-- +-----------------------------+
-- | Copyright 2008 DOULOS       |
-- |       Library: memory       |
-- |    designer : Tim Pagden    |
-- +-----------------------------+
--
-- Ram write                    Ram Read
--
--       ___                ___               ___________
-- nCS      \______________/   \_____________/
--       _______     ____________________________________
-- nWE          \___/
--       ____________________________     _______________
-- nOE                               \___/
--       ____ _______________ __________________ ________
-- Addr  ____X_______________X__________________X________
--
--       ____ _______________          __
-- Data  ____X_______________X------- X__X---------------
--


library IEEE;
use IEEE.std_logic_1164.all;
use WORK.RamPack.all;

entity BigRam is
  port (Address: in AddrType;
        Data: inout DataType;
        nCS, nWE, nOE: in Std_logic);
end;

architecture Lists of BigRam is
  -- Model using single linked list
begin
  process (Address, nCS, nWE, nOE)
    type Item;

    type ItemPtr is access Item;

    type Item is
      record
        NextItem: ItemPtr;
        Address:  AddrType;
        Word:     DataType;
      end record;

    variable Head: ItemPtr;

    procedure Get (Addr: in AddrType; Word: out DataType) is
      -- Get the contents of the ram with address = Addr
      variable Ptr: ItemPtr;
    begin
      Ptr := Head;
      while Ptr /= null loop
        if Ptr.Address = Addr then
          Word := Ptr.Word;
          return;
        end if;
        Ptr := Ptr.NextItem;
      end loop;
      Word := (others => 'U');
    end Get;


    procedure Set (Addr: in AddrType; Word: in DataType) is
      -- Set the contents of the ram with address = Addr to Word
      variable Ptr, PreviousPtr: ItemPtr;
    begin
      Ptr := Head;
      PreviousPtr := null;
      while Ptr /= null loop
        if Ptr.Address = Addr then
          if Word = DataType'(others => 'U') then

            -- Delete item from list...
            if PreviousPtr = null then
              Head := Ptr.NextItem;
            else
              PreviousPtr.NextItem := Ptr.NextItem;
            end if;
            DEALLOCATE (Ptr);

          else
            Ptr.Word := Word;
          end if;
          return;
        end if;
        PreviousPtr := Ptr;
        Ptr := Ptr.NextItem;
      end loop;

      if Word /= DataType'(others => 'U') then
        -- Insert new item into list...
        Ptr := new Item'(NextItem => null, Address => Addr, Word => Word);
        if PreviousPtr = null then
          Head := Ptr;
        else
          PreviousPtr.NextItem := Ptr;
        end if;
      end if;

    end Set;


    procedure Diagnose is
      variable Ptr: ItemPtr;
      variable Count: NATURAL := 0;
    begin
      if Diagnostics then
        Ptr := Head;
        while Ptr /= null loop
          Count := Count + 1;
          Ptr := Ptr.NextItem;
        end loop;
        assert FALSE
          report "List length = " & INTEGER'IMAGE(Count)
          severity Note;
      end if;
    end Diagnose;


    variable D: DataType;
  begin
    Data <= (others => 'Z');

    if nCS  = '0' then

      if nOE = '0' then			-- Read operation

        Get(Address, D);
        Data <= D;
        Diagnose;

      elsif nWE = '0' then		-- Write operation

        Set (Address, Data);
        Diagnose;

      end if;
    end if;
  end process;
end;