ld bc, $20BF ld hl, $4020 ; Set VRAM address to $0020 out (c), l out (c), h dec c ld a, $C3 ; New bitplane #3 value loop: in f, (c) ; Skip bitplane #0 in f, (c) ; Skip bitplane #1 in f, (c) ; Skip bitplane #2 out (c), a ; Update bitplane #3 djnz loop