Write dot

        Entry:
          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)
        Return:
          AX,SI,DI,BP destroyed by call

        Note: Video modes 4-6 only