AH = 0Ch
        AL = color of dot (0/1 in mode 6, 0-3 in modes 4 and 5)
             if bit 7 set, new color will be XORed with current pixel
        DX = row (0 - 199)
        CX = column (0 - 319 in modes 4/5, 0 - 639 in mode 6)
        Note: video modes 4-6 only
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