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
triggered.

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

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INT 10H 04H: Query Light Pen