[AT]
Expects: AH    85H
AL    00H = key make (pressed down)
01H = key break (released)
CF    NC (0)
                                                                  
Returns: AX    0085H
CF    NC (0) if no error; CY (1) if invalid value in AL on entry
                                                                  
Info: Called by INT 01H upon press and release of the SysReq key.

A multitasking operating system might intercept this fn in order
to monitor user-requests to switch tasks.

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 85H: SysReq Key