请教异步fifo设计时的一个问题

assign    usedword=(wptr>rptr)?(wptr-rptr)full?4'hFempty?4'h016-rptr+wptr)));
always @ (posedge w_clk or posedge clear)
if(clear)
  begin
   wptr<=4'h0;
   pwptr<=4'h0;
  end
else if(!full && wr) begin
  mem[wptr]<=data_in;
  pwptr<=wptr;
  wptr<=(&wptr)?4'h0wptr+1);
end   
always @ (posedge r_clk or posedge clear)
if(clear)
  begin
   rptr<=4'h0;
   prptr<=4'h0;
   data_out<=8'h00;
  end
else if(!empty && rd) begin
  data_out<=mem[rptr];
  prptr<=rptr;
  rptr<=(&rptr)?4'h0rptr+1);
end
reg  n_full=1'b0;
assign full=(wptr==rptr)?n_full:1'b0;
assign empty=(wptr==rptr)?(~n_full):1'b0;
always @ (posedge w_clk or posedge r_clk)
if(w_clk)    n_full<=(wptr==prptr);
else          n_full<=(wptr==prptr);