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  Interrupt:   21h     Function:   33h
 
  Title:       Get or Set Break Flag
 
  Description:
 
     Obtains or changes the status of the operating system's Break
     flag, which influences CTRL+C checking during function calls. Also
     returns the system boot drive in version 4.0.
 
     Input                                      Output
 
     If getting Break flag                      DL = 00h (Break flag
       AH = 33h                                           is OFF)
       AL = 00h                                    = 01h (Break flag
                                                          is ON)
     If setting Break flag
       AH = 33h
       AL = 01h
       DL = 00h (if turning Break flag OFF)
          = 01h (if turning Break flag ON)
 
     Notes:
          AL will be FFh on exit if an invalid function number was
             entered in AL on entry.
          To get the boot drive, put 5 in AL on entry. The boot drive
             number will be in DL on exit (drive A = 1, drive B = 2)
                                    -♦-