Obtain the MS-DOS version number with Int 21H Function 30H during your program's initialization and be sure that all of the functions your program requires are actually available. If you find that the host MS-DOS version is inadequate, be careful about which functions you call to display an error message and to terminate.
Use the enhanced capabilities of MS-DOS versions 3 and 4 when your program is running under those versions. For example, you can specify a sharing mode when opening a file with Int 21H Function 3DH, you can create temporary or unique files with Int 21H Functions 5AH and 5BH, and you can obtain extended error information (including a recommended recovery strategy) with Int 21H Function 59H. Section 2 of this book contains version-dependency information for each MS-DOS function.