Changes the default directory for a mass memory file system. The <directory_name>
parameter is a string. If no parameter is specified, the directory is set to the *RST
value. At *RST, this value is set to the default user data storage area, that is defined
as System.Environment.SpecialFolder.Personal
e.g. C:\Users\Name\Documents
MMEMory:CDIRectory? — Query returns full path of the default directory.