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  Service 0DH (decimal 13) is the reverse of service 0CH: It reads a pixel's
  color value rather than writing it. A pixel has only a single color
  attribute, which is returned through service 0DH. (The read-character
  service 08H returns both a color and an ASCII character code.) The row is
  specified in DX, the column in CX, and the display page in BH. The pixel
  color value is returned in AL. (See Figure 9-13.) All high-order bits of
  the value returned in AL are set to 0, as you would expect.

  Service Number Parameters                   Returns
  --------------------------------------------------------------------------
  AH = 0DH       BH = display page number     AL = pixel color value
                 DX = row number of pixel
                 CX = column number of pixel
  --------------------------------------------------------------------------

  Figure 9-13.  The registers used to read a pixel using service 0DH.