// Synchronous FIFO.  4 x 16 bit words.
//
module fifo (clk, rstp, din, writep, readp, dout, emptyp, fullp);
input		clk;
input		rstp;
input [15:0]	din;
input		readp;
input		writep;
output [15:0]	dout;
output		emptyp;
output		fullp;

// Defines sizes in terms of bits.
//
parameter	DEPTH = 2,		// 2 bits, e.g. 4 words in the FIFO.
		MAX_COUNT = 2'b11;	// topmost address in FIFO.

reg 		emptyp;
reg		fullp;

// Registered output.
reg [15:0]	dout;

// Define the FIFO pointers.  A FIFO is essentially a circular queue.
//
reg [(DEPTH-1):0]	tail;
reg [(DEPTH-1):0]	head;

// Define the FIFO counter.  Counts the number of entries in the FIFO which
// is how we figure out things like Empty and Full.
//
reg [(DEPTH-1):0]	count;

// Define our regsiter bank.  This is actually synthesizable!
//
reg [15:0] fifomem[0:MAX_COUNT];

// Dout is registered and gets the value that tail points to RIGHT NOW.
//
always @(posedge clk) begin
   if (rstp == 1) begin
      dout <= 16'h0000;
   end
   else begin
      dout <= fifomem[tail];
   end
end 
     
// Update FIFO memory.
always @(posedge clk) begin
   if (rstp == 1'b0 && writep == 1'b1 && fullp == 1'b0) begin
      fifomem[head] <= din;
   end
end
                  
// Update the head register.
//
always @(posedge clk) begin
   if (rstp == 1'b1) begin
      head <= 2'b00;
   end
   else begin
      if (writep == 1'b1 && fullp == 1'b0) begin
         // WRITE
         head <= head + 1;
      end
   end
end

// Update the tail register.
//
always @(posedge clk) begin
   if (rstp == 1'b1) begin
      tail <= 2'b00;
   end
   else begin
      if (readp == 1'b1 && emptyp == 1'b0) begin
         // READ               
         tail <= tail + 1;
      end
   end
end

// Update the count regsiter.
//
always @(posedge clk) begin
   if (rstp == 1'b1) begin
      count <= 2'b00;
   end
   else begin
      case ({readp, writep})
         2'b00: count <= count;
         2'b01: 
            // WRITE
            if (count != MAX_COUNT) 
               count <= count + 1;
         2'b10: 
            // READ
            if (count != 2'b00)
               count <= count - 1;
         2'b11:
            // Concurrent read and write.. no change in count
            count <= count;
      endcase
   end
end

         
// *** Update the flags
//
// First, update the empty flag.
//
always @(count) begin
   if (count == 2'b00)
      emptyp <= 1'b1;
   else
      emptyp <= 1'b0;
end


// Update the full flag
//
always @(count) begin
   if (count == MAX_COUNT)
      fullp <= 1'b1;
   else
      fullp <= 1'b0;
end

endmodule

// synopsys translate_off

`define TEST_FIFO
// synopsys translate_off
`ifdef TEST_FIFO
module test_fifo;

reg		clk;
reg		rstp;
reg [15:0]	din;
reg		readp;
reg		writep;
wire [15:0]	dout;
wire		emptyp;
wire		fullp;

reg [15:0]	value;

fifo U1 (
   .clk		(clk),
   .rstp	(rstp),
   .din		(din),
   .readp	(readp),
   .writep	(writep),
   .dout	(dout),
   .emptyp	(emptyp),
   .fullp	(fullp)
);

task read_word;
begin
   @(negedge clk);
   readp = 1;
   @(posedge clk) #5;
   $display ("Read %0h from FIFO", dout);
   readp = 0;
end
endtask
   
task write_word;
input [15:0]	value;
begin
   @(negedge clk);
   din = value;
   writep = 1;
   @(posedge clk);
   $display ("Write %0h to FIFO", din);
   #5;
   din = 16'hzzzz;
   writep = 0;
end
endtask

initial begin
   clk = 0;
   forever begin
      #10 clk = 1;
      #10 clk = 0;
   end
end

initial begin
   $shm_open ("./fifo.shm");
   $shm_probe (test_fifo, "AS");
   
   //test1;
   test2;
   
   $shm_close;
   $finish;
end

task test1;
begin
   din = 16'hzzzz;
   writep = 0;
   readp = 0;

   // Reset
   rstp = 1;
   #50;
   rstp = 0;
   #50;
   
   // ** Write 3 values.
   write_word (16'h1111);
   write_word (16'h2222);
   write_word (16'h3333);
   
   // ** Read 2 values
   read_word;
   read_word;
   
   // ** Write one more
   write_word (16'h4444);
   
   // ** Read a bunch of values
   repeat (6) begin
      read_word;
   end
   
   // *** Write a bunch more values
   write_word (16'h0001);
   write_word (16'h0002);
   write_word (16'h0003);
   write_word (16'h0004);
   write_word (16'h0005);
   write_word (16'h0006);
   write_word (16'h0007);
   write_word (16'h0008);

   // ** Read a bunch of values
   repeat (6) begin
      read_word;
   end
   
   $display ("Done TEST1.");
end
endtask

// TEST2
//
// This test will operate the FIFO in an orderly manner the way it normally works.
// 2 threads are forked; a reader and a writer.  The writer writes a counter to
// the FIFO and obeys the fullp flag and delays randomly.  The reader likewise
// obeys the emptyp flag and reads at random intervals.  The result should be that
// the reader reads the incrementing counter out of the FIFO.  The empty/full flags
// should bounce around depending on the random delays.  The writer repeats some
// fixed number of times and then terminates both threads and kills the sim.
//
task test2;
reg [15:0] writer_counter;
begin
   writer_counter = 16'h0001;
   din = 16'hzzzz;
   writep = 0;
   readp = 0;

   // Reset
   rstp = 1;
   #50;
   rstp = 0;
   #50;
   
   fork
      // Writer
      begin
         repeat (500) begin
            @(negedge clk);
            if (fullp == 1'b0) begin
               write_word (writer_counter);
               #5;
               writer_counter = writer_counter + 1;
            end
            else begin
               $display ("WRITER is waiting..");
            end
            // Delay a random amount of time between 0ns and 100ns
            #(50 + ($random % 50));
         end
         $display ("Done with WRITER fork..");
         $finish;
      end
      
      // Reader
      begin
         forever begin
            @(negedge clk);
            if (emptyp == 1'b0) begin
               read_word;
            end  
            else begin
               $display ("READER is waiting..");
            end
            // Delay a random amount of time between 0ns and 100ns
            #(50 + ($random % 50));
         end
      end
   join
end
endtask

always @(fullp)
   $display ("fullp = %0b", fullp);
   
always @(emptyp)
   $display ("emptyp = %0b", emptyp);

always @(U1.head)
   $display ("head = %0h", U1.head);

always @(U1.tail)
   $display ("tail = %0h", U1.tail);

endmodule
`endif