MOVEQ #0,D0
  MOVEQ #0,D1
  MOVE.W #320,D0 ; width
  ASR.W #4,D0 ; in words
  MOVE.W #256,D1 ; height
  ASL.W #6,D1 ; shifted 6 bits
  ADD.W D1,D0
  MOVE.W D0,BLTSIZE ; start blit