33 byte X

gistfile1.txt

ov = $90 ; == $40, initial value for the overflow counter
ct = $D5 ; == $27 / 39, number of passes. Decrementing, finished at -1
sp = $D1 ; == $07C0, pointer to the bottom line (only set after scolling)

* = $02E5

scroll: jsr $E8EA
loop:   ldy ct
        lda #$A0
        sta $D020, y
p1:     sta $07C0
        sta (sp), y
        inc p1 + 1
        adc ov
        sta ov
        dec ct
        bmi *
        bcc loop
        jmp scroll