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  Service 0FH (decimal 15) returns the current video mode and two other
  useful pieces of information: the screen width in characters (80 or 40)
  and the display page number.

  The video mode number, as explained under service 00H, is returned in AL.
  The screen width is returned in AH as a number of characters per line. The
  display page number will be returned in BH. (See Figure 9-15.)

  Service Number           Returns
  --------------------------------------------------------------------------
  AH = 0FH                 AL = current display mode
                           AH = number of characters per line
                           BH = active display page
  --------------------------------------------------------------------------

  Figure 9-15.  Information returned by service 0FH.

  See page 72 for more on video modes. See page 58, memory location
  0040:0049H, for more on how a record of the mode is kept.