====== Bresenham Circle Routine ======

Implementation by Scout/Silicon Ltd.

This example is textmode (40x25) only but can easily be converted into any other mode.
Enjoy and don't forget to credit me if you use it ;-)

I hope those solid circle tunnels show up now in a demo.

<code>
; Draws a circle using the Bresenham algorithm in textmode 
; [C]2006 Scout/Silicon Ltd.

Radius	= 12
CenterX	= 20
CenterY = 12
Char    = 160
Cx	= $50
Cy	= $51
Cd	= $52

*=$0810

circle
		lda #Radius
		sta Cy
		asl	 ; 2 * radius
		sta CalcD

		lda #0
		sta Cx

		lda #3
		sec
CalcD = *+1
		sbc #$00
		sta Cd

C_loop
		lda Cx
		cmp Cy
		bmi C_Points
		jmp E_rts
C_points

		lda Cx
		sta x1
		sta y2
		sta x4
		sta y7

		eor #$ff	; Maak 
		clc		; getal
		adc #1		; negatief
		sta y3
		sta x5
		sta y6
		sta x8

		lda Cy

		sta y1
		sta x2
		sta x3
		sta y8

		eor #$ff
		clc
		adc #1
		sta y4
		sta y5
		sta x6
		sta x7

y1=*+1
		lda #0			; y
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
x1=*+1
		lda #0			; x
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y
;-----

y2=*+1
		lda #0			; x
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
x2=*+1
		lda #0			; y
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y

;-----

y3=*+1
		lda #0			; x
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
x3=*+1
		lda #0			; y
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y
;----

y4=*+1
		lda #0			; x
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
x4=*+1
		lda #0			; y
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y
		
;----

y5=*+1
		lda #0			; x
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
x5=*+1
		lda #0			; y
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y

y6=*+1
		lda #0			; x
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
x6=*+1
		lda #0			; y
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y

y7=*+1
		lda #0			; x
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
x7=*+1
		lda #0			; y
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y

y8=*+1
		lda #0			; x
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
x8=*+1
		lda #0			; y
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y

		lda Cd
		bpl D_pos	; d >= 0
		
		lda Cx
		asl
		asl	; 4*x
		clc
		adc #6
		adc Cd
		sta Cd
		inc Cx
		jmp C_loop

D_pos
		lda Cx
		sec
		sbc Cy
		asl
		asl	;4*(x-y)
		clc
		adc #10
		clc
		adc Cd
		sta Cd
		dec Cy

E_next
		inc Cx
		jmp C_loop

E_rts
		lda Cx
		sta xx1
		sta yy2
		sta xx4
		sta yy7

		eor #$ff
		clc
		adc #1
		sta yy3
		sta xx5
		sta yy6
		sta xx8

		lda Cy

		sta yy1
		sta xx2
		sta xx3
		sta yy8

		eor #$ff
		clc
		adc #1
		sta yy4
		sta yy5
		sta xx6
		sta xx7

yy1=*+1
		lda #0			; y
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
xx1=*+1
		lda #0			; x
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y
;-----

yy2=*+1
		lda #0			; x
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
xx2=*+1
		lda #0			; y
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y

;-----

yy3=*+1
		lda #0			; x
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
xx3=*+1
		lda #0			; y
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y
;----

yy4=*+1
		lda #0			; x
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
xx4=*+1
		lda #0			; y
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y
		
;----

yy5=*+1
		lda #0			; x
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
xx5=*+1
		lda #0			; y
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y

yy6=*+1
		lda #0			; x
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
xx6=*+1
		lda #0			; y
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y

yy7=*+1
		lda #0			; x
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
xx7=*+1
		lda #0			; y
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y

yy8=*+1
		lda #0			; x
		clc
		adc #CenterY
		tax
		lda screenlo,x
		sta $f0
		lda screenhi,x
		sta $f1 
xx8=*+1
		lda #0			; y
		clc
		adc #CenterX
		tay
		lda #Char
		sta ($f0),y

		rts

.align 256

screenlo	; you could also get this from kernal address $EFCO, if kernal is enabled.

		.byte <$0400+(40*0), <$0400+(40*1),<$0400+(40*2),<$0400+(40*3),<$0400+(40*4),<$0400+(40*5)
		.byte <$0400+(40*6), <$0400+(40*7),<$0400+(40*8),<$0400+(40*9),<$0400+(40*10),<$0400+(40*11)
		.byte <$0400+(40*12), <$0400+(40*13),<$0400+(40*14),<$0400+(40*15),<$0400+(40*16),<$0400+(40*17)
		.byte <$0400+(40*18), <$0400+(40*19),<$0400+(40*20),<$0400+(40*21),<$0400+(40*22),<$0400+(40*23)
		.byte <$0400+(40*24)
      	
screenhi
		.byte >$0400+(40*0), >$0400+(40*1),>$0400+(40*2),>$0400+(40*3),>$0400+(40*4),>$0400+(40*5)
		.byte >$0400+(40*6), >$0400+(40*7),>$0400+(40*8),>$0400+(40*9),>$0400+(40*10),>$0400+(40*11)
		.byte >$0400+(40*12), >$0400+(40*13),>$0400+(40*14),>$0400+(40*15),>$0400+(40*16),>$0400+(40*17)
		.byte >$0400+(40*18), >$0400+(40*19),>$0400+(40*20),>$0400+(40*21),>$0400+(40*22),>$0400+(40*23)
		.byte >$0400+(40*24)

</code>