Subversion Repositories t400

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

--

-- The decoder unit.

-- Implements the instruction opcodes and controls all units of the T400 core.

--

-- $Id: t400_decoder.vhd,v 1.6 2006-06-05 14:20:34 arniml Exp $

--

-- Copyright (c) 2006 Arnim Laeuger (arniml@opencores.org)

--

-- All rights reserved

--

-- Redistribution and use in source and synthezised forms, with or without

-- modification, are permitted provided that the following conditions are met:

--

-- Redistributions of source code must retain the above copyright notice,

-- this list of conditions and the following disclaimer.

--

-- Redistributions in synthesized form must reproduce the above copyright

-- notice, this list of conditions and the following disclaimer in the

-- documentation and/or other materials provided with the distribution.

--

-- Neither the name of the author nor the names of other contributors may

-- be used to endorse or promote products derived from this software without

-- specific prior written permission.

--

-- THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"

-- AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,

-- THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR

-- PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE

-- LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR

-- CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF

-- SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS

-- INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN

-- CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)

-- ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE

-- POSSIBILITY OF SUCH DAMAGE.

--

-- Please report bugs to the author, but before you do so, please

-- make sure that this is not a derivative work and that

-- you have the latest version of this file.

--

-- The latest version of this file can be found at:

--      http://www.opencores.org/cvsweb.shtml/t400/

--

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

library ieee;

use ieee.std_logic_1164.all;

use work.t400_opt_pack.all;

use work.t400_pack.all;

entity t400_decoder is

  generic (

    opt_type_g : integer := t400_opt_type_420_c

  );

  port (

    -- System Interface -------------------------------------------------------

    ck_i       : in  std_logic;

    ck_en_i    : in  boolean;

    por_i      : in  boolean;

    res_i      : in  boolean;

    out_en_i   : in  boolean;

    in_en_i    : in  boolean;

    icyc_en_i  : in  boolean;

    -- Module Control Interface -----------------------------------------------

    pc_op_o    : out pc_op_t;

    stack_op_o : out stack_op_t;

    dmem_op_o  : out dmem_op_t;

    b_op_o     : out b_op_t;

    skip_op_o  : out skip_op_t;

    alu_op_o   : out alu_op_t;

    io_l_op_o  : out io_l_op_t;

    io_d_op_o  : out io_d_op_t;

    io_g_op_o  : out io_g_op_t;

    io_in_op_o : out io_in_op_t;

    sio_op_o   : out sio_op_t;

    dec_data_o : out dec_data_t;

    en_o       : out dw_t;

    -- Skip Interface ---------------------------------------------------------

    skip_i     : in  boolean;

    skip_lbi_i : in  boolean;

    is_lbi_o   : out boolean;

    int_i      : in  boolean;

    -- Program Memory Interface -----------------------------------------------

    pm_addr_i  : in  pc_t;

    pm_data_i  : in  byte_t

  );

end t400_decoder;

library ieee;

use ieee.numeric_std.all;

use work.t400_comp_pack.t400_opc_table;

architecture rtl of t400_decoder is

  signal cyc_cnt_q    : unsigned(2 downto 0);

  signal ibyte1_q,

         ibyte2_q     : byte_t;

  signal opcode_s     : byte_t;

  signal second_cyc_q : boolean;

  signal mnemonic_s,

         mnemonic_q   : mnemonic_t;

  signal multi_byte_s,

         multi_byte_q : boolean;

  signal last_cycle_s : boolean;

  signal force_mc_s   : boolean;

  signal en_q         : dw_t;

  signal set_en_s     : boolean;

  signal ack_int_s    : boolean;

begin

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

  -- Theory of operation:

  --

  -- a) One instruction cycle lasts at least 4 ck_i cycles.

  -- b) PC for instruction/parameter fetch must be valid during cycle 2.

  --      => cycle 2 is the opcode fetch cycle

  -- c) Cycle 3 is the opcode decode cycle.

  --      => opcode_s is valid with cycle 3

  -- d) mnemonic_q is then valid with cycle 0 until end of instruction.

  --    So is ibyte1_q.

  -- e) PC for is incremented during last instruction cycle.

  --      => fetch of either new instruction or second instruction byte

  -- f) Second instruction byte is saved in ibyte2_q for cycle 0.

  --    Valid until end of instruction.

  --

  -- Constraints:

  --

  -- a) PC of next instruction must be pushed in cycle 0 or 1.

  -- b) PC for next instruction must be poped latest in cycle 1.

  -- c) PC for next instruction can only be calculated latest in cycle 1.

  -- d) IO output is enabled by out_en_i

  -- e) IO inputs are sampled with in_en_i

  --

  -- d) and e) are required for proper timing in relation to phi1

  -- (SK clock/sync output).

  --

  -- Conventions:

  --

  -- a) ALU operations take place in cycle 1.

  --

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

  last_cycle_s <= (not multi_byte_q and

                   not second_cyc_q and not force_mc_s)

                  or

                  second_cyc_q;

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

  -- Process seq

  --

  -- Purpose:

  --   Implements the various sequential elements.

  --     Cycle counter:

  --       It identifies the execution cycle of the

  --       current instruction.

  --     Instruction registers:

  --       They save the first and second byte of an instruction for

  --       further processing.

  --     New instruction flag:

  --       Indicates when a new instruction is fetched from the program

  --       memory. Implemented as a flip-flop to control the multiplexer

  --       which saves power by gating the combinational opcode decoder.

  --     Mnemonic register:

  --       Latches the decoded mnemonic of the current instruction.

  --     Multi byte flag:

  --       Latches the decoded multi byte status information.

  --

  seq: process (ck_i, por_i)

  begin

    if por_i then

      cyc_cnt_q    <= to_unsigned(1, cyc_cnt_q'length);

      second_cyc_q <= false;

      ibyte1_q     <= (others => '0');

      ibyte2_q     <= (others => '0');

      mnemonic_q   <= MN_CLRA;

      multi_byte_q <= false;

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

    elsif ck_i'event and ck_i = '1' then

      if    res_i then

        -- synchronous reset upon external reset event

        mnemonic_q     <= MN_CLRA;

        multi_byte_q   <= false;

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

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

      elsif ck_en_i then

        -- cycle counter ------------------------------------------------------

        if    icyc_en_i then

          -- new instruction cycle started

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

        elsif cyc_cnt_q /= 4 then

          cyc_cnt_q    <= cyc_cnt_q + 1;

        end if;

        -- second cycle flag --------------------------------------------------

        if icyc_en_i then

          if not last_cycle_s then

            second_cyc_q <= true;

          else

            second_cyc_q <= false;

          end if;

        end if;

        -- instruction byte 1 and mnemonic info -------------------------------

        if icyc_en_i and last_cycle_s then

          if not ack_int_s then

            -- update instruction descriptors in normal mode

            ibyte1_q     <= pm_data_i;

            mnemonic_q   <= mnemonic_s;

            multi_byte_q <= multi_byte_s;

          else

            -- force NOP instruction when vectoring to interrupt routine

            ibyte1_q     <= "01000100";

            mnemonic_q   <= MN_NOP;

            multi_byte_q <= false;

          end if;

        end if;

        -- instruction byte 2 -------------------------------------------------

        if icyc_en_i and not last_cycle_s then

          ibyte2_q     <= pm_data_i;

        end if;

        -- EN register --------------------------------------------------------

        if    set_en_s then

          en_q         <= ibyte2_q(dw_range_t);

        elsif ack_int_s then

          -- reset interrupt enable when INT has been acknowledged

          en_q(1)      <= '0';

        end if;

      end if;

    end if;

  end process seq;

  --

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

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

  -- Opcode multiplexer

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

  opcode_s <=   pm_data_i

              when icyc_en_i else

                ibyte1_q;

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

  -- Opcode decoder table

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

  opc_table_b : t400_opc_table

    generic map (

      opt_type_g   => opt_type_g

    )

    port map (

      opcode_i     => opcode_s,

      mnemonic_o   => mnemonic_s,

      multi_byte_o => multi_byte_s

    );

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

  -- Process decoder_ctrl

  --

  -- Purpose:

  --   Implements the controlling logic of the decoder module.

  --

  decoder_ctrl: process (icyc_en_i,

                         out_en_i, in_en_i,

                         cyc_cnt_q,

                         mnemonic_q, second_cyc_q, last_cycle_s,

                         ibyte1_q, ibyte2_q,

                         skip_i, skip_lbi_i,

                         en_q, int_i,

                         pm_addr_i, pm_data_i)

    variable cyc_v        : natural range 0 to 4;

    variable t41x_type_v,

             t420_type_v  : boolean;

    variable en_int_v     : boolean;

  begin

    -- default assignments

    pc_op_o     <= PC_NONE;

    stack_op_o  <= STACK_NONE;

    dmem_op_o   <= DMEM_RB;             -- default is read via B

    b_op_o      <= B_NONE;

    skip_op_o   <= SKIP_NONE;

    alu_op_o    <= ALU_NONE;

    io_l_op_o   <= IOL_NONE;

    io_d_op_o   <= IOD_NONE;

    io_g_op_o   <= IOG_NONE;

    io_in_op_o  <= IOIN_NONE;

    sio_op_o    <= SIO_NONE;

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

    is_lbi_o    <= false;

    set_en_s    <= false;

    force_mc_s  <= false;

    en_int_v    := true;

    ack_int_s   <= false;

    cyc_v       := to_integer(cyc_cnt_q);

    -- determine type

    t41x_type_v := opt_type_g = t400_opt_type_410_c;

    t420_type_v := opt_type_g = t400_opt_type_420_c;

    if icyc_en_i then

      -- immediately increment program counter

      -- this happens at two occasions:

      -- a) right before new mnemonic becomes valid

      -- b) before the second instruction cycle begins

      pc_op_o   <= PC_INC_PC;

    end if;

    if icyc_en_i and last_cycle_s then

      -- update skip state when last instruction cycle ends

      skip_op_o <= SKIP_UPDATE;

    end if;

    -- skip instruction execution

    if not skip_i then

      -- implement instruction control

      case mnemonic_q is

        -- Mnemonic ASC -------------------------------------------------------

        when MN_ASC =>

          if cyc_v = 1 then

            alu_op_o     <= ALU_ADD_C;

            skip_op_o    <= SKIP_CARRY;

          end if;

        -- Mnemonic ADD -------------------------------------------------------

        when MN_ADD =>

          if cyc_v = 1 then

            alu_op_o     <= ALU_ADD;

          end if;

        -- Mnemonic ADT -------------------------------------------------------

        when MN_ADT =>

          if cyc_v = 1 then

            alu_op_o     <= ALU_ADD_10;

          end if;

        -- Mnemonic AISC ------------------------------------------------------

        when MN_AISC =>

          dec_data_o(dw_range_t) <= ibyte1_q(dw_range_t);

          if cyc_v = 1 then

            alu_op_o     <= ALU_ADD_DEC;

            skip_op_o    <= SKIP_CARRY;

          end if;

        -- Mnemonic CASC ------------------------------------------------------

        when MN_CASC =>

          case cyc_v is

            when 0 =>

              alu_op_o   <= ALU_COMP;

            when 1 =>

              alu_op_o   <= ALU_ADD_C;

              skip_op_o  <= SKIP_CARRY;

            when others =>

              null;

          end case;

        -- Mnemonic CLRA ------------------------------------------------------

        when MN_CLRA =>

          if cyc_v = 1 then

            alu_op_o     <= ALU_CLRA;

          end if;

        -- Mnemonic COMP ------------------------------------------------------

        when MN_COMP =>

          if cyc_v = 1 then

            alu_op_o     <= ALU_COMP;

          end if;

        -- Mnemonic NOP -------------------------------------------------------

        when MN_NOP =>

          -- do nothing

          null;

        -- Mnemonic C ---------------------------------------------------------

        when MN_C =>

          if cyc_v = 1 then

            if ibyte1_q(4) = '1' then

              alu_op_o   <= ALU_RC;

            else

              alu_op_o   <= ALU_SC;

            end if;

          end if;

        -- Mnemonic XOR -------------------------------------------------------

        when MN_XOR =>

          if cyc_v = 1 then

            alu_op_o     <= ALU_XOR;

          end if;

        -- Mnemonic JID -------------------------------------------------------

        when MN_JID =>

          force_mc_s     <= true;

          en_int_v       := false;

          dec_data_o(byte_t'range) <= pm_data_i;

          if cyc_v = 1 then

            if not second_cyc_q then

              -- first cycle: load PC from A and M

              pc_op_o    <= PC_LOAD_A_M;

            else

              -- second cycle: load PC from program memory

              pc_op_o    <= PC_LOAD_8;

            end if;

          end if;

          if icyc_en_i and not second_cyc_q then

            -- do not increment PC for second instruction cycle

            pc_op_o      <= PC_NONE;

          end if;

        -- Mnemonic JMP -------------------------------------------------------

        when MN_JMP =>

          en_int_v       := false;

          dec_data_o <= ibyte1_q(1) & ibyte1_q(0) & ibyte2_q;

          if second_cyc_q and cyc_v = 1 then

            pc_op_o      <= PC_LOAD;

          end if;

        -- Mnemonic JP_JSRP ---------------------------------------------------

        when MN_JP_JSRP =>

          en_int_v       := false;

          -- universal decoder data

          dec_data_o <= '0' & "01" & ibyte1_q(6 downto 0);

          if cyc_v = 1 then

            if    pm_addr_i(9 downto 7) = "001" then

              -- JP within pages 2 & 3

              pc_op_o    <= PC_LOAD_7;

            elsif ibyte1_q(6) = '1' then

              -- JP outside of pages 2 & 3

              pc_op_o    <= PC_LOAD_6;

            else

              -- JSRP to page 2

              pc_op_o    <= PC_LOAD;

              stack_op_o <= STACK_PUSH;

            end if;

          end if;

        -- Mnemonic JSR -------------------------------------------------------

        when MN_JSR =>

          en_int_v       := false;

          dec_data_o <= ibyte1_q(1) & ibyte1_q(0) & ibyte2_q;

          if second_cyc_q and cyc_v = 1 then

            pc_op_o      <= PC_LOAD;

            stack_op_o   <= STACK_PUSH;

          end if;

        -- Mnemonic RET -------------------------------------------------------

        when MN_RET =>

          en_int_v       := false;

          if cyc_v = 1 then

            pc_op_o      <= PC_POP;

            stack_op_o   <= STACK_POP;

            if t420_type_v then

              -- always restore skip state in case this was an interrupt

              skip_op_o  <= SKIP_POP;

            end if;

          end if;

        -- Mnemonic RETSK -----------------------------------------------------

        when MN_RETSK =>

          en_int_v       := false;

          if cyc_v = 1 then

            pc_op_o      <= PC_POP;

            stack_op_o   <= STACK_POP;

            skip_op_o    <= SKIP_NOW;

          end if;

        -- Mnemonic LD --------------------------------------------------------

        when MN_LD =>

          dec_data_o(br_range_t) <= ibyte1_q(br_range_t);

          if cyc_v = 1 then

            alu_op_o     <= ALU_LOAD_M;

            b_op_o       <= B_XOR_BR;

          end if;

        -- Mnemonic LDD_XAD ---------------------------------------------------

        when MN_LDD_XAD =>

          -- preload decoder data

          dec_data_o(b_range_t) <= ibyte2_q(b_range_t);

          if second_cyc_q then

            case ibyte2_q(7 downto 6) is

              -- LDD

              when "00" =>

                if not t41x_type_v then

                  case cyc_v is

                    when 1 =>

                      dmem_op_o <= DMEM_RDEC;

                    when 2 =>

                      alu_op_o  <= ALU_LOAD_M;

                    when others =>

                      null;

                  end case;

                end if;

                -- XAD

              when "10" =>

                if not t41x_type_v                   or

                  unsigned(ibyte2_q(b_range_t)) = 63 then

                  case cyc_v is

                    when 1 =>

                      dmem_op_o <= DMEM_RDEC;

                    when 2 =>

                      alu_op_o  <= ALU_LOAD_M;

                      dmem_op_o <= DMEM_WDEC_SRC_A;

                    when others =>

                      null;

                  end case;

                end if;

              when others =>

                null;

            end case;

          end if;

        -- Mnemonic LQID ------------------------------------------------------

        when MN_LQID =>

          force_mc_s     <= true;

          en_int_v       := false;

          if not second_cyc_q then

            -- first cycle: push PC and set PC from A/M,

            --              read IOL from program memory

            if cyc_v = 1 then

              stack_op_o <= STACK_PUSH;

              pc_op_o    <= PC_LOAD_A_M;

            end if;

            if out_en_i then

              io_l_op_o  <= IOL_LOAD_PM;

            end if;

          else

            if cyc_v = 1 then

              -- second cycle: pop PC

              stack_op_o <= STACK_POP;

              pc_op_o    <= PC_POP;

            end if;

          end if;

          if icyc_en_i and not second_cyc_q then

            -- do not increment PC for second instruction cycle

            pc_op_o      <= PC_NONE;

          end if;

        -- Mnemonic RMB -------------------------------------------------------

        when MN_RMB =>

          if cyc_v = 1 then

            dmem_op_o  <= DMEM_WB_RES_BIT;

            -- select bit to be reset

            case ibyte1_q(dw_range_t) is

              when "1100" =>

                dec_data_o(dw_range_t) <= "0001";

              when "0101" =>

                dec_data_o(dw_range_t) <= "0010";

              when "0010" =>

                dec_data_o(dw_range_t) <= "0100";

              when "0011" =>

                dec_data_o(dw_range_t) <= "1000";

              when others =>

                null;

            end case;

          end if;

        -- Mnemonic SMB -------------------------------------------------------

        when MN_SMB =>

          if cyc_v = 1 then

            dmem_op_o  <= DMEM_WB_SET_BIT;

            -- select bit to be set

            case ibyte1_q(dw_range_t) is

              when "1101" =>

                dec_data_o(dw_range_t) <= "0001";

              when "0111" =>

                dec_data_o(dw_range_t) <= "0010";

              when "0110" =>

                dec_data_o(dw_range_t) <= "0100";

              when "1011" =>

                dec_data_o(dw_range_t) <= "1000";

              when others =>

                null;

            end case;

          end if;

        -- Mnemonic STII ------------------------------------------------------

        when MN_STII =>

          dec_data_o(dw_range_t) <= ibyte1_q(dw_range_t);

          if cyc_v = 1 then

            dmem_op_o    <= DMEM_WB_SRC_DEC;

            b_op_o       <= B_INC_BD;

          end if;

        -- Mnemonic X ---------------------------------------------------------

        when MN_X =>

          dec_data_o(br_range_t) <= ibyte1_q(br_range_t);

          if cyc_v = 1 then

            alu_op_o     <= ALU_LOAD_M;

            dmem_op_o    <= DMEM_WB_SRC_A;

            b_op_o       <= B_XOR_BR;

          end if;

        -- Mnemonic XDS -------------------------------------------------------

        when MN_XDS =>

          dec_data_o(br_range_t) <= ibyte1_q(br_range_t);

          case cyc_v is

            when 1 =>

              alu_op_o   <= ALU_LOAD_M;

              dmem_op_o  <= DMEM_WB_SRC_A;

              b_op_o     <= B_DEC_BD;

            when 2 =>

              b_op_o     <= B_XOR_BR;

              skip_op_o  <= SKIP_BD_UFLOW;

            when others =>

              null;

          end case;

        -- Mnemonic XIS -------------------------------------------------------

        when MN_XIS =>

          dec_data_o(br_range_t) <= ibyte1_q(br_range_t);

          case cyc_v is

            when 1 =>

              alu_op_o   <= ALU_LOAD_M;

              dmem_op_o  <= DMEM_WB_SRC_A;

              b_op_o     <= B_INC_BD;

            when 2 =>

              b_op_o     <= B_XOR_BR;

              skip_op_o  <= SKIP_BD_OFLOW;

            when others =>

              null;

          end case;

        -- Mnemonic CAB -------------------------------------------------------

        when MN_CAB =>

          if cyc_v = 1 then

            b_op_o       <= B_SET_BD;

          end if;

        -- Mnemonic CBA -------------------------------------------------------

        when MN_CBA =>

          if cyc_v = 1 then

            alu_op_o     <= ALU_LOAD_BD;

          end if;

        -- Mnemonic LBI -------------------------------------------------------

        when MN_LBI =>

          is_lbi_o       <= true;

          en_int_v       := false;

          dec_data_o(br_range_t) <= ibyte1_q(br_range_t);

          dec_data_o(bd_range_t) <= ibyte1_q(bd_range_t);

          if cyc_v = 1 and not skip_lbi_i then

            -- increment Bd by 1

            b_op_o       <= B_SET_B_INC;

            skip_op_o    <= SKIP_LBI;

          end if;

        -- Mnemonic XABR ------------------------------------------------------

        when MN_XABR =>

          if cyc_v = 1 then

            alu_op_o     <= ALU_LOAD_BR;

            b_op_o       <= B_SET_BR;

          end if;

        -- Mnemonic SKC -------------------------------------------------------

        when MN_SKC =>

          if cyc_v = 1 then

            skip_op_o    <= SKIP_C;

          end if;

        -- Mnemonic SKE -------------------------------------------------------

        when MN_SKE =>

          if cyc_v = 1 then

            skip_op_o    <= SKIP_A_M;

          end if;

        -- Mnemonic SKMBZ -----------------------------------------------------

        when MN_SKMBZ =>

          if cyc_v = 1 then

            skip_op_o  <= SKIP_M_BIT;

            -- select bit to be checked

            case ibyte1_q is

              when "00000001" =>

                dec_data_o(dw_range_t) <= "0001";

              when "00010001" =>

                dec_data_o(dw_range_t) <= "0010";

              when "00000011" =>

                dec_data_o(dw_range_t) <= "0100";

              when "00010011" =>

                dec_data_o(dw_range_t) <= "1000";

              when others =>

                null;

            end case;

          end if;

        -- Mnemonic SKT -------------------------------------------------------

        when MN_SKT =>

          if cyc_v = 1 then

            skip_op_o  <= SKIP_TIMER;

          end if;

        -- Mnemonic XAS -------------------------------------------------------

        when MN_XAS =>

          if out_en_i then

            sio_op_o <= SIO_LOAD;

            alu_op_o <= ALU_LOAD_SIO;

          end if;

        -- Mnemonic EXT -------------------------------------------------------

        when MN_EXT =>