clr  Px.y
   setb Px.y
   clr  Px.y
   setb Px.y
   [...repeat as many times as needed...]
   [... throw HBLANK/HSYNC/whatever...]
   [... repeat as many times as needed for VSIZE...]
   [... throw VBLANK/VSYNC/whatever...]
   end   ;-) <---- see Russ'es recent thread on usage of smileys - is this OK?