op_format(
    oparg_T	*oap,
    int		keep_cursor)		// keep cursor on same text char
{
    long	old_line_count = curbuf->b_ml.ml_line_count;

    // Place the cursor where the "gq" or "gw" command was given, so that "u"
    // can put it back there.
    curwin->w_cursor = oap->cursor_start;

    if (u_save((linenr_T)(oap->start.lnum - 1),
				       (linenr_T)(oap->end.lnum + 1)) == FAIL)
	return;
    curwin->w_cursor = oap->start;

    if (oap->is_VIsual)
	// When there is no change: need to remove the Visual selection
	redraw_curbuf_later(INVERTED);

    if ((cmdmod.cmod_flags & CMOD_LOCKMARKS) == 0)
	// Set '[ mark at the start of the formatted area
	curbuf->b_op_start = oap->start;

    // For "gw" remember the cursor position and put it back below (adjusted
    // for joined and split lines).
    if (keep_cursor)
	saved_cursor = oap->cursor_start;

    format_lines(oap->line_count, keep_cursor);

    // Leave the cursor at the first non-blank of the last formatted line.
    // If the cursor was moved one line back (e.g. with "Q}") go to the next
    // line, so "." will do the next lines.
    if (oap->end_adjusted && curwin->w_cursor.lnum < curbuf->b_ml.ml_line_count)
	++curwin->w_cursor.lnum;
    beginline(BL_WHITE | BL_FIX);
    old_line_count = curbuf->b_ml.ml_line_count - old_line_count;
    msgmore(old_line_count);

    if ((cmdmod.cmod_flags & CMOD_LOCKMARKS) == 0)
	// put '] mark on the end of the formatted area
	curbuf->b_op_end = curwin->w_cursor;

    if (keep_cursor)
    {
	curwin->w_cursor = saved_cursor;
	saved_cursor.lnum = 0;

	// formatting may have made the cursor position invalid
	check_cursor();
    }

    if (oap->is_VIsual)
    {
	win_T	*wp;

	FOR_ALL_WINDOWS(wp)
	{
	    if (wp->w_old_cursor_lnum != 0)
	    {
		// When lines have been inserted or deleted, adjust the end of
		// the Visual area to be redrawn.
		if (wp->w_old_cursor_lnum > wp->w_old_visual_lnum)
		    wp->w_old_cursor_lnum += old_line_count;
		else
		    wp->w_old_visual_lnum += old_line_count;
	    }
	}
    }
}
The software reads data past the end, or before the beginning, of the intended buffer.