INT 10H 04H: Query Light Pen

                                                          Compatibility: All
 Expects: AH    04H
 Returns: AH    0=not triggered (ignore other registers)
                1=is triggered (other regs have valid info)
          DH,DL text mode row,column at which pen was triggered
          CX,BX graphics mode row,column (in pixels)
                (on oldest BIOSes, row returned in CL and CH not used)
    Info: Checks to see if the light pen has been triggered and returns the
          screen location at which the pen was pointing when it was

   Notes: Old CGA cards always included a light pen connector, but later
          EGA/VGA cards often do not.

See Also: INT 10H: Video Services