They're raster lines. At some point in the code, or multiple points, the program increments the register holding the border color so the video chip starts drawi

They're raster lines. At some point in the code, or multiple points, the program increments the register holding the border color so the video chip starts drawing a different color. This can happen in the middle of a line, which is why you get multiple "columns". Edit: vardump is right, the screen would be "turned off" in this case so just the border color is used. This gives a slight performance boost.