// 異步比較器
module async_cmp (aempty_n, afull_n, wptr, rptr, wrst_n);
	parameter ADDR_WIDTH = 4;		
	parameter N = ADDR_WIDTH-1;
	output aempty_n, afull_n;
	input [N:0] wptr, rptr;
	input wrst_n;
	reg direction;
	wire high = 1'b1;
	wire dirset_n = ~( (wptr[N]^rptr[N-1]) & ~(wptr[N-1]^rptr[N]));
	wire dirclr_n = ~((~(wptr[N]^rptr[N-1])&(wptr[N-1]^rptr[N]))|~wrst_n);
	
	always @(posedge high or negedge dirset_n or negedge dirclr_n)
		if (!dirclr_n) direction <= 1'b0;
		else if (!dirset_n) direction <= 1'b1;
		else direction <= high;

	assign aempty_n = ~((wptr == rptr) && !direction);	// "接近空"標志
	assign afull_n = ~((wptr == rptr) && direction);	// "接近滿"標志
endmodule