c64 scrolling hello world. To be assembled using `dasm`. https://youtu.be/l_xk4MDTjOo

scrolling_helloworld.asm

        processor 6502          ; define processor family for dasm
        org $C000               ; memory location for our code

CR = $0d
clear = $e544                   ; clear screen
screenbeg = $0400               ; beginning of screen memory
screenend = $07e7               ; end of screen memory
screenpos = $8000               ; current position in screen
screenwidth = #$20

main:
        jsr initscreen
        jsr loop
        rts

loop:
        jsr clear
        jsr drawmsg

        inc screenpos           ; increment current screen position
        jmp loop                ; loop
        rts

initscreen:
        lda #$00
        sta $d020               ; border
        sta $d021               ; background
        sta screenpos           ; screen position
        rts

return:
        rts

msg:
        .byte "HELLO WORLD!"
        .byte 0
        rts

drawmsg:
        ldx #$00                  ; regX keeps msg index
        ldy screenpos             ; regY keeps screen position

        cpy #$20
        bcs screenposgreatherthan

        jmp drawmsghelper

drawmsghelper:
        jsr drawmsgloop
        rts

drawmsgloop:
        lda msg,x               ; char at msg index
        beq return              ; exit when zero byte

        and #$3f                ; ascii to petscii
        sta screenbeg,y         ; modify screen memory
        inx
        iny

        cpy #$20
        bcs ycountergreatherthan

        jmp drawmsgloop

screenposgreatherthan:
        ldy #$00
        sty screenpos

        jmp drawmsghelper

ycountergreatherthan:
        tya
        sbc #$20
        tay

        jmp drawmsgloop