list    p=16C84,t=ON,c=132,n=80,st=off


        include "P16C84.INC"

  cblock   0x0C
    temp
    x,y,a
    x1,y1,cnt
    result
  endc


        ORG     0               ;Reset Vector

        GOTO    Main

        ORG     4               ;Interrupt Vector


Main


        CLRF    x
	clrf	x1
	movlw	0x30
	movwf	y1
	movwf	x1
l1
	movf	y1,w
	movwf	y

	movlw	3
	addwf   x1,w
	movwf	x1
	movwf	x


        CALL    FRAC_DIV

	movf	a,w
	movwf	x
	CALL	arctan

        goto    l1

FRAC_DIV:
;-------------------
;Fractional division
;
; Given x,y this routine finds:
;  a = 256 * y / x
;

    movlw  8    ;number of bits in the result
    movwf  cnt
    clrf   a    ; the result
    movf   x,w

L1:

    clrc
    rlf    y,f   ;if msb of y is set we know x<y
    rlf    a,f   ;and that the lsb of 'a' should be set
    subwf  y,f   ;But we still need to subtract the
                 ;divisor from the dividend just in
                 ;case y is less than 256.
    skpnc        ;If y>x, but y<256
     bsf   a,0   ; we still need to set a:0


    btfss  a,0   ;If y<x then we shouldn't have
     addwf y,f   ;done the subtraction

    decfsz cnt,f
     goto  L1

    return


;----------------------------------------------------------
;
;arctan (as adapted from the similar arcsin function)
;
;  The purpose of this routine is to take the arctan of an
;8-bit number that ranges from 0 < x < 255/256. In other
;words, the input, x, is an unsigned fraction whose implicit
;divisor is 256.
;  The output is in a conveniently awkward format of binary
;radians (brads?). The output corresponds to the range of zero
;to pi/4 for the normal arctan function. Specifically, this
;algorithm computes:
;
; arctan(x) = real_arctan(x/256) * 256 / (pi/4)
;  for 0 <= x <= 255
;  
;  where, real_arctan returns the real arctan of its argument
;in radians.
;
;  The algorithm is a table look-up algorithm plus first order
;linear interpolation. The psuedo code is:
;
;unsigned char arctan(unsigned char x)
;{
;  unsigned char i;
;
;  i = x >> 4;
;  return(arctan[i] + ((arctan[i+1] - arctan[i]) * (x & 0xf))/16);
;}
;
;


arctan

        SWAPF   x,W
        ANDLW   0xf
        ADDLW   1
        MOVWF   temp                    ;Temporarily store the index
        CALL    arc_tan_table           ;Get a2=atan( (x>>4) + 1)
        MOVWF   result                  ;Store temporarily in result

        DECF    temp,W                  ;Get the saved index
        CALL    arc_tan_table           ;Get a1=atan( (x>>4) )

        SUBWF   result,W                ;W=a2-a1, This is always positive.
        SUBWF   result,F                ;a1 = a1 - (a1-W) = W

        CLRF    temp                    ;Clear the product
        CLRC

        BTFSC   x,0
         ADDWF  temp,F
        RRF     temp,F
        CLRC

        BTFSC   x,1
         ADDWF  temp,F
        RRF     temp,F
        CLRC

        BTFSC   x,2
         ADDWF  temp,F
        RRF     temp,F
        CLRC

        BTFSC   x,3
         ADDWF  temp,F
        RRF     temp,W

        ADDWF   result,F

        RETURN

arc_tan_table
        ADDWF   PCL,F
        RETLW   0
        RETLW   20     ;atan(1/16) = 3.576deg * 256/45
        RETLW   41
        RETLW   60
        RETLW   80
        RETLW   99
        RETLW   117
        RETLW   134
        RETLW   151
        RETLW   167
        RETLW   182
        RETLW   196
        RETLW   210
        RETLW   222
        RETLW   234
        RETLW   245
        RETLW   0	;atan(32/32) = 45deg * 256/45

        END