Screen =$0400

; [BASIC LOADER] --------------------------------------------------------------

; The following bytes generate the SYS call below to execute the Assembler.
; 10 SYS (2065)  = $0811

*=$0801
        BYTE    $0E, $08, $0A, $00, $9E, $20, $28,  $32, $30, $36, $35, $29, $00, $00, $00



; [CODE START] ----------------------------------------------------------------

*=$0811
        ldx #250                        ; Clear screen
        lda #" "
@clr    sta Screen,x
        sta Screen+250,x
        sta Screen+500,x
        sta Screen+750,x
        dex
        bne @clr
        txa

        jsr initRaster                  ; Init Raster IRQ
@loop
;        jsr waitRaster
        sta Screen+120,x                ; Draw character to screen
        inx
        cpx #40
        bne @nzero
        ldx #0
        clc
        adc #1
@nzero
        jmp @loop
;;; -----------------------------------------------------------------------------
;;; install raster interrupt handler
;;; -----------------------------------------------------------------------------

initRaster
        sei                     ; turn off interrupts

        ldx #1                  ; enable raster interrupts
        stx $d01a

        lda #<intHandler       ; set raster interrupt vector
        ldx #>intHandler
        sta $fffe
        stx $ffff

        ldy #$50                ; set scanline on which to trigger interrupt
        sty $d012
        lda $d011                ; scanline hi bit
        and #%01111111
        sta $d011

        lda #$35                ; disable kernal and BASIC memory ($e000 - $ffff)
        sta $01

        asl $d019               ; acknowledge VIC interrupts
        cli
        rts

;;; -----------------------------------------------------------------------------
;;; raster IRQ handler
;;; -----------------------------------------------------------------------------

intHandler

        pha
        txa
        pha
        tya
        pha

        lda #$ff
        sta $d019

;        inc $d020
;        inc rasterCount

        pla
        tay
        pla
        tax
        pla

        rti

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

waitRaster
        pha
        lda rasterCount
@wait   cmp rasterCount
        beq @wait
        pla
        rts


rasterCount byte       00