c64.asm

!filescope c64
!macro basic_start(addr) {
* = $801
    !byte $0c, $08, $00, $00, $9e
    !for d in [10000, 1000, 100, 10, 1] {
        !if (addr >= d) {
            !byte $30 + (addr/d)%10
        }
    }
    !byte 0, 0, 0
}

main.asm

!include "c64.asm"

+c64::basic_start(entry)

!let yf = 249
!let x0 = $d5 ; initial value == 39
!let x1 = $62 ; 253 guessing this is zero

entry: {
        ; assume A=0 on entry
        sta $d020
        sta $d021

        lda #80
        sta yf

        ; Assume 'xx' contains 39 here
xloop:
        lda #$a0
        ldx x0
        sta $0400 + 24*40, x
        ldx x1
        sta $0400 + 24*40, x

        inc x1
        dec x0
        bmi xloop_done

        ; A=#$a0 from above
        adc yf
        sta yf
        bcc no_scroll

        jsr $e8ea ; scroll screen up
no_scroll:
        jmp xloop
xloop_done:

inf: 	bne inf   ; BNE on byte shorter than JMP, Z flag constant here
}