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INT 15h,  85h (133)      System Request Pressed                          many

    Provides an interface for special processing of the Sys Req key.

       On entry:      AH         85h
                      AL         Subservice
                                 00h - System Request key Make (pressed)
                                 01h - System Request key Break (released)

       Returns:       Carry      Set if service not supported, else cleared

  --------------------------------------------------------------------------

    By default, this service does nothing but clear the Carry Flag. It is
    up to the operating system or user to provide a handler to process the
    Sys Req key. This service is called by INT 09h (Keyboard) after the
    Sys Req key has been pressed ("made") or released ("broken").

       Notes:         The PC, PCjr, and the XT dated 11/08/82 do not
                      support this service.

                      Currently there is no standard use for the System
                      Request key. Future operating systems will probably
                      use it to switch between tasks.