For those options that require a file name, you may specify a file name with a full path name, or you may specify just a directory path; in this case, the default file name is used with that directory. The default file name is the name of the first object file specified with any extension removed.
A default extension is always placed on a file unless one is explicitly specified.
If an output file name consists only of a directory, it should be terminated by a "\". If not, it is treated as a file name. Thus, /M=txt\ outputs the map to file txt\cat.map (assuming cat.obj is the input file). /M=txt outputs the map to txt.map.