as.info: File

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`.file STRING'

   `.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.  *Note
Machine Dependencies::.