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        Read light pen position (all but PS)

        Entry:
          AH = 04h
        Return:
          AH = 0: Light pen switch not activated
          AH = 1: Light pen values in registers
                    DH = row of current position
                    DL = column of current position
                    CH = raster line (0-199) (EGA) old graphics modes
                    CX = (EGA) raster line (0-nnn) new graphics modes
                    BX = pixel column (0-319 or 0-639)
          SI,DI,BP destroyed by call