 1,FAD
+ ARG PIOVR2
+NOPI2, TAD SIGN
+ TAD ACH /SET SIGN OF RESULT = SIGN OF ARGUMENT
+ DCA ACH
+EXIT, RETRN ATAN
+
+INVRS, 0 /INVERSION SUBROUTINE
+ TAD (2014 /ENTERED WITH AC=0 OR 20
+ DCA L4 /AC=0 MEANS 1/FAC, AC=20 MEANS 4/FAC
+ CALL 1,STO
+ ARG T
+ TAD L4
+ DCA ACH /ACM AND ACL WERE CLEARED BY STORE
+ CALL 1,FDV
+ ARG T
+ JMP I INVRS
+
+FMPT, 0 /SUBROUTINE TO MULTIPLY BY T
+ CALL 1,FMP /SAVES A FEW MEASLY LOCATIONS
+ ARG T
+ JMP I FMPT
+
+SIGN, 0 /CELL FOR HOLDING SIGN OF ARG
+T, BLOCK 3 /FLOATING TEMPORARY
+
+ PAGE
+
+PIOVR6, 2004;1405;2216 /PI/6
+PIOVR2, 2016;2207;7325
+SQRT3, 2016;7331;7272 /SQRT(3)
+/THE NEXT 4 NUMBERS MUST IMMEDIATELY FOLLOW SQRT3
+ 1756;0462;4562 /APPROXIMATELY 1/9
+ 5764;4221;3403 /APPROXIMATELY -1/7
+ 1766;3141;6672 /APPROXIMATELY 1/5
+ 5775;2525;2337 /APPROXIMATELY -1/3
+
+ END
+\f
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