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INT 10h,  04h (4)        Read Light-Pen Position

    Reports the position and status of the light pen.

       On entry:      AH         04h

       Returns:       AH         Status (0=not triggered, 1=triggered)
                      BX         Pixel column number (0-639)
                      CH         Pixel line number (0-199)
                      CX         Pixel line number for modes 0Fh-10h (0-nnn)
                      DH         Character row number (0-24)
                      DL         Character column number (0-39 or 0-79)

       Registers destroyed:      AX, SP, BP, SI, DI

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       Notes:         The position is reported in two forms: the character
                      position (row in DH, column in DL), and the pixel
                      location (raster line in CH, column/dot in BX).

                      For the PC Convertible, if AH is 0 upon return, the
                      light pen is not supported.

                      The pixel coordinates for the light pen position are
                      not precise. The y coordinate is always a multiple
                      of 2, while the x coordinate is a multiple of 4 for
                      the 320 x 200 graphics mode and a multiple of 8 for
                      the 640 x 200 graphics mode.