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Horizontal Retrace
From Higher Intellect Vintage Wiki
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.