Function 33h (51)        Get or Set Ctrl-Break Status

    Reports the status of or sets extended Ctrl-Break checking.

       On entry:      AH         33h
                      AL         00h      to get BREAK state
                      ------------------------------------
                      AL         01h      to set BREAK state
                                 05       get boot drive (DOS 4+)
                      DL         00h      to set BREAK OFF
                                 01h      to set BREAK ON

       Returns:       DL         00h      BREAK is OFF
                                 01h      BREAK is ON

        [Note: Subfunction 5 returns boot drive (A = 1) in DL.]

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

    With DOS versions 2.00 and later, Ctrl-Break checking can be extended
    to take place any time input or output is requested via DOS. (DOS
    normally acts on Ctrl-Break only under a limited set of
    circumstances--during certain keyboard and screen functions.) Use
    Function 33h to toggle extended Ctrl-Break monitoring on or off.
    You can also use this function to find the boot disk under DOS 4+. Use
    Subfunction 5. The boot drive (A=1, C=3, etc.) is returned in DL.

       Note:          The default for Ctrl-Break monitoring is off.
                      The state of the control break flag can be set at
                      the DOS prompt with the "break on" command, so it is
                      a nice idea to save the state of the break flag and
                      restore it before returning to DOS.