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-- $Id: pdp11_bram.vhd 314 2010-07-09 17:38:41Z mueller $

--

-- Copyright 2008- 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:    pdp11_bram - syn

-- Description:    pdp11: BRAM based ext. memory dummy

--

-- Dependencies:   memlib/ram_2swsr_rfirst_gen

-- Test bench:     -

-- Target Devices: generic

-- Tool versions:  xst 8.1, 8.2, 9.1, 9.2; ghdl 0.18-0.25

-- Revision History: 

-- Date         Rev Version  Comment

-- 2008-03-01   120   1.0.2  add addrzero constant to avoid XST errors

-- 2008-02-23   118   1.0.1  AWIDTH now a generic port

-- 2008-02-17   117   1.0    Initial version 

------------------------------------------------------------------------------

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_arith.all;

use work.slvtypes.all;

use work.memlib.all;

use work.pdp11.all;

entity pdp11_bram is                   -- cache

  generic (

    AWIDTH : positive := 14);           -- address width

  port (

    CLK : in slbit;                     -- clock

    GRESET : in slbit;                  -- global reset

    EM_MREQ : in em_mreq_type;          -- em request

    EM_SRES : out em_sres_type          -- em response

  );

end pdp11_bram;

architecture syn of pdp11_bram is

  type regs_type is record

    req_r : slbit;                      -- read request

    req_w : slbit;                      -- write request

    be : slv2;                          -- byte enables

    addr : slv(AWIDTH-1 downto 1);      -- address

  end record regs_type;

  constant addrzero : slv(AWIDTH-1 downto 1) := (others=>'0');

  constant regs_init : regs_type := (

    '0','0',                            -- req_r,w

    (others=>'0'),                      -- be

    addrzero                            -- addr

  );

  signal R_REGS : regs_type := regs_init;  -- state registers

  signal N_REGS : regs_type := regs_init;  -- next value state regs

  signal MEM_ENB : slbit := '0';

  signal MEM_WEA : slv2  := "00";

  signal MEM_DOA : slv16 := (others=>'0');

begin

  MEM_BYT0 : ram_2swsr_rfirst_gen

    generic map (

      AWIDTH => AWIDTH-1,

      DWIDTH =>  8)

    port map (

      CLKA  => CLK,

      CLKB  => CLK,

      ENA   => EM_MREQ.req,

      ENB   => MEM_ENB,

      WEA   => MEM_WEA(0),

      WEB   => R_REGS.be(0),

      ADDRA => EM_MREQ.addr(AWIDTH-1 downto 1),

      ADDRB => R_REGS.addr,

      DIA   => EM_MREQ.din(7 downto 0),

      DIB   => MEM_DOA(7 downto 0),

      DOA   => MEM_DOA(7 downto 0),

      DOB   => open

      );

  MEM_BYT1 : ram_2swsr_rfirst_gen

    generic map (

      AWIDTH => AWIDTH-1,

      DWIDTH =>  8)

    port map (

      CLKA  => CLK,

      CLKB  => CLK,

      ENA   => EM_MREQ.req,

      ENB   => MEM_ENB,

      WEA   => MEM_WEA(1),

      WEB   => R_REGS.be(1),

      ADDRA => EM_MREQ.addr(AWIDTH-1 downto 1),

      ADDRB => R_REGS.addr,

      DIA   => EM_MREQ.din(15 downto 8),

      DIB   => MEM_DOA(15 downto 8),

      DOA   => MEM_DOA(15 downto 8),

      DOB   => open

      );

  proc_regs: process (CLK)

  begin

    if CLK'event and CLK='1' then

      if GRESET = '1' then

        R_REGS <= regs_init;

      else

        R_REGS <= N_REGS;

      end if;

    end if;

  end process proc_regs;

  N_REGS.req_r  <= EM_MREQ.req and not EM_MREQ.we;

  N_REGS.req_w  <= EM_MREQ.req and EM_MREQ.we;

  N_REGS.be     <= EM_MREQ.be;

  N_REGS.addr   <= EM_MREQ.addr(N_REGS.addr'range);

  MEM_WEA(0) <= EM_MREQ.we and EM_MREQ.be(0);

  MEM_WEA(1) <= EM_MREQ.we and EM_MREQ.be(1);

  MEM_ENB    <= EM_MREQ.cancel and R_REGS.req_w;

  EM_SRES.ack_r <= R_REGS.req_r;

  EM_SRES.ack_w <= R_REGS.req_w;

  EM_SRES.dout  <= MEM_DOA;

end syn;