.file string
.file (which may also be spelled `.app-file') tells
as that we are about to start a new logical file.
string is the new file name. In general, the filename is
recognized whether or not it is surrounded by quotes `"'; but if
you wish to specify an empty file name, you must give the
quotes--"". This statement may go away in future: it is only
recognized to be compatible with old as programs.
In some configurations of as, .file has already been
removed to avoid conflicts with other assemblers. See section Machine Dependent Features.