====== Stable IRQ with DMA ======

ChristopherJam's stable IRQ routine using DMA, posted on http://noname.c64.org/csdb/forums/?roomid=11&topicid=88971 (cleaned up and converted to ca65 syntax).

<code asm>
	.segment "STARTUP"

	.word basicstub		; load address

basicstub:
	.word @nextline
	.word 1970 + (.time / 31557600)
	.byte $9e
	.byte <(((init / 1000) .mod 10) + $30)
	.byte <(((init / 100 ) .mod 10) + $30)
	.byte <(((init / 10  ) .mod 10) + $30)
	.byte <(((init       ) .mod 10) + $30)
	.byte 0
@nextline:
	.word 0

init:
	lda #$04
	sta $d021
	sta 646
	lda #147
	jsr $ffd2
	lda #0
	sta $d021
	lda #1		; something to see
	sta $0400
	lda #2
	sta $0428

	lda #$7f	; kill CIA irq
	sta $dc0d
	
	sei
	
	lda #$35	; disable ROM
	sta $01
	
	lda #$01	; enable VIC irq
	sta $d01a
	lda #$1b	; clear high bit of irq rasterline
	sta $d011
	lda #$32	; last invisible line
	sta $d012
	
	lda #<irq1	; set irq vector
	sta $fffe
	lda #>irq1
	sta $ffff
	
	cli
	
loop:
	inc $07e7	; a nice untidy main loop to ensure instability!
	bpl loop
	jmp loop

irq1:
	pha
	lda $d019
	sta $d019
	txa
	pha
	tya
	pha
	
	lda #$0f   ; this is unstable!
	sta $d020
	lda #$1a   ; force partial badline before screen starts
	sta $d011
	lda #$0b   ; this is stable ^_^
	sta $d020
	lda #$1b   ; trigger normal re-fetch of first row of chars
	sta $d011
	lda #$ff   ; set end-of-screen irq
	sta $d012
	
	lda #<irq2
	sta $fffe
	lda #>irq2
	sta $ffff
	
	pla
	tay
	pla
	tax
	pla
	rti

irq2:
	pha
	lda $d019
	sta $d019
	txa
	pha
	tya
	pha
	
	;ensure first row of chars is already being displayed when badline forced by irq1
	lda #$18
	sta $d011
	lda #$32
	sta $d012
	
	lda #<irq1
	sta $fffe
	lda #>irq1
	sta $ffff
	
	pla
	tay
	pla
	tax
	pla
	rti
</code>