INT 10 - VIDEO - READ LIGHT PEN POSITION (except VGA)
AH = 04h
Return: AH = light pen trigger flag
00h not down/triggered
01h down/triggered
DH,DL = row,column of character light pen is on
CH = pixel row (graphics modes 04h-06h)
CX = pixel row (graphics modes with >200 rows)
BX = pixel column
Notes:  on a CGA, returned column numbers are always multiples of 2 (320-
column modes) or 4 (640-column modes)
returned row numbers are only accurate to two lines


1004 - VIDEO - READ LIGHT PEN POSITION (except VGA)