怎么在vim里设置一行有80个字符呢

'textwidth' 'tw'        number  (default 0)
                        local to buffer
                        {not in Vi}
        Maximum width of text that is being inserted.  A longer line will be
        broken after white space to get this width.  A zero value disables
        this.  'textwidth' is set to 0 when the 'paste' option is set.  When
        'textwidth' is zero, 'wrapmargin' may be used.  See also
        'formatoptions' and |ins-textwidth|.
        When 'formatexpr' is set it will be used to break the line.
        NOTE: This option is set to 0 when 'compatible' is set.