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--!

--! PDP-8 Processor

--!

--! \brief

--!      CPU User Buffer (UB) KM8x Time Share Register

--!

--! \details

--!      The User Buffer Register UB) is temporary storage for the

--!      User Flag (UF) register.

--!

--!      The UB register ia modified under the following conditions:

--!      -# set to the contents of AC(5) when the unit

--!         configured for PDP8 (not HD-6120) and a Restore Flags

--!         (RTF) instruction is executed, or

--!      -# set to the contents of SF(0) when a Restore Memory

--!         Field (RMF) instruction is executed, or

--!      -# set when a Set User Flag (SUF) instruction is

--!         executed, or

--!      -# cleared when a Clear User Flag (CUF) instruction is

--!         executed, or

--!      -# cleared when an interrupt occurs.

--!

--!      The UB register is transfered into the UF register under

--!      the following conditions:

--!      -# JMS Instruction executed, or

--!      -# JMP Instruction executed.

--!

--! \file

--!      ub.vhd

--!

--! \author

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

--!

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--

--  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

--

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--

-- 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 Buffer (UB) KM8x Time Share Register Entity

--

entity eUB is port (

    sys  : in  sys_t;                           --! Clock/Reset

    ubOP : in  ubOP_t;                          --! UB Operation

    AC5  : in  std_logic;                       --! LAC Regiter

    SF0  : in  std_logic;                       --! SF Register bit 0

    UB   : out std_logic                        --! UB Output

);

end eUB;

--

--! CPU User Buffer (UB) KM8x Time Share Register RTL

--

architecture rtl of eUB is

    signal ubREG : std_logic;                   --! User Buffer Flag

    signal ubMUX : std_logic;                   --! User Buffer Flag Multiplexer

begin

    --

    -- UB Multiplexer

    --

    with ubOP select

         ubMUX <= ubREG when ubopNOP,

                  '0'   when ubopCLR,

                  '1'   when ubopSET,

                  AC5   when ubopAC5,

                  SF0   when ubopSF;

    --

    --! REG_UB Register

    --

    REG_UB : process(sys)

    begin

        if sys.rst = '1' then

            ubREG <= '0';

        elsif rising_edge(sys.clk) then

            ubREG <= ubMUX;

        end if;

    end process REG_UB;

    UB <= ubREG;

end rtl;