OLD_NTSC = 1
NTSC = 0
PAL = 2
DREAN = 3

        !to "detect.prg",cbm
	!cpu 6510

	* = $0801
	!by $0b,$08,$00,$00,$9e,$32,$30,$36,$31,$00,$00,$00
		
detect:	sei
w0:  	lda $d012
w1:  	cmp $d012
	beq w1
	bmi w0
	and #$03
	cmp #$01
	bne w2
	lda #OLD_NTSC
	bne detected ; unconditional
w2:	cmp #$03
	lda #$00
	rol
	beq detected	;ntsc

	;check for drean or pal
	ldx #$00
	lda #$10
l	inx
	cmp $d012
	bne l
	lda #PAL
	cpx #$70
	bcc detected
	;is a drean
	lda #DREAN
detected:
	tay
	lax offset,y
l2:	lda base_text,x
	beq end
	jsr $ffd2
	inx
	bne l2
end:	lda #$0d
	jmp $ffd2

base_text:
old_ntsc_text:	!text "OLD "
ntsc_text:	!text "NTSC",0
pal_text:	!text "PAL",0
drean_text:	!text "DREAN",0

offset:		
	!byte ntsc_text-base_text
	!byte old_ntsc_text-base_text
	!byte pal_text-base_text
	!byte drean_text-base_text

    sei              // Set Interrupt Flag (MPU Ignore IRQ line)
    cld              // No stupid tricks ;-)
    lda #$7f
    sta $dc0d        // No more IRQ Interrupts from CIA #1
    lda #$00
    sta $d01a        // No more IRQ Interrupts from VICII
    lda $dc0d        // Ack any latent CIA #1 Interrupts
    lda #$01
    sta $d019        // Ack any latent VICII Interrupts
    lda #$35
    sta $01          // Switch off the MPUs access to BASIC and KERNAL ROM and access the RAM instead (dec/inc $01 allows access to IO RAM).

    // Ignore NMIs
    lda #<nmivector
    sta $fffa
    lda #>nmivector
    sta $fffb        // Set NMI HW Vector to point to RTI
    jmp *+4
nmivector:
    rti
    lda #$00
    sta $dd0e  // Stop CIA #2 Timer A
    sta $dd04
    sta $dd05  // Set Timer A = 0 (NMI will trigger immediately when Timer A is restarted)
    lda #$81
    sta $dd0d  // Set Timer A as NMI source
    lda #$01
    sta $dd0e  // Start Timer A (triggers NMI and unless the NMI get's acked, any new NMI's will be ignored).

    // From this point on we will not be disturbed and can commence testing.

    lda #$00
    sta $d015  // No Sprites
    lda #$00
    sta $d011  // Turn off screen to avoid badlines


    // Detect the model
    //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    // 312 rasterlines -> 63 cycles per line PAL        => 312 * 63 = 19656 Cycles / VSYNC  => #>76  %00
    // 262 rasterlines -> 64 cycles per line NTSC V1    => 262 * 64 = 16768 Cycles / VSYNC  => #>65  %01
    // 263 rasterlines -> 65 cycles per line NTSC V2    => 263 * 65 = 17095 Cycles / VSYNC  => #>66  %10
    // 312 rasterlines -> 65 cycles per line PAL DREAN  => 312 * 65 = 20280 Cycles / VSYNC  => #>79  %11

    // Use CIA #1 Timer A to count cycled in a frame
    lda #$ff
    sta $dc06
    sta $dc07  // Latch #$ffff to Timer B

    lda $d011
    bmi *-3
    lda $d012
    bne *-3    // VSYNC

    lda #%00011001
    sta $dc0f  // Start Timer B (One shot mode (Timer stops automatically when underflow))

    lda $d011
    bpl *-3    // Wait untill we cross the 256 rasterline

    lda $d011
    bmi *-3
    lda $d012
    bne *-3    // VSYNC again

    lda #$ff
    sec
    sbc $dc07  // Hibyte number of cycles used
    and #%00000011

    sta $0400

    // Turn back on screen to see result (#$00, #$01, #$02 or #$03)
    lda #$1b
    sta $d011

    jmp *