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▀ Because this function was not defined under MS-DOS 1.x, it should al-
  ways 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 (Termin-
  ate Process with Return Code), Int 21H Function 40H (Write to File or
  Device), and the standard error handle 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.
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