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  Service 05H (decimal 5) selects the active display page for text modes 0
  through 3 and also for 16-color EGA and VGA graphics modes. You specify
  the page number in register AL. (See Figure 9-6.) In text modes, page
  numbers range from 0 through 7. Don't forget, however, that the CGA
  hardware can display only four different 80-column pages, so CGA pages 4
  through 7 overlap pages 0 through 3 when you're in 80 x 25 text mode. On
  the EGA and in the PS/2 video subsystems, you can also select among
  multiple display pages in 16-color graphics modes.

  Service Number           Parameters
  --------------------------------------------------------------------------
  AH = 05H                 AL = new display page number
  --------------------------------------------------------------------------

  Figure 9-6.  The registers used to set the active display page using
  service 05H.

  In all video modes, page 0 is used by default. Page 0 is located at the
  beginning of display memory, with higher page numbers in higher memory
  locations. See page 87 for more on display pages.