Без особых выкрутасов
Код:
;a- lines to clean
clean_lines
  ld hl,#4000
  ld c,l
clcyc
  ex af,af'
  ld a,l
  ld b,#20
cl_line
  ld (hl),c
  inc l
  djnz cl_line
  ld l,a
  inc h
  ld a,h
  and 7
  jr nz,nxl
  ld a,l
  add a,32
  ld l,a
  jr c,nxl
  ld a,h
  sub 8
  ld h,a
nxl
  ex af,af
  dec a
  jr nz,clcyc