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-- $Id: ram_1swsr_wfirst_gen.vhd 422 2011-11-10 18:44:06Z mueller $
--
-- Copyright 2006-2011 by Walter F.J. Mueller <W.F.J.Mueller@gsi.de>
--
-- This program is free software; you may redistribute and/or modify it under
-- the terms of the GNU General Public License as published by the Free
-- Software Foundation, either version 2, or at your option any later version.
--
-- This program is distributed in the hope that it will be useful, but
-- WITHOUT ANY WARRANTY, without even the implied warranty of MERCHANTABILITY
-- or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
-- for complete details.
--
------------------------------------------------------------------------------
-- Module Name:    ram_1swsr_rfirst_gen - syn
-- Description:    Single-Port RAM with with one synchronous read/write port
--                 and 'read-through' semantics (as block RAM).
--                 The 'ram_style' attribute is set to 'block', this will
--                 force in XST a synthesis as block RAM.
--
-- Notes:          For xst 8.1.03i: can be written with a signal or a shared
--                 variable declared at the architecture level. Use variable
--                 because this seemed better for simulation. Using a simple
--                 variable declared at process level leads to an array of
--                 registers and a big mux.
--
-- Dependencies:   -
-- Test bench:     -
-- Target Devices: generic Spartan, Virtex
-- Tool versions:  xst 8.2, 9.1, 9.2, 13.1; ghdl 0.18-0.29
-- Revision History: 
-- Date         Rev Version  Comment
-- 2011-11-08   422   1.0.4  now numeric_std clean
-- 2010-06-03   299   1.0.3  use sv_ prefix for shared variables
-- 2008-03-08   123   1.0.2  use std_..._arith, not _unsigned; use unsigned();
-- 2008-03-02   122   1.0.1  change generic default for BRAM models
-- 2007-06-03    45   1.0    Initial version 
------------------------------------------------------------------------------
 
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
 
use work.slvtypes.all;
 
entity ram_1swsr_wfirst_gen is          -- RAM, 1 sync r/w ports, write first
  generic (
    AWIDTH : positive := 11;            -- address port width
    DWIDTH : positive :=  9);           -- data port width
  port(
    CLK  : in slbit;                    -- clock
    EN   : in slbit;                    -- enable
    WE   : in slbit;                    -- write enable
    ADDR : in slv(AWIDTH-1 downto 0);   -- address port
    DI   : in slv(DWIDTH-1 downto 0);   -- data in port
    DO   : out slv(DWIDTH-1 downto 0)   -- data out port
  );
end ram_1swsr_wfirst_gen;
 
 
architecture syn of ram_1swsr_wfirst_gen is
 
  constant memsize : positive := 2**AWIDTH;
  constant datzero : slv(DWIDTH-1 downto 0) := (others=>'0');
  type ram_type is array (0 to memsize-1) of slv(DWIDTH-1 downto 0);
  shared variable sv_ram : ram_type := (others=>datzero);
 
  attribute ram_style : string;
  attribute ram_style of sv_ram : variable is "block";
 
  signal R_DO : slv(DWIDTH-1 downto 0) := datzero;
 
begin
 
  proc_clk: process (CLK)
  begin
    if rising_edge(CLK) then
      if EN = '1' then
        if WE = '1' then
          sv_ram(to_integer(unsigned(ADDR))) := DI;
        end if;
        R_DO <= sv_ram(to_integer(unsigned(ADDR)));
      end if;
    end if;
  end process proc_clk;
 
  DO <= R_DO;
 
end syn;