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--!

--! PDP-8 Processor

--!

--! \brief

--!      CPU User Flag (UF) KM8x Time Share Register

--!

--! \details

--!      The KM8x Time Share facility allows certain of the PDP8

--!      operations to be virtualized by trapping the execution of

--!      specific instructions to an interrupt.

--!     

--!      The UF register is modified under the following

--!      conditions:

--!      -# UF is cleared when the instruction trap occurs, or

--!      -# UF is set to the contents of the UB register when a JMS

--!         instruction is executed, or

--!      -# UF is set to the contents of the UB register when a JMP

--!         instruction is executed.

--!

--!      When the UF is set and KM8E Time Sharing is enabled, all

--!      IOTs, the HLT instruction, and Switch Register input

--!      instructions trap to a User Mode Interrupt when executed

--!      otherwise the execute normally.

--!

--! \file

--!      uf.vhd

--!

--! \author

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

--!

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

--

--  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 User Flag (UF) KM8x Time Share Register Entity

--

entity eUF is

    port (

        sys  : in  sys_t;                       --! Clock/Reset

        ufOP : in  ufOP_t;                      --! UF Operation

        UB   : in  std_logic;                   --! UB Register

        UF   : out std_logic                    --! UF Output

    );

end eUF;

--

--! CPU User Flag (UF) KM8x Time Share Register RTL

--

architecture rtl of eUF is

    signal ufREG : std_logic;                   --! User Flag Register

    signal ufMUX : std_logic;                   --! User Flag Multiplexer

begin

    --

    -- UF Multiplexer

    --

    with ufOP select

        ufMUX <= ufREG when ufopNOP,            --! UF <- UF

                 '0'   when ufopCLR,            --! UF <- '0'

                 '1'   when ufopSET,            --! UF <- '1'

                  UB   when ufopUB;             --! UF <- UB

    --

    --! UF Register

    --

    REG_UF : process(sys)

    begin

        if sys.rst = '1' then

            ufREG <= '0';

        elsif rising_edge(sys.clk) then

            ufREG <= ufMUX;

        end if;

    end process REG_UF;

    UF <= ufREG;

end rtl;