ErrorMode Flag

  If you have control when a critical disk error occurs you must not call
  any system functions (including INT 21H and other DOS services).

  This situation can arise if your program hooks INT 24H (Critical Error
  handler) or if your TSR gets control via a timer interrupt such as
  INT 08H or INT 1cH while a critical disk error is taking place.

  Note that a similar caution is described about the InDOS Flag, but when
  that flag is set, it is still OK to use DOS fns 01H-0cH.  The ErrorMode
  Flag indicates a more dire situation, with DOS being in a very unstable
  state.  When it is set, you must avoid all DOS fns.

█▌ErrorMode Detection▐█
  Your program can check the state of the ErrorMode Flag as follows:

    ■ Early on (before you need it!) use DOS fn 34H (get InDOS addr).  The
      address of the ErrorMode Flag is one byte before the address returned
      by fn 34H.  Save it.

    ■ Before popping up, check the byte at the ErrorMode Flag address.
      A non-zero value means beware!.  When ErrorMode is non-zero, DOS is
      processing a critical disk error and you must NOT use any DOS

█▌INT 28H: Toggling ErrorMode▐█
  It is common for a pop-up TSR to take control by hooking INT 28H (DOS Idle

  When INT 28H is called, it is OK to use most DOS services (including disk
  I/O and all higher-level fns.  But you must avoid fns 01H-0cH.  If you
  simply must use fns 01H-0cH, the DOS tech ref suggests that you first set
  ErrorMode to 01H.

  In general, it is pretty easy to avoid fns 01H-0cH by accessing CON or
  PRN, etc. via Handle-Oriented File I/O or by dropping down to BIOS-level
  services such as INT 16H (keyboard services).

See Also: DOS fn 34H (get InDOS and ErrorMode address)
          DOS fns 01H-0CH (low-level character I/O)
          InDOS Flag
          INT 24H (Critical Error Handler)
          Process Control Functions
          TSR Functions