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--!

--! PDP-8 Processor

--!

--! \brief

--!      CPU Extended Memory (XMA) Register

--!

--! \file

--!      xma.vhd

--!

--! \author

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

--!

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

--

--  Copyright (C) 2009, 2010, 2011, 2012 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 Memory (XMA) Register Entity

--

entity eXMA is

    port (

        sys   : in  sys_t;                      --! Clock/Reset

        xmaOP : in  xmaOP_t;                    --! XMA Operation

        swCPU : in  swCPU_t;                    --! CPU Configuration

        DF    : in  field_t;                    --! DF register

        INF   : in  field_t;                    --! INF register

        IB    : in  field_t;                    --! IB register

        XMA   : out field_t                     --! XMA Output

    );

end eXMA;

--

--! CPU Extended Memory (XMA) Register RTL

--

architecture rtl of eXMA is

    signal xmaREG : field_t;                    --! XMA Register

    signal xmaMUX : field_t;                    --! XMA Multiplexer

begin

    --

    -- XMA Multiplexer

    --

    with xmaOP select

        xmaMUX <= xmaREG when xmaopNOP,         -- XMA <- XMA

                  "000"  when xmaopCLR,         -- XMA <- "000"

                  DF     when xmaopDF,          -- XMA <- DF

                  INF    when xmaopIF,          -- XMA <- IF

                  IB     when xmaopIB;          -- XMA <- IB

    --

    --! XMA Register

    --

    REG_XMA : process(sys)

    begin

        if sys.rst = '1' then

            xmaREG <= "000";

        elsif rising_edge(sys.clk) then

            if swCPU = swHD6120 then

                xmaREG <= "000";

            else

                xmaREG <= xmaMUX;

            end if;

        end if;

    end process REG_XMA;

    XMA <= xmaREG;

end rtl;