INT 21 - DOS 2+ - GET ADDRESS OF INDOS FLAG
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.
the critical error flag is the byte immediately following InDOS in
DOS 2.x, and the byte BEFORE the InDOS flag in DOS 3+ (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.
SeeAlso: AX=5D0Bh,INT 28

See also: 215D06

2134 - DOS 2+ - GET ADDRESS OF INDOS FLAG