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     Because this function was not defined under MS-DOS 1.x, it should
     always be called with AL = 00. In an MS-DOS 1.x environment, AL
     will be returned unchanged.
 
     Care must be taken not to exit in an unacceptable fashion if an
     MS-DOS 1.x environment is detected. For example, Int 21h Function
     4Ch (Terminate Process with Return Code), Int 21h Function 40h
     (Write to File or Device), and the standard error handler are not
     available in MS-DOS 1.x. In such cases a program should display an
     error message using Int 21h Function 09h and then terminate with
     Int 20h or Int 21h Function 00h.
                                    -♦-