Routines 
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The address of this routine is found in the table of addresses. It is called via the calculator literal +22 by the routine at acs. It is also called indirectly via fp_calc_2.
This subroutine handles the function ASN X and returns a real number from π/2 to π/2 inclusive which is equal to the value in radians of the angle whose sine is X. Thereby if Y=ASN X then X=SIN Y.
This subroutine uses the trigonometric identity TAN (Y/2)=SIN Y/(1+COS Y) to obtain TAN (Y/2) and hence (using ATN) Y/2 and finally Y.


asn  3833  RST $28  X  
3834  DEFB $31  duplicate: X, X  
3835  DEFB $31  duplicate: X, X, X  
3836  DEFB $04  multiply: X, X*X  
3837  DEFB $A1  stk_one: X, X*X, 1  
3838  DEFB $03  subtract: X, X*X1  
3839  DEFB $1B  negate: X, 1X*X  
383A  DEFB $28  sqr: X, SQR (1X*X)  
383B  DEFB $A1  stk_one: X, SQR (1X*X), 1  
383C  DEFB $0F  addition: X, 1+SQR (1X*X)  
383D  DEFB $05  division: X/(1+SQR (1X*X))=TAN (Y/2)  
383E  DEFB $24  atn: Y/2  
383F  DEFB $31  duplicate: Y/2, Y/2  
3840  DEFB $0F  addition: Y=ASN X  
3841  DEFB $38  end_calc  
3842  RET  Finished: 'last value'=ASN X. 
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