cc65/libsrc/tgi/tgidrv_line.inc

;
; Oliver Schmidt <ol.sc@web.de>
;
; Generic LINE implementation based on SETPIXEL for TGI driver inclusion
; Code previously present in a2.lo.tgi, c128-vdc.tgi and c128-vdc2.tgi
;

; ------------------------------------------------------------------------

; LINE: Draw a line from X1/Y1 to X2/Y2, where X1/Y1 = ptr1/ptr2 and
; X2/Y2 = ptr3/ptr4 using the current drawing color.
; Must set an error code: NO

	.proc 	LINE

; Used for passing parameters to the driver.

X1	:=	ptr1
Y1	:=	ptr2
X2	:=	ptr3
Y2	:=	ptr4

; These ones must be on zpage.

TEMP1	:=	tmp3
TEMP2	:=	tmp4
TEMP3	:=	sreg
TEMP4	:=	sreg+1
PB 	:=	ptr3
UB	:=	ptr4
ERR	:=	regsave
NX	:=	regsave+2

; ------------------------------------------------------------------------

	.bss

COUNT:	.res	2
NY:	.res	2
DX:	.res	1
DY:	.res	1
AX:	.res	1
AY:	.res	1

; ------------------------------------------------------------------------

	.code

	; nx = abs (x2 - x1)
	sec
	lda	X2
	sbc	X1
	sta	NX
	lda	X2+1
	sbc	X1+1
	tay
	lda	NX
	jsr	abs
	sta	NX
	sty	NX+1

	; ny = abs (y2 - y1)
	sec
	lda	Y2
	sbc	Y1
	sta	NY
	lda	Y2+1
	sbc	Y1+1
	tay
	lda	NY
	jsr	abs
	sta	NY
	sty	NY+1

	; if (x2 >= x1)
	ldx	#X2
	lda	X1
	ldy	X1+1
	jsr	icmp
	bcc	:+

	;    dx = 1
	lda	#$01
	bne	:++

	; else
	;    dx = -1
:	lda	#$FF
:	sta	DX

	; if (y2 >= y1)
	ldx	#Y2
	lda	Y1
	ldy	Y1+1
	jsr	icmp
	bcc	:+

	;    dy = 1
	lda	#$01
	bne	:++

	; else
	;    dy = -1
:	lda	#$FF
:	sta	DY

	; err = ax = ay = 0
	lda	#$00
	sta	ERR
	sta	ERR+1
	sta	AX
	sta	AY

	; if (nx < ny) {
	ldx	#NX
	lda	NY
	ldy	NY+1
	jsr	icmp
	bcs	:+

	;    nx <-> ny
	lda	NX
	ldx	NY
	sta	NY
	stx	NX
	lda	NX+1
	ldx	NY+1
	sta	NY+1
	stx	NX+1

	;    ax = dx
	lda	DX
	sta	AX

	;    ay = dy
	lda	DY
	sta	AY

	;    dx = dy = 0 }
	lda	#$00
	sta	DX
	sta	DY

	; ny = - ny
:	lda	NY
	ldy	NY+1
	jsr	neg
	sta	NY
	sty	NY+1

	; for (count = nx; count > 0; --count) {
	lda	NX
	ldx	NX+1
	sta	COUNT
	stx	COUNT+1
for:	lda	COUNT		; count > 0
	ora	COUNT+1
	bne	:+
	rts

	;    setpixel (X1, Y1)
:	jsr	SETPIXEL

	;    pb = err + ny
	clc
	lda	ERR
	adc	NY
	sta	PB
	lda	ERR+1
	adc	NY+1
	sta	PB+1
	tax

	;    ub = pb + nx
	clc
	lda	PB
	adc	NX
	sta	UB
	txa
	adc	NX+1
	sta	UB+1

	;    x1 = x1 + dx
	ldx	#$00
	lda	DX
	bpl	:+
	dex
:	clc
	adc	X1
	sta	X1
	txa
	adc	X1+1
	sta	X1+1

	;    y1 = y1 + ay
	ldx	#$00
	lda	AY
	bpl	:+
	dex
:	clc
	adc	Y1
	sta	Y1
	txa
	adc	Y1+1
	sta	Y1+1

	;    if (abs (pb) < abs (ub)) {
	lda	PB
	ldy	PB+1
	jsr	abs
	sta	TEMP3
	sty	TEMP4
	lda	UB
	ldy	UB+1
	jsr	abs
	ldx	#TEMP3
	jsr	icmp
	bpl	:+

	;       err = pb }
	lda	PB
	ldx	PB+1
	jmp	next

	;    else { x1 = x1 + ax
:	ldx	#$00
	lda	AX
	bpl	:+
	dex
:	clc
	adc	X1
	sta	X1
	txa
	adc	X1+1
	sta	X1+1

	;       y1 = y1 + dy
	ldx	#$00
	lda	DY
	bpl	:+
	dex
:	clc
	adc	Y1
	sta	Y1
	txa
	adc	Y1+1
	sta	Y1+1

	;	err = ub }
	lda	UB
	ldx	UB+1
next:	sta	ERR
	stx	ERR+1

	; } (--count)
	lda	COUNT
	sec
	sbc	#$01
	sta	COUNT
	bcc	:+
	jmp	for
:	dec	COUNT+1
	jmp	for


; Copies of some runtime routines

abs:
	; A/Y := abs (A/Y)
	cpy	#$00
	bpl	:+
	
	; A/Y := neg (A/Y)
neg:	clc
	eor	#$FF
	adc	#$01
	pha
	tya
	eor	#$FF
	adc	#$00
	tay
	pla
:	rts

icmp:
	; Compare A/Y to zp,X
	sta	TEMP1		; TEMP1/TEMP2 - arg2
	sty	TEMP2
	lda	$00,x
	pha
	lda	$01,x
	tay
	pla
	tax
	tya			; X/A - arg1 (a = high)

	sec
	sbc	TEMP2
	bne	:++
	cpx	TEMP1
	beq	:+
	adc	#$FF
	ora	#$01
:	rts
:	bvc	:+
	eor	#$FF
	ora	#$01
:	rts

	.endproc