-- A First-in First-out Memory
-- a first-in first out memory, uses a synchronising clock
-- generics allow fifos of different sizes to be instantiated
-- download from: www.fpga.com.cn & www.pld.com.cn

library IEEE;
use IEEE.Std_logic_1164.all;

entity FIFOMXN is
   generic(m, n : Positive := 8); --m is fifo depth, n is fifo width
   port(RESET, WRREQ, RDREQ, CLOCK : in Std_logic;
         DATAIN : in Std_logic_vector((n-1) downto 0);
         DATAOUT : out Std_logic_vector((n-1) downto 0);
         FULL, EMPTY : inout Std_logic);
end FIFOMXN;

architecture V2 of FIFOMXN is

   type Fifo_array is array(0 to (m-1)) of Bit_vector((n-1) downto 0);
   signal Fifo_memory : Fifo_array;
   signal Wraddr, Rdaddr, Offset : Natural range 0 to (m-1);
   signal Rdpulse, Wrpulse, Q1, Q2, Q3, Q4 : Std_logic;
   signal Databuffer : Bit_vector((n-1) downto 0);

begin

--pulse synchronisers for WRREQ and RDREQ
--modified for Synplify to a process

sync_ffs : process
        begin
                wait until rising_edge(CLOCK);
                Q1 <= WRREQ;
                Q2 <= Q1;
                Q3 <= RDREQ;
                Q4 <= Q3;
end process;

--concurrent logic to generate pulses

Wrpulse <= Q2 and not(Q1);
Rdpulse <= Q4 and not(Q3);   


Fifo_read : process
   begin
      wait until rising_edge(CLOCK);
      if RESET = '1' then
         Rdaddr <= 0;
         Databuffer <= (others => '0');
      elsif (Rdpulse = '1' and EMPTY = '0') then
         Databuffer <= Fifo_memory(Rdaddr);
         Rdaddr <= (Rdaddr + 1) mod m;
      end if;
   end process;


Fifo_write : process
   begin
      wait until rising_edge(CLOCK);
      if RESET = '1' then
         Wraddr <= 0;
      elsif (Wrpulse = '1' and FULL = '0') then
         Fifo_memory(Wraddr) <= To_Bitvector(DATAIN);
         Wraddr <= (Wraddr + 1) mod m;
      end if;
   end process;

Offset <= (Wraddr - Rdaddr) when (Wraddr > Rdaddr) 
            else (m - (Rdaddr - Wraddr)) when (Rdaddr > Wraddr)
            else 0;

EMPTY <= '1' when (Offset = 0) else '0';
FULL <= '1' when (Offset = (m-1)) else '0';

DATAOUT <= To_Stdlogicvector(Databuffer) when RDREQ = '0' 
            else (others => 'Z');

end V2;
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