Horizontal Retrace

   When a monitor has finished displaying a single scan line, it must
move it electron beam(s) back to the left edge of the CRT, during which time
it turns off the beam. On the original CGA (and some early clones), the
only time one could access the display memory without causing "snow" was
during the horizontal or vertical retrace periods, as the display adapter
was not itself accessing the display memory during those times.     See also
Vertical Retrace.