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-- 

-- Copyright (C) 2009, 2010 Dr. Juergen Sauermann

-- 

--  This code is free software: you can redistribute it and/or modify

--  it under the terms of the GNU General Public License as published by

--  the Free Software Foundation, either version 3 of the License, or

--  (at your option) any later version.

--

--  This code 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 more details.

--

--  You should have received a copy of the GNU General Public License

--  along with this code (see the file named COPYING).

--  If not, see http://www.gnu.org/licenses/.

--

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--

-- Module Name:    opc_fetch - Behavioral 

-- Create Date:    13:00:44 10/30/2009 

-- Description:    the opcode fetch stage of a CPU.

--

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

--

library IEEE;

use IEEE.std_logic_1164.ALL;

use IEEE.std_logic_ARITH.ALL;

use IEEE.std_logic_UNSIGNED.ALL;

entity opc_fetch is

    port (  I_CLK       : in  std_logic;

            I_CLR       : in  std_logic;

            I_INTVEC    : in  std_logic_vector( 5 downto 0);

            I_LOAD_PC   : in  std_logic;

            I_NEW_PC    : in  std_logic_vector(15 downto 0);

            I_PM_ADR    : in  std_logic_vector(11 downto 0);

            I_SKIP      : in  std_logic;

            Q_OPC       : out std_logic_vector(31 downto 0);

            Q_PC        : out std_logic_vector(15 downto 0);

            Q_PM_DOUT   : out std_logic_vector( 7 downto 0);

            Q_T0        : out std_logic);

end opc_fetch;

architecture Behavioral of opc_fetch is

component prog_mem

    port (  I_CLK       : in  std_logic;

            I_WAIT      : in  std_logic;

            I_PC        : in  std_logic_vector (15 downto 0);

            I_PM_ADR    : in  std_logic_vector (11 downto 0);

            Q_OPC       : out std_logic_vector (31 downto 0);

            Q_PC        : out std_logic_vector (15 downto 0);

            Q_PM_DOUT   : out std_logic_vector ( 7 downto 0));

end component;

signal P_OPC            : std_logic_vector(31 downto 0);

signal P_PC             : std_logic_vector(15 downto 0);

signal L_INVALIDATE     : std_logic;

signal L_LONG_OP        : std_logic;

signal L_NEXT_PC        : std_logic_vector(15 downto 0);

signal L_PC             : std_logic_vector(15 downto 0);

signal L_T0             : std_logic;

signal L_WAIT           : std_logic;

begin

    pmem : prog_mem

    port map(   I_CLK       => I_CLK,

                I_WAIT      => L_WAIT,

                I_PC        => L_NEXT_PC,

                I_PM_ADR    => I_PM_ADR,

                Q_OPC       => P_OPC,

                Q_PC        => P_PC,

                Q_PM_DOUT   => Q_PM_DOUT);

   lpc: process(I_CLK)

    begin

        if (rising_edge(I_CLK)) then

            L_PC <= L_NEXT_PC;

            L_T0 <= not L_WAIT;

        end if;

    end process;

    L_NEXT_PC <= X"0000"        when (I_CLR     = '1')

            else L_PC           when (L_WAIT    = '1')

            else I_NEW_PC       when (I_LOAD_PC = '1')

            else L_PC + X"0002" when (L_LONG_OP = '1')

            else L_PC + X"0001";

    -- Two word opcodes:

    --

    --        9       3210

    -- 1001 000d dddd 0000 kkkk kkkk kkkk kkkk - LDS

    -- 1001 001d dddd 0000 kkkk kkkk kkkk kkkk - SDS

    -- 1001 010k kkkk 110k kkkk kkkk kkkk kkkk - JMP

    -- 1001 010k kkkk 111k kkkk kkkk kkkk kkkk - CALL

    --

    L_LONG_OP <= '1' when (((P_OPC(15 downto  9) = "1001010") and

                            (P_OPC( 3 downto  2) = "11"))       -- JMP, CALL

                       or  ((P_OPC(15 downto 10) = "100100") and

                            (P_OPC( 3 downto  0) = "0000")))    -- LDS, STS

            else '0';

    -- Two cycle opcodes:

    --

    -- 1001 000d dddd .... - LDS etc.

    -- 1001 0101 0000 1000 - RET

    -- 1001 0101 0001 1000 - RETI

    -- 1001 1001 AAAA Abbb - SBIC

    -- 1001 1011 AAAA Abbb - SBIS

    -- 1111 110r rrrr 0bbb - SBRC

    -- 1111 111r rrrr 0bbb - SBRS

    --

    L_WAIT <= '0'  when ((L_INVALIDATE = '1') or (I_INTVEC(5)  = '1'))

         else L_T0 when ((P_OPC(15 downto   9) = "1001000" )    -- LDS etc.

                     or ((P_OPC(15 downto   8) = "10010101")    -- RET etc.

                      and (P_OPC(3 downto 0) /= "1010"))        -- but not DEC

                     or  ((P_OPC(15 downto 10) = "100110")      -- SBIC, SBIS

                      and (P_OPC(8) = '1'))

                     or  (P_OPC(15 downto  10) = "111111"))     -- SBRC, SBRS

         else '0';

    L_INVALIDATE <= I_CLR or I_SKIP;

    Q_OPC <= X"00000000" when (L_INVALIDATE = '1')

        else P_OPC       when (I_INTVEC(5) = '0')

        else (X"000000" & "00" & I_INTVEC);     -- "interrupt opcode"

    Q_PC <= P_PC;

    Q_T0 <= L_T0;

end Behavioral;