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--!

--! PDP-8 Processor

--!

--! \brief

--!      CPU Panel Data Flag (PDF) Register

--!

--! \details

--!      The Panel Data Flag (PDF) Register controls whether

--!      indirect data memory accesses by the Control Panel AND,

--!      TAD, ISZ or DCA instructions reference Panel Memory or

--!      Main Memory.

--!

--!      If the PDF Register is set, indirect data memory

--!      references as described above access Panel Memory

--!      (LXPAR asserted).  If the PDF Register is cleared,

--!      indirect data memory references as described above

--!      access Main Memory (LXMAR asserted).

--!

--!      The PDF Register is modified under the following

--!      conditions:

--!      -# the PDF Register is cleared on entry to the Panel

--!         Mode Interrupt, and

--!      -# the PDF Register is cleared if the unit is

--!         configured as a HD-6120 and the unit is in Panel

--!         Mode (CTRLFF asserted) and a Clear Panel Data Flag

--!         (CPD) instruction is executed.

--!      -# the PDF Register is set if the unit is

--!         configured as a HD-6120 and the unit is in Panel

--!         Mode (CTRLFF asserted) and a Set Panel Data Flag

--!         (SPD) instruction is executed.

--!

--! \file

--!      pdf.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 Panel Data Flag (PDF) Register Entity

--

entity ePDF is port (

    sys   : in  sys_t;                          --! Clock/Reset

    pdfOP : in  pdfOP_t;                        --! PDF Operation

    PDF   : out std_logic                       --! PDF Output

);

end ePDF;

--

--! CPU Panel Data Flag (PDF) Register RTL

--

architecture rtl of ePDF is

    signal pdfREG : std_logic;                  --! Panel Data Flag

    signal pdfMUX : std_logic;                  --! Panel Data Flag Multiplexer

begin

    --

    -- PDF Multiplexer

    --

    with pdfOP select

        pdfMUX <= pdfREG when pdfopNOP,

                  '0'    when pdfopCLR,

                  '1'    when pdfopSET;

    --

    --! PDF Register

    --

    REG_PDF : process(sys)

    begin

        if sys.rst = '1' then

            pdfREG <= '0';

        elsif rising_edge(sys.clk) then

            pdfREG <= pdfMUX;

        end if;

    end process REG_PDF;

    PDF <= pdfREG;

end rtl;