Int 21 Fn 34  - DOS 2+ - Get Address Of Indos Flag                         [D]

   AH = 34h

Return: ES:BX -> one-byte InDOS flag

Notes: the value of InDOS is incremented whenever an INT 21 function begins
     and decremented whenever one completes
   during an INT 28 call, it is safe to call some INT 21 functions even
     though InDOS may be 01h instead of zero
   InDOS alone is not sufficient for determining when it is safe to
     enter DOS, as the critical error handling decrements InDOS and
     increments the critical error flag for the duration of the critical
     error.  Thus, it is possible for InDOS to be zero even if DOS is
     busy.
   SMARTDRV 4.0 sets the InDOS flag while flushing its buffers to disk,
     then zeros it on completion
   the critical error flag is the byte immediately following InDOS in
     DOS 2.x, and the byte BEFORE the InDOS flag in DOS 3+ and
     DR-DOS 3.41+ (except COMPAQ DOS 3.0, where the critical error flag
     is located 1AAh bytes BEFORE the critical section flag)
   for DOS 3.1+, an undocumented call exists to get the address of the
     critical error flag (see AX=5D06h)
   this function was undocumented prior to the release of DOS 5.0.

See Also: AX=5D06h,AX=5D0Bh,INT 15/AX=DE1Fh,INT 28