Int 2F Fn 0500  U - DOS 3+ Critical Error Handler - Installation Check     [l]

   AX = 0500h

Return: AL = 00h not installed, OK to install
        01h not installed, can't install
        FFh installed
Desc:  determine whether a critical error message override is installed

Note:  this set of functions allows a user program to partially or completely
     override the default critical error handler's message in COMMAND.COM

See Also: AH=05h,INT 24