yosys/docs/source/code_examples/fifo/fifo.v

// address generator/counter
module addr_gen 
#(  parameter MAX_DATA=256,
	localparam AWIDTH = $clog2(MAX_DATA)
) ( input en, clk, rst,
	output reg [AWIDTH-1:0] addr
);
	initial addr = 0;

	// async reset
	// increment address when enabled
	always @(posedge clk or posedge rst)
		if (rst)
			addr <= 0;
		else if (en) begin
			if ({'0, addr} == MAX_DATA-1)
				addr <= 0;
			else
				addr <= addr + 1;
		end
endmodule //addr_gen

// Define our top level fifo entity
module fifo 
#(  parameter MAX_DATA=256,
	localparam AWIDTH = $clog2(MAX_DATA)
) ( input wen, ren, clk, rst,
	input [7:0] wdata,
	output reg [7:0] rdata,
	output reg [AWIDTH:0] count
);
	// fifo storage
	// sync read before write
	wire [AWIDTH-1:0] waddr, raddr;
	reg [7:0] data [MAX_DATA-1:0];
	always @(posedge clk) begin
		if (wen)
			data[waddr] <= wdata;
		rdata <= data[raddr];
	end // storage

	// addr_gen for both write and read addresses
	addr_gen #(.MAX_DATA(MAX_DATA))
	fifo_writer (
		.en     (wen),
		.clk    (clk),
		.rst    (rst),
		.addr   (waddr)
	);

	addr_gen #(.MAX_DATA(MAX_DATA))
	fifo_reader (
		.en     (ren),
		.clk    (clk),
		.rst    (rst),
		.addr   (raddr)
	);

	// status signals
	initial count = 0;

	always @(posedge clk or posedge rst) begin
		if (rst)
			count <= 0;
		else if (wen && !ren)
			count <= count + 1;
		else if (ren && !wen)
			count <= count - 1;
	end

endmodule