INT 10 - VIDIO - READ LIGHT PEN POSITION
        AH = 4
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)
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