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--!

--! PDP-8 Processor

--!

--! \brief

--!      CPU Multiplier Quotient Auxillary Register (MQA)

--!

--! \details

--!      The Multiplier Quotient Auxillary Register (MQA) is a

--!      temporary register used during EAE shift and divide

--!      instructions.

--!

--! \todo

--!      Is the MQA register really necessary?  Can it be

--!      deleted and the instructions be re-written?

--!

--! \file

--!      mqa.vhd

--!

--! \author

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

--!

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--

--  Copyright (C) 2009, 2010, 2011 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

--

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--

-- Comments are formatted for doxygen

--

library ieee;                                   --! IEEE Library

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

use work.cpu_types.all;                         --! Types

--

--! CPU Auxillary Multiplier Quotient Register (MQA) Entity

--

entity eMQA is port (

    sys   : in  sys_t;                          --! Clock/Reset

    mqaOP : in  mqaop_t;                        --! MQA Operation

    MQ    : in  data_t;                         --! MQ Register

    MQA   : out data_t                          --! MQA Output

);

end eMQA;

--

--! CPU Auxillary Multiplier Quotient Register (MQA) RTL

--

architecture rtl of eMQA is

    signal mqaREG : data_t;                     --! Aux Multiplier Quotient Register

    signal mqaMUX : data_t;                     --! Aux Multiplier Quotient Multiplexer

begin

    --

    -- MQ Multiplexer

    --

    with mqaOP select

        mqaMUX <= mqaREG                when mqaopNOP,  -- MQA <- MQ

                  o"0000"               when mqaopCLR,  -- MQA <- "0000"

                  MQ                    when mqaopMQ,   -- MQA <- MQ

                  mqaREG(1 to 11) & '0' when mqaopSHL;  -- MQA <- (MQA << 1)

    --

    --! MQA Register

    --

    REG_MQA : process(sys)

    begin

        if sys.rst = '1' then

            mqaREG <= o"0000";

        elsif rising_edge(sys.clk) then

            mqaREG <= mqaMUX;

        end if;

    end process REG_MQA;

    MQA <= mqaREG;

end rtl;