Bugs and Errata
The higher bandwidth used by 80-column text mode results in random short horizontal lines appearing onscreen (known as "snow") if a program writes directly to video memory, as the CPU has priority when accessing it. This can be avoided by only accessing the memory during the period of vertical or horizontal retrace. The "snow" problem does not occur on any other video adapter, or on most CGA clones.
Composite color monitors will only display proper color in 80-column mode if the border color is set to color 6, which produces a phase identical to and serves as a substitute for the color burst that in 80-column mode is incorrectly generated by the composite signal generating circuit.
The video controller 6845's row counter being only seven bits wide, display RAM in graphics modes is laid out in a 2:1 "interlace" pattern, first laying out the data for rows 0, 2, 4, etc., then the data for rows 1, 3, 5, etc., adding additional software overhead for display RAM manipulation. This is unrelated to the NTSC field interlace.
Read more about this topic: Color Graphics Adapter
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