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  Service 0AH (decimal 10) is the same as service 09H (write character and
  attribute to cursor location) with one exception: Service 09H lets you
  change the existing screen color attribute in text mode but service 0AH
  does not.

  However, in graphics mode you must still specify a color in BL (see
  Figure 9-10), which makes the description of this service as only a
  character-writing service partly incorrect. Service 0AH has the same
  graphics color rules as services 09H and 0CH: The color can be used
  directly or used with XOR and the existing color. (See service 09H for an
  explanation.)

  See page 82 for more on display attributes in text modes. See page 84
  for more on color attributes in graphics modes.

  Service Number           Parameters
  --------------------------------------------------------------------------
  AH = 0AH                 AL = ASCII character to write to screen
                           BL = foreground color (graphics modes only)
                           BH = background color (video mode 13H only) or
                           display page number (all other modes)
                           CX = number of times to write character
  --------------------------------------------------------------------------

  Figure 9-10.  The registers used to write a character using service 0AH.