Subversion Repositories t400

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

--

-- The skip unit.

-- Skip conditions are checked here and communicated to the decoder unit.

--

-- $Id: t400_skip.vhd 179 2009-04-01 19:48:38Z arniml $

--

-- Copyright (c) 2006 Arnim Laeuger (arniml@opencores.org)

--

-- All rights reserved

--

-- Redistribution and use in source and synthezised forms, with or without

-- modification, are permitted provided that the following conditions are met:

--

-- Redistributions of source code must retain the above copyright notice,

-- this list of conditions and the following disclaimer.

--

-- Redistributions in synthesized form must reproduce the above copyright

-- notice, this list of conditions and the following disclaimer in the

-- documentation and/or other materials provided with the distribution.

--

-- Neither the name of the author nor the names of other contributors may

-- be used to endorse or promote products derived from this software without

-- specific prior written permission.

--

-- THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"

-- AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,

-- THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR

-- PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE

-- LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR

-- CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF

-- SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS

-- INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN

-- CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)

-- ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE

-- POSSIBILITY OF SUCH DAMAGE.

--

-- Please report bugs to the author, but before you do so, please

-- make sure that this is not a derivative work and that

-- you have the latest version of this file.

--

-- The latest version of this file can be found at:

--      http://www.opencores.org/cvsweb.shtml/t400/

--

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

library ieee;

use ieee.std_logic_1164.all;

use work.t400_opt_pack.all;

use work.t400_pack.all;

entity t400_skip is

  generic (

    opt_type_g : integer := t400_opt_type_420_c

  );

  port (

    -- System Interface -------------------------------------------------------

    ck_i       : in  std_logic;

    ck_en_i    : in  boolean;

    por_i      : in  boolean;

    res_i      : in  boolean;

    -- Control Interface ------------------------------------------------------

    op_i       : in  skip_op_t;

    dec_data_i : in  dec_data_t;

    carry_i    : in  std_logic;

    c_i        : in  std_logic;

    bd_i       : in  dw_t;

    is_lbi_i   : in  boolean;

    skip_o     : out boolean;

    skip_lbi_o : out boolean;

    -- Data Interface ---------------------------------------------------------

    a_i        : in  dw_t;

    m_i        : in  dw_t;

    g_i        : in  dw_t;

    tim_c_i    : in  boolean

  );

end t400_skip;

library ieee;

use ieee.numeric_std.all;

architecture rtl of t400_skip is

  signal skip_q,

         skip_next_q : boolean;

  signal skip_lbi_q  : boolean;

  signal skip_int_q  : boolean;

begin

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

  -- Process skip

  --

  -- Purpose:

  --   Implements the skip logic.

  --

  skip: process (ck_i, por_i)

    variable t420_type_v : boolean;

  begin

    if por_i then

      skip_next_q <= false;

      skip_q      <= false;

      skip_lbi_q  <= false;

      skip_int_q  <= false;

    elsif ck_i'event and ck_i = '1' then

      if    res_i then

        -- synchronous reset upon external reset event

        skip_next_q    <= false;

        skip_q         <= false;

        skip_lbi_q     <= false;

        skip_int_q     <= false;

      elsif ck_en_i then

        t420_type_v := opt_type_g = t400_opt_type_420_c;

        if ck_en_i then

          case op_i is

            -- update skip information ----------------------------------------

            when SKIP_UPDATE =>

              skip_q       <= skip_next_q;

              -- also reset skip_next flag

              skip_next_q  <= false;

              -- reset skip-on-lbi flag when this was not an LBI

              if not is_lbi_i then

                skip_lbi_q <= false;

              end if;

            -- skip always ----------------------------------------------------

            when SKIP_NOW =>

              skip_next_q <= true;

            -- skip on carry --------------------------------------------------

            when SKIP_CARRY =>

              skip_next_q <= carry_i = '1';

            -- skip on C ------------------------------------------------------

            when SKIP_C =>

              skip_next_q <= c_i = '1';

            -- skip on BD underflow ------------------------------------------

            when SKIP_BD_UFLOW =>

              skip_next_q <= unsigned(bd_i) = 15;

            -- skip on BD overflow -------------------------------------------

            when SKIP_BD_OFLOW =>

              skip_next_q <= unsigned(bd_i) = 0;

            -- skip on LBI instruction ----------------------------------------

            when SKIP_LBI =>

              skip_lbi_q  <= true;

            -- skip on A and M equal ------------------------------------------

            when SKIP_A_M =>

              skip_next_q <= unsigned(a_i) = unsigned(m_i);

            -- skip on G zero -------------------------------------------------

            when SKIP_G_ZERO =>

              skip_next_q <= unsigned(g_i) = 0;

            -- skip on G bit --------------------------------------------------

            when SKIP_G_BIT =>

              skip_next_q <= unsigned(g_i and dec_data_i(dw_range_t)) = 0;

            -- skip on M bit --------------------------------------------------

            when SKIP_M_BIT =>

              skip_next_q <= unsigned(m_i and dec_data_i(dw_range_t)) = 0;

            -- skip on timer carry --------------------------------------------

            when SKIP_TIMER =>

              skip_next_q <= tim_c_i;

              null;

            -- push skip state when vectoring to interrupt routine ------------

            when SKIP_PUSH =>

              if t420_type_v then

                -- save next skip flag

                skip_int_q  <= skip_next_q;

                skip_next_q <= false;

                -- never skip first instruction of interrupt routine

                skip_q      <= false;

              end if;

            -- pop skip state for RET from interrupt routine ------------------

            when SKIP_POP =>

              if t420_type_v then

                -- push'ed info must be pop'ed to skip_next_q as pop'ing

                -- happens during RET of interrupt routine

                -- skip info is valid for next instruction

                skip_next_q <= skip_int_q;

                skip_int_q  <= false;

              end if;

            when others =>

              null;

          end case;

        end if;

      end if;

    end if;

  end process skip;

  --

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

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

  -- Output mapping

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

  skip_o     <= skip_q;

  skip_lbi_o <= skip_lbi_q;

end rtl;