Function 34h            Get InDOS Flag Address

    Returns the address of the InDOS flag.

        Entry   AH = 34h

        Return  ES:BX = Address of InDOS flag

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    While DOS is processing an interrupt 21h function, the value of
    the InDOS flag is non-zero.

    With DOS 5.0, function 34h has been documented for DOS version
    2.0 and later. The DOS 5.0 Programmer's Reference also documents
    that the address of the ErrorMode flag can be determined by
    subtracting 1 (one) from the InDOS flag address.