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  Service 1AH (decimal 26) was introduced in the ROM BIOS in the PS/2s, but
  it is also part of the ROM BIOS of the VGA. This service returns a 2-byte
  code that indicates which combination of video subsystems and video
  displays is found in your computer. The display combination codes
  recognized by this ROM BIOS service are listed in Figure 9-21. Service
  1AH lets you select either of two subservices using the value in register
  AL; subservice 00H or subservice 01H.

  Subservice 00H (decimal 0) returns a 2-byte display combination code in
  register BX. If your computer has two different video subsystems, the
  value in BL indicates which one is active; that is, which is currently
  being updated by the video ROM BIOS. The value in BH indicates the
  inactive subsystem. If your computer has only video subsystem, the value
  in BH is zero.

  Subservice 01H (decimal 1) performs the reverse function of subservice
  00H. It lets you change the current display combination code known to the
  ROM BIOS. Don't use this subservice, however, unless you know exactly what
  you're doing. It's a rare program indeed that requires you to change the
  ROM BIOS's idea of what the video hardware actually is.

  Code               Video Subsystem
  --------------------------------------------------------------------------
  00H                (No display)
  01H                MDA
  02H                CGA
  03H                (Reserved)
  04H                EGA with color display
  05H                EGA with monochrome display
  06H                Professional Graphics Controller
  07H                VGA with monochrome display
  08H                VGA with color display
  09H,0AH            (Reserved)
  0BH                MCGA with monochrome display
  0CH                MCGA with color display
  0FFH               (Unknown)
  --------------------------------------------------------------------------

  Figure 9-21.  Display combination codes returned by video BIOS service
  1AH.