<code>
;-------------------------------------------------------------------------
RasterRow = $2d ; May not be a line with sprites or a badline
_rx = $03
_ry = $04
_pageflip = $05
;-------------------------------------------------------------------------
* = $0801
;-------------------------------------------------------------------------
;- SYS $0810
;-------------------------------------------------------------------------
	.byte $0c, $08, $00, $00, $9e, $20, $32, $30
	.byte $36, $34, $00, $00, $00, $00, $00
;-------------------------------------------------------------------------
* = $0810
;-------------------------------------------------------------------------
	sei
;-------------------------------------------------------------------------
; Turn all sprites off
;-------------------------------------------------------------------------
	lda #$00
	sta $d021
	sta $d020
	sta $d015
;-------------------------------------------------------------------------
; Wait for VBLANC
;-------------------------------------------------------------------------
	bit $d011
	bpl * - 3
	bit $d011
	bmi * - 3
;-------------------------------------------------------------------------
; Perform halfvariance polling
;-------------------------------------------------------------------------
.page
	ldx $d012
	inx
	cpx $d012
	bne * - 3
	ldy #$0a
	dey
	bne * - 1
	inx
	cpx $d012
	nop
	beq Time_1
	nop
	bit $24
Time_1	ldy #$09
	dey
	bne * - 1
	nop
	nop
	inx
	cpx $d012
	nop
	beq Time_2
	bit $24
Time_2	ldy #$0a
	dey
	bne * - 1
	inx
	cpx $d012
	bne Time_3
Time_3	nop
	nop
	nop
	nop
	nop
;-------------------------------------------------------------------------
; Raster is stable here.
; Launch a continous timer which a count of 63.
;-------------------------------------------------------------------------
	lda #$3e
	sta $dc04
	sty $dc05
	lda #$11
	sta $dc0e
.endp
;-------------------------------------------------------------------------
; Initialize everything needed to be inited here
;-------------------------------------------------------------------------
	; Set multicolor-mode and turn on the bitmap, in the process set
	; the high end of the raster to 0, point the screenmemory to
	; $0000 - $3fff, allocated positions are:
	; Bitmap @ $4000
	; Screen 1 = $6000
	; Screen 2 = $6400
	; Screen 3 = $6800 (For doublebuffering)
	; Screen 4 = $6c00 (For doublebuffering)
	lda #$3b
	sta $d011
	lda $d016
	ora #%00010000
	sta $d016
	lda $dd00
	and #%11111100
	ora #%00000010
	sta $dd00
	ldx #0
	stx _pageflip
	lda #%10100101
FillScreen
	sta $4000, x
	sta $4100, x
	sta $4200, x
	sta $4300, x
	sta $4400, x
	sta $4500, x
	sta $4600, x
	sta $4700, x
	sta $4800, x
	sta $4900, x
	sta $4a00, x
	sta $4b00, x
	sta $4c00, x
	sta $4d00, x
	sta $4e00, x
	sta $4f00, x
	sta $5000, x
	sta $5100, x
	sta $5200, x
	sta $5300, x
	sta $5400, x
	sta $5500, x
	sta $5600, x
	sta $5700, x
	sta $5800, x
	sta $5900, x
	sta $5a00, x
	sta $5b00, x
	sta $5c00, x
	sta $5d00, x
	sta $5e00, x
	sta $5f00, x
	inx
	bne FillScreen
SetColorloop
	lda #$9e
	sta $6000, x
	lda #$2f
	sta $6400, x
	lda #$57
	sta $6100, x
	lda #$8a
	sta $6500, x
	lda #$c1
	sta $6200, x
	lda #$bd
	sta $6600, x
	lda #$10
	sta $6300, x
	lda #$f6
	sta $6700, x
	lda #$69
	sta $6000 + $800, x
	lda #$2b
	sta $6400 + $800, x
	lda #$58
	sta $6100 + $800, x
	lda #$83
	sta $6500 + $800, x
	lda #$f1
	sta $6200 + $800, x
	lda #$be
	sta $6600 + $800, x
	lda #$10
	sta $6300 + $800, x
	lda #$a6
	sta $6700 + $800, x
	inx
	bne SetColorloop
;-------------------------------------------------------------------------
; Setup irq-code
;-------------------------------------------------------------------------
	lda #$7f
	sta $dc0d
	sta $dd0d
	lda #1
	sta $d01a
	sta $d019
;-------------------------------------------------------------------------
; Setup interrupt position
;-------------------------------------------------------------------------
	lda #RasterRow
	sta $d012
;-------------------------------------------------------------------------
	lda #$35
	sta $01
	lda #<irq
	sta $fffe
	lda #>irq
	sta $ffff
	lda $dc0d
	lda $dd0d
	asl $d019
;-------------------------------------------------------------------------
; Setup the nmi-irq
;-------------------------------------------------------------------------
	lda #<nmiirq
	sta $fffa
	lda #>nmiirq
	sta $fffb
;-------------------------------------------------------------------------
; Setup the timer adjusted to work every eight row, (63 * 8) - 1 cycles
;-------------------------------------------------------------------------
	lda #$f7
	sta $dd04
	lda #$01
	sta $dd05
	cli
;-------------------------------------------------------------------------
; Here is the maincode executed
;-------------------------------------------------------------------------
MainCode
	lda _pageflip
	eor #$ff
	sta _pageflip
	jmp MainCode
;-------------------------------------------------------------------------
irq
;-------------------------------------------------------------------------
; Save the registers
;-------------------------------------------------------------------------
	pha
	php
	stx _rx
	sty _ry
;-------------------------------------------------------------------------
; Stabilize the raster
;-------------------------------------------------------------------------
	lda $dc04
	and #7
	sta Time_4 + 1
	lda #7
	sec
Time_4	sbc #4
	sta Time_5 + 1
.page
Time_5	bpl * + 2
	cmp #$c9
	cmp #$c9
	cmp #$c9
	cmp #$24
	nop
.endp
;-------------------------------------------------------------------------
; Stable raster here, start the nmi-irq if the raster is on the beginning of the screen
;-------------------------------------------------------------------------
.page
	lda $d012
	cmp #RasterRow + 8
	bcs StopNMI
	lda _pageflip	;3	 3
	bmi _page_2	;2 3	 5  6
	lda #$81	;2	 7
	sta _p_2 + 1	;4	11
	lda #$91	;2	13
	sta _p_1 + 1	;4	17
	nop		;2	19
	jmp start_nmi	;3	22
_page_2	lda #$a1	;  2	    8
	sta _p_2 + 1	;  4	   12
	lda #$b1	;  2	   14
	sta _p_1 + 1	;  4	   18
	nop		;  2	   20
	nop		;  2	   22
start_nmi
	bit $ea		;3 3	25 25 
.endp
	lda #$11
	sta $dd0e
	lda #$81
	sta $dd0d
	lda $dd0d
;-------------------------------------------------------------------------
; Set raster-row for ending the NMI here
;-------------------------------------------------------------------------
	lda #$f8
	sta $d012
;-------------------------------------------------------------------------
; Restore all registers
;-------------------------------------------------------------------------
	plp
	pla
	ldx _rx
	ldy _ry
;-------------------------------------------------------------------------
; Return from irq here
;-------------------------------------------------------------------------
	asl $d019
	rti
;-------------------------------------------------------------------------
; Stop the NMI-irq, play music and stuff
;-------------------------------------------------------------------------
StopNMI
	lda _p_2 + 1	; Restore $d018 for the first row
	sta $d018
	lda #RasterRow
	sta $d012
	lda #$00
	sta $dd0e
;-------------------------------------------------------------------------
; Restore all registers
;-------------------------------------------------------------------------
	plp
	pla
	ldx _rx
	ldy _ry
;-------------------------------------------------------------------------
; Return from irq here
;-------------------------------------------------------------------------
	asl $d019
	rti
nmiirq
;-------------------------------------------------------------------------
; Stabilize the raster
;-------------------------------------------------------------------------
	sta reg_a + 1
	lda $dd04
	eor #$e7
	sta time_6 + 1
.page
time_6	bpl *
	cmp #$c9
	cmp #$c9
	cmp #$24
	nop
.endp
;-------------------------------------------------------------------------
; Stable raster here, create badline for fli and restore to next badline
;-------------------------------------------------------------------------
_p_1	lda #$91
	sta $d018
	lda #$3f
	sta $d011
_p_2	lda #$81
	sta $d018
	lda #$3b
	sta $d011
;-------------------------------------------------------------------------
; Trigger new NMI, restore A and return from IRQ here
;-------------------------------------------------------------------------
	lda $dd0d
reg_a	lda #0
	rti

;	.byte $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9
;	.byte $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9
;	.byte $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9
;	.byte $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9
;	.byte $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9
;	.byte $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9, $c9
;	bit $ea
</code>

=== Pitfalls ===

by Bitbreaker/Oxyron* (the code example is not mine, it was here before, i just added some rant)

FIXME
isn't it a bad idea to do the following, as the asl clobbers the carry:
<code>
plp
pla
asl $d019
rti
</code>
So better pull P and A after asl $d019. However the processor status is anyway pushed to the stack on an interrupt and restored by the rti.

Also the NMIIRQ does not work with both CIA types, one might use two different NMI-handlers:
In case of using a new CIA you better waste an additional cycle by:

<code>
sta $06
nop
lda $dd04
eor #$e7
...
lda $06
rti
</code>