Function 04h    Read Light Pen - fetch light pen information 

entry   AH      04h 

return  AH      00h     light pen not triggered 

	AH      01h     light pen is triggered, values in resgisters 

	BX      pixel column               (X=0-319 or 0-639)  graphics mode 

	CH      raster line                (Y=0-199)        old graphics modes 

	CX      (EGA) raster line (0-nnn)                   new graphics modes 

	DH      row of current position    (Y=0-24)            text mode 

	DL      column of current position (X=0-79 or 0-39)    text mode 

note    Not supported on PS/2 

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