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--!

--! PDP-8 Processor

--!

--! \brief

--!      CPU Save Flag (SF) Memory Extension Register

--!

--! \details

--!      The Save Flags (SF) Register is a temporary register

--!      that is used for saving the Memory Extension Registers

--!      context during an interrupt.

--!

--!      When an interrupt occurs, the contents of the UF, IF,

--!      and DF registers are saved into the SF Register.

--!

--!      The contents of the SF register is saved into the

--!      AC register during the RIB instruction.

--!

--!      The RMF instruction restores the UB, IB, and DF

--!      from the SF register; i.e., it restores what the

--!      interrupt did.

--!

--!      When a JMP, JMS, RET1 or RET2 instruction is executed,

--!      the IF and UF registers are updated with contents of

--!      the IB and UB registers.

--!

--!      The SF Register is set to zero when the Front Panel

--!      CLEAR Switch is asserted.

--!

--! \file

--!      sf.vhd

--!

--! \author

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

--!

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

--

--  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

--

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

--

-- Comments are formatted for doxygen

--

library ieee;                                   --! IEEE Library

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

use work.cpu_types.all;                         --! Types

--

--! CPU Save Flag (SF) Memory Extension Register Entity

--

entity eSF is port (

    sys  : in  sys_t;                           --! Clock/Reset

    sfOP : in  sfop_t;                          --! SF Operation

    DF   : in  field_t;                         --! DF Register

    IB   : in  field_t;                         --! IB Register

    UB   : in  std_logic;                       --! UF Register

    SF   : out sf_t                             --! SF Output

);

end eSF;

--

--! CPU Save Flag (SF) Memory Extension Register RTL

--

architecture rtl of eSF is

    signal sfREG : sf_t;                        --! Save Flag

    signal sfMUX : sf_t;                        --! Save Flag Multiplexer

begin

    --

    -- SF Multiplexer

    --

    with sfOP select

        sfMUX <= sfREG        when sfopNOP,

                 UB & IB & DF when sfopUBIBDF;

    --

    --! SF Register

    --

    REG_SF : process(sys)

    begin

        if sys.rst = '1' then

            sfREG <= (others => '0');

        elsif rising_edge(sys.clk) then

            sfREG <= sfMUX;

        end if;

    end process REG_SF;

    SF <= sfREG;

end rtl;