[AT]
Expects: AH    81H
BX    device ID
CX    process ID
CF    NC (0) (clear the Carry Flag before when invoking this fn)
                                                                  
Returns: AX    0081H
                                                                  
Info: This service is designed to be intercepted by a multitasking
operating system.  It would be used, along with INT 15H fns 80H
and 82H to avoid device conflicts.

When unintercepted, the BIOS simply sets AH to 0 and clears CF.

See Also: INT 15H (extended AT services)
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INT 15H 81H: Device Close