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-- ion_cpu.vhdl -- MIPS-I(tm) compatible CPU core

-- ion_cpu.vhdl -- MIPS-I(tm) compatible CPU core

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-- project:       ION (http://www.opencores.org/project,ion_cpu)

-- project:       ION (http://www.opencores.org/project,ion_cpu)

-- author:        Jose A. Ruiz (ja_rd@hotmail.com)

-- author:        Jose A. Ruiz (ja_rd@hotmail.com)

-- created:       Jan/11/2011

-- created:       Jan/11/2011

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-- Please read file /doc/ion_project.txt for usage instructions.

-- Please read file /doc/ion_project.txt for usage instructions.

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--### MIPS-I things not implemented

--### MIPS-I things not implemented

--

--

-- 

-- 

-- 1.- Load interlocks: the pipeline is stalled for every load instruction, even

-- 1.- Load interlocks: the pipeline is stalled for every load instruction, even

--     if the target register is not used in the following instruction. So that

--     if the target register is not used in the following instruction. So that

--     every load takes two cycles.

--     every load takes two cycles.

--     The interlock logic should check register indices (@note2)

--     The interlock logic should check register indices (@note2)

--     Invalid opcodes do trap but the logic that prevents bad opcodes from

--     Invalid opcodes do trap but the logic that prevents bad opcodes from

--     having side affects has not been tested yet.

--     having side affects has not been tested yet.

--     When in user mode, COP* instructions will trigger a 'CpU' exception.

--     When in user mode, COP* instructions will trigger a 'CpU' exception.

--     BUT there's no address checking and user code can still access kernel 

--     BUT there's no address checking and user code can still access kernel 

--     space in this version.

--     space in this version.

--

--

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-- KNOWN BUGS:

-- KNOWN BUGS:

--

--

-- 1.- The instruction executed right after entering user mode (i.e. the 

-- 1.- The instruction executed right after entering user mode (i.e. the 

--     instruction after the MTC0 or RFE that clears the KU flag) is executed 

--     instruction after the MTC0 or RFE that clears the KU flag) is executed 

--     in case it makes any sense to speak of security in this project at this

--     in case it makes any sense to speak of security in this project at this

--     stage. 

--     stage. 

--     This can be easily fixed but is not very urgent.

--     This can be easily fixed but is not very urgent.

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-- Copyright (C) 2011 Jose A. Ruiz

-- Copyright (C) 2011 Jose A. Ruiz

        XILINX_REGBANK  : string    := "distributed" -- {distributed|block}

        XILINX_REGBANK  : string    := "distributed" -- {distributed|block}

    );

    );

    port(

    port(

        clk             : in std_logic;

        clk             : in std_logic;

        reset           : in std_logic;

        reset           : in std_logic;

        data_addr       : out std_logic_vector(31 downto 0);

        data_addr       : out std_logic_vector(31 downto 0);

        data_rd         : in std_logic_vector(31 downto 0);

        data_rd         : in std_logic_vector(31 downto 0);

        data_rd_vma     : out std_logic;

        data_rd_vma     : out std_logic;