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--

-- AT90Sxxxx compatible microcontroller core

--

-- Version : 0224

--

-- Copyright (c) 2001-2002 Daniel Wallner (jesus@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/ax8/

--

-- Limitations :

--      Four level stack

--

-- File history :

--

library IEEE;

use IEEE.std_logic_1164.all;

use IEEE.numeric_std.all;

entity AX_PCS is

        generic(

                HW_Stack        : boolean

        );

        port(

                Clk                     : in std_logic;

                Reset_n         : in std_logic;

                Offs_In         : in std_logic_vector(11 downto 0);

                Z                       : in unsigned(15 downto 0);

                Data_In         : in std_logic_vector(7 downto 0);

                Pause           : in std_logic;

                Push            : in std_logic;

                Pop                     : in std_logic;

                HRet            : in std_logic;

                LRet            : in std_logic;

                ZJmp            : in std_logic;

                RJmp            : in std_logic;

                CInt            : in std_logic_vector(3 downto 0);

                IPending        : in std_logic;

                IPush           : out std_logic;

                NPC                     : out std_logic_vector(15 downto 0);

                PC                      : out std_logic_vector(15 downto 0)

        );

end AX_PCS;

architecture rtl of AX_PCS is

        signal  PC_i    : unsigned(15 downto 0);

        signal  NPC_i   : unsigned(15 downto 0);

        signal  IPush_i : std_logic;

        type Stack_Image is array (3 downto 0) of unsigned(15 downto 0);

        signal  Stack : Stack_Image;

        signal  StackPtr : unsigned(1 downto 0);

begin

        NPC <= std_logic_vector(NPC_i);

        PC <= std_logic_vector(PC_i);

        IPush <= IPush_i;

        process (PC_i, Pause, IPending, IPush_i, Push, Pop, Stack, Data_In, Offs_In, CInt, HRet, LRet, RJmp, ZJmp, Z)

        begin

                NPC_i <= PC_i;

                if Pause = '0' then

                        if IPending = '0' then

                                NPC_i <= PC_i + 1;

                        end if;

                        if IPending = '0' and IPush_i = '1' then

                                NPC_i(15 downto 4) <= "000000000000";

                                NPC_i(3 downto 0) <= unsigned(CInt);

                        end if;

                end if;

                if Pop = '1' and HW_Stack then

                        NPC_i <= Stack(to_integer(StackPtr - 1));

                end if;

                if HRet = '1' then

                        NPC_i(15 downto 8) <= unsigned(Data_In);

                end if;

                if LRet = '1' then

                        NPC_i(7 downto 0) <= unsigned(Data_In);

                end if;

                if ZJmp = '1' then

                        NPC_i <= Z;

                end if;

                if RJmp = '1' then

                        NPC_i <= PC_i + unsigned(resize(signed(Offs_In), 16));

                end if;

        end process;

        process (Reset_n, Clk)

        begin

                if Reset_n = '0' then

                        PC_i <= (others => '0');

                        IPush_i <= '0';

                        if HW_Stack then

                                Stack <= (others => (others => '0'));

                                StackPtr <= "00";

                        end if;

                elsif Clk'event and Clk = '1' then

                        PC_i <= NPC_i;

                        if Pause = '0' then

                                IPush_i <= IPending;

                                if IPending = '0' and IPush_i = '1' then

                                        if HW_Stack then

                                                Stack(to_integer(StackPtr)) <= PC_i;

                                                StackPtr <= StackPtr + 1;

                                        end if;

                                end if;

                        end if;

                        if Push = '1' and HW_Stack then

                                Stack(to_integer(StackPtr)) <= PC_i;

                                StackPtr <= StackPtr + 1;

                        end if;

                        if Pop = '1' and HW_Stack then

                                StackPtr <= StackPtr - 1;

                        end if;

                end if;

        end process;

end;