;=====================================================================
; Name:          	ATAN_DIV.ASM
; Module name:   	atan_div(int x, int y)
; Description:   	This module implements a C-callable assembly routine
;		     		for ArcTan function
;=====================================================================
                .include "atan_div.inc"   	; atan lookup table
						 					; atan macro

;--------------------------------------------------------------------
; int atan_div(int x, int y);
;--------------------------------------------------------------------
; Function:      	inverse tanget of the divident (x/y) 
; 									
; Arguments:     	farctional Q15 sine, fractional Q15 cosine
;                	min: -1.0   <--> 8000h
;                	max: 0.9999 <--> 7FFFh
; 									
; Return value:  	fractional Q15 angle (-1 .. 0,9999)
;                	scaling factor: PI  
;
; Error:         < 3 LSB  
;--------------------------------------------------------------------
                .def    _atan_div
                .text
_atan_div:

        ;context save
        ;------------
                popd    *+              	;push return address
                sar     AR0, *+         	;push old frame pointer
                sar     AR1, *          
                lar     AR0,*           	;init new frame pointer  
                adrk    #3              	;alocate space for three local 
；variables

        ;init pointer to (most left) argument
                mar     *,AR2
                lar     AR2,#-3      
                mar     *0+             	;AR2 = &parameter1

        ;store absolute values to local #1,#2
                setc    sxm
                lacc    *               	;ACC = sin
                abs                    	;ACC = abs(sin)
                adrk    #3             ;AR2 = local #1
                sacl    *             	;local #1 = abs(sin)
                sbrk    #4             ;AR2 = 2nd parameter
                lacc    *              ;ACC = cos
                abs                    	;ACC = abs(cos)
                adrk    #5             ;AR2 = local #2
                sacl    *-             ;local #2 = abs(cos), AR2 = abs(sin)

       ;calculate phase within 1st quadrant
                sub     *               ;ACC = abs(cos) - abs(sin), AR2 -> sin 
                bcnd    DEG_0_45, GT    
                bcnd    DEG_45_90, LT

DEG_45:         
lacc    #2000h
                b       GET_QUADRANT

DEG_0_45:       
;Calc atan(abs(sin)/abs(cos)), abs(cos)>abs(sin)
                lacc    *+,16         ;ACC = abs(sin), AR2 -> abs(cos)
                rpt     #13
                subc    *               
                subc    *-            	;AR2 = local #1
                ATAN                   	;on entry: ACC = argument
                                        ;          AR2 --> local #1
                                        ;on exit:  return = ACC
                                        ;          AR2 --> local #1
                b       GET_QUADRANT

DEG_45_90:      
;Calc atan(abs(cos)/abs(sin)), abs(sin)>abs(cos) or 
；		actan(abs(sin)/abs(sin))
                mar     *+             ;AR2 -> abs(cos)
                lacc    *-,16         ;ACC = abs(cos), AR2 -> abs(sin) 
                rpt     #14
                subc    *
                ATAN                   ;on entry: ACC = argument
                                        ;          AR2 --> local #1
                                        ;on exit:  return = ACC
                                        ;          AR2 --> local #1
                neg
                add     #4000h        ;atan = pi/2 - actan
                b       GET_QUADRANT


GET_QUADRANT:   
sacl    *               ;local #1 = phase(1Q)
                sbrk    #3              ;AR2 -> sin
                lacc    *-              ;ACC = sin, AR2 -> cos
                bcnd    Q34,LT

Q12:            lacc    *               ;ACC = cos
                adrk    #4              ;AR2 -> phase(1Q)

                	bcnd    Q2,LT
Q1:            	lacc    *               ;ACC = phase(1Q)
                	b       epio
Q2:             lacc    #7fffH          
                	sub     *               ;ACC = pi - phase(1Q)
                	b       epio

Q34:            lacc    *               ;ACC = cos
                	adrk    #4              ;AR2 -> phase(1Q)
                	bcnd    Q4,GT
Q3:             lacc    *
                	add     #8000h          ;ACC = phase(1Q) - pi
                	b       epio
Q4:             lacc    *
                	neg                      ;ACC = -phase(1Q)
                	b       epio

        ;context restore                
        ;---------------
epio:           mar     *,AR1
                	sbrk    #(3+1)          ;pop local var's+1 from stack
                	lar     ar0, *-         ;restore old frame pointer
                	pshd    *                ;restore return address
                	ret