Stable raster without I/O

gistfile1.asm

.pc = $0900
main:
    sei
    ldx #1
    txs

   outer:
    ldx #24
   badline:
    cmp ($00,x)
    cmp ($00,x)
    cmp ($00,x)
    clc
   rmw:
    dec $0420
    ldy #85
   !:
    dey
    bpl !-
    dex
    bpl badline

    ldy #312-25*8

    tsx
    bne inc_s
    lda #$d0
    sta rmw+2,x
    bne skip_14
  inc_s:
    pla
    bcc skip_12

   waste:
    cmp ($00,x)
    cmp ($00,x)
   skip_12:
    nop
   skip_14:
    cmp ($00,x)
    cmp ($00,x)
    cmp ($00,x)
    cmp ($00,x)
    cmp ($00,x)
    cmp ($00,x)
    cmp ($00,x)
    dey
    beq outer
    bne waste