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--!

--! PDP-8 Processor

--!

--! \brief

--!      CPU Extended Arithmetic Mode A (EMODE) Register

--!

--! \file

--!      emode.vhd

--!

--! \author

--!      Rob Doyle - doyle (at) cox (dot) net

--!

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--

--  Copyright (C) 2009 Rob Doyle

--

-- This source file may be used and distributed without

-- restriction provided that this copyright statement is not

-- removed from the file and that any derivative work contains

-- the original copyright notice and the associated disclaimer.

--

-- This source file is free software; you can redistribute it

-- and/or modify it under the terms of the GNU Lesser General

-- Public License as published by the Free Software Foundation;

-- version 2.1 of the License.

--

-- This source 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 Lesser General Public License for more

-- details.

--

-- You should have received a copy of the GNU Lesser General

-- Public License along with this source; if not, download it

-- from http://www.gnu.org/licenses/lgpl.txt

--

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

--

-- Comments are formatted for doxygen

--

library ieee;                                   --! IEEE Library

use ieee.std_logic_1164.all;                    --! IEEE 1164

use ieee.numeric_std.all;                       --! IEEE Numeric Standard

use work.cpu_types.all;                         --! Types

--

--! CPU Extended Arithmetic Mode A (EMODE) Register Entity

--

entity eEMODE is port (

    sys     : in  sys_t;                        --! Clock/Reset

    emodeOP : in  emodeOP_t;                    --! emode Operation

    EMODE   : out std_logic                     --! emode Output

);

end eEMODE;

--

--! CPU Extended Arithmetic Mode A (EMODE) Register RTL

--

architecture rtl of eEMODE is

    signal emodeREG : std_logic;                --! EAE Mode Flip-Flop

    signal emodeMUX : std_logic;                --! EAE Mode Multiplexer

begin

    --

    -- EMODE Multiplexer

    --

    with emodeop select

        emodeMUX <= emodeREG when emodeopNOP,   -- EMODE <- EMODE

                    '0'      when emodeopCLR,   -- EMODE <- '0'

                    '1'      when emodeopSET;   -- EMODE <- '1'

    --

    --! EMODE Register

    --

    REG_EMODE : process(sys)

    begin

        if sys.rst = '1' then

            emodeREG <= '0';

        elsif rising_edge(sys.clk) then

            emodeREG <= emodeMUX;

        end if;

    end process REG_EMODE;

    EMODE <= emodeREG;

end rtl;