Raster line black and white

; asm 6502/6510 - C64
FIRST   = 58

    *=$C000

        sei                   ; disabling interrupts

        ldy #$7f              ; $7f = %01111111
        sty $dc0d             ; disabling CIA's interrupts
        sty $dd0d             ; settings $DC0D and $DD0D
        lda $dc0d             ; ... so wipe the queue
        lda $dd0d             ; of unprocessed interrupts

        lda #<RasterIrq       ; Interrupts now points to myroutine
        ldx #>RasterIrq       ;

        sta $314              ; Lo-Byte
        stx $315              ; Hi-Byte


        lda newRow            ; Set line of raster that triggers interrupt
        sta $d012             ; placing it in $d012

        lda $d011             ; bit 7 of D011 + gli i bit di $D012 identifies
        and #$7f              ; the whole raster line, infact raster line number
        sta $d011             ; could be > 255, so this bit must be set in
                              ; right way according to purpose
                              ; this time must be reset


        lda #$01              ; Interrupt mask setting
        sta $d01a             ; for raster interrupt manage

        cli                   ; re-enabling interrupts

        rts


BufPtr  byte $00
newRow  byte FIRST
color   byte $00


RasterIrq

        lda color
        and #$01            ; test if odd or even
        bne RI_Odd
        lda #$01            ; White for Even
        jmp RI_IncColoc
RI_Odd
        lda #$00            ; Black for Odd
RI_IncColoc
        inc color

        sta $d021           ; Set Background
                            ; Irq pointers doesn't change
        lda newRow          ; Load Newrow byte
        clc
        adc #$08            ; add 8
        cmp #$FA            ;
        bcc RI_SetD012      ; if not beyond last line it'OK
        lda #FIRST          ; else set next interrupt to first text line
RI_SetD012
        sta newRow
        sta $d012           ; set next interrupt raster line
        asl $d019           ; "Acknowledge" the interrupt by clearing the VIC's interrupt flag.

        jmp $ea31           ; return to basic 8)





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