Subversion Repositories pdp8

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

--!

--! PDP-8 Processor

--!

--! \brief

--!      CPU Interrupt Enable (IE) Register

--!

--!      The IE register is modified under the following

--!      conditions:

--!      -# the IE Register is cleared on entry to an interrupt,

--!         and

--!      -# the IE Register is cleared when the Front Panel CLEAR

--!         switch is asserted, and

--!      -# the IE Register is cleared when the Skip If Interupt

--!         System Is On (SKON) instruction is executed, and

--!      -# the IE Register is cleared when the Interrupt Disable

--!         (IOF) instruction is executed, and

--!      -# the IE Register is set when the Interrupt Enable

--!         (ION) instruction is executed, and

--!

--! \note

--!      Per tribal knowledge, several unimplemented

--!      instructions have the side-effect of performing an

--!      interrupt Enable (ION) instruction which also sets

--!      the IE Register.  These are enumerated in the CPU

--!      VHDL code, but not summarized here.

--!

--! \file

--!      ie.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 Interrupt Enable (IE) Register Entity

--

entity eIE is port (

    sys  : in  sys_t;                           --! Clock/Reset

    ieOP : in  ieOP_t;                          --! IE Operation

    IE   : out std_logic                        --! IE Output

);

end eIE;

--

--! CPU Interrupt Enable (IE) Register RTL

--

architecture rtl of eIE is

    signal ieREG   : std_logic;               --! Interrupt Enable Flip-Flop

    signal ieMUX   : std_logic;               --! Interrupt Enable Flip-Flop Multiplexer

begin

    --

    -- IE Multiplexer

    --

    with ieOP select

        ieMUX <= ieREG when ieopNOP,

                 '0'   when ieopCLR,

                 '1'   when ieopSET;

    --

    --! IE Register

    --

    REG_IE : process(sys)

    begin

        if sys.rst = '1' then

            ieREG <= '0';

        elsif rising_edge(sys.clk) then

            ieREG <= ieMUX;

        end if;

    end process REG_IE;

    IE <= ieREG;

end rtl;