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-- #################################################################################################

-- # << NEORV32 - Arithmetical/Logical Unit >>                                                     #

-- # ********************************************************************************************* #

-- # Main data and address ALU. Includes comparator unit and co-processor interface/arbiter.       #

-- # The shifter sub-unit uses an iterative approach.                                              #

-- # ********************************************************************************************* #

-- # BSD 3-Clause License                                                                          #

-- #                                                                                               #

-- # Copyright (c) 2020, Stephan Nolting. All rights reserved.                                     #

-- #                                                                                               #

-- # Redistribution and use in source and binary forms, with or without modification, are          #

-- # permitted provided that the following conditions are met:                                     #

-- #                                                                                               #

-- # 1. Redistributions of source code must retain the above copyright notice, this list of        #

-- #    conditions and the following disclaimer.                                                   #

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-- # 2. Redistributions in binary 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.                                                            #

-- #                                                                                               #

-- # 3. Neither the name of the copyright holder nor the names of its contributors may be used to  #

-- #    endorse or promote products derived from this software without specific prior written      #

-- #    permission.                                                                                #

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

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-- # ********************************************************************************************* #

-- # The NEORV32 Processor - https://github.com/stnolting/neorv32              (c) Stephan Nolting #

-- #################################################################################################

library ieee;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

library neorv32;

use neorv32.neorv32_package.all;

entity neorv32_cpu_alu is

  generic (

    CPU_EXTENSION_RISCV_M : boolean := true -- implement muld/div extension?

  );

  port (

    -- global control --

    clk_i       : in  std_ulogic; -- global clock, rising edge

    rstn_i      : in  std_ulogic; -- global reset, low-active, async

    ctrl_i      : in  std_ulogic_vector(ctrl_width_c-1 downto 0); -- main control bus

    -- data input --

    rs1_i       : in  std_ulogic_vector(data_width_c-1 downto 0); -- rf source 1

    rs2_i       : in  std_ulogic_vector(data_width_c-1 downto 0); -- rf source 2

    pc2_i       : in  std_ulogic_vector(data_width_c-1 downto 0); -- delayed PC

    imm_i       : in  std_ulogic_vector(data_width_c-1 downto 0); -- immediate

    csr_i       : in  std_ulogic_vector(data_width_c-1 downto 0); -- csr read data

    -- data output --

    cmp_o       : out std_ulogic_vector(1 downto 0); -- comparator status

    add_o       : out std_ulogic_vector(data_width_c-1 downto 0); -- OPA + OPB

    res_o       : out std_ulogic_vector(data_width_c-1 downto 0); -- ALU result

    -- co-processor interface --

    cp_opa_o    : out std_ulogic_vector(data_width_c-1 downto 0); -- co-processor operand a

    cp_opb_o    : out std_ulogic_vector(data_width_c-1 downto 0); -- co-processor operand b

    cp0_start_o : out std_ulogic; -- trigger co-processor 0

    cp0_data_i  : in  std_ulogic_vector(data_width_c-1 downto 0); -- co-processor 0 result

    cp0_valid_i : in  std_ulogic; -- co-processor 0 result valid

    cp1_start_o : out std_ulogic; -- trigger co-processor 1

    cp1_data_i  : in  std_ulogic_vector(data_width_c-1 downto 0); -- co-processor 1 result

    cp1_valid_i : in  std_ulogic; -- co-processor 1 result valid

    -- status --

    wait_o      : out std_ulogic -- busy due to iterative processing units

  );

end neorv32_cpu_alu;

architecture neorv32_cpu_cpu_rtl of neorv32_cpu_alu is

  -- operands --

  signal opa, opb, opc : std_ulogic_vector(data_width_c-1 downto 0);

  -- results --

  signal add_res : std_ulogic_vector(data_width_c-1 downto 0);

  signal alu_res : std_ulogic_vector(data_width_c-1 downto 0);

  signal cp_res  : std_ulogic_vector(data_width_c-1 downto 0);

  -- comparator --

  signal cmp_opx   : std_ulogic_vector(data_width_c downto 0);

  signal cmp_opy   : std_ulogic_vector(data_width_c downto 0);

  signal cmp_sub   : std_ulogic_vector(data_width_c downto 0);

  signal sub_res   : std_ulogic_vector(data_width_c-1 downto 0);

  signal cmp_equal : std_ulogic;

  signal cmp_less  : std_ulogic;

  -- shifter --

  type shifter_t is record

    cmd    : std_ulogic;

    cmd_ff : std_ulogic;

    start  : std_ulogic;

    run    : std_ulogic;

    halt   : std_ulogic;

    cnt    : std_ulogic_vector(4 downto 0);

    sreg   : std_ulogic_vector(data_width_c-1 downto 0);

  end record;

  signal shifter : shifter_t;

  -- co-processor arbiter and interface --

  type cp_ctrl_t is record

    cmd_ff : std_ulogic;

    busy   : std_ulogic;

    start  : std_ulogic;

    halt   : std_ulogic;

  end record;

  signal cp_ctrl : cp_ctrl_t;

begin

  -- Operand Mux ----------------------------------------------------------------------------

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

  input_op_mux: process(ctrl_i, csr_i, pc2_i, rs1_i, rs2_i, imm_i)

  begin

    -- operand a (first ALU input operand) --

    case ctrl_i(ctrl_alu_opa_mux_msb_c downto ctrl_alu_opa_mux_lsb_c) is

      when "00"   => opa <= rs1_i;

      when "01"   => opa <= pc2_i;

      when others => opa <= csr_i;

    end case;

    -- operand b (second ALU input operand) --

    if (ctrl_i(ctrl_alu_opb_mux_c) = '0') then

      opb <= rs2_i;

    else

      opb <= imm_i;

    end if;

    -- operand c (third ALU input operand for comparison and SUB) --

    if (ctrl_i(ctrl_alu_opc_mux_c) = '0') then

      opc <= rs2_i;

    else

      opc <= imm_i;

    end if;

  end process input_op_mux;

  -- Comparator Unit ------------------------------------------------------------------------

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

  -- less than (x < y) --

  cmp_opx  <= (rs1_i(rs1_i'left) and (not ctrl_i(ctrl_alu_unsigned_c))) & rs1_i;

  cmp_opy  <= (opc(opc'left)     and (not ctrl_i(ctrl_alu_unsigned_c))) & opc;

  cmp_sub  <= std_ulogic_vector(signed(cmp_opx) - signed(cmp_opy));

  cmp_less <= cmp_sub(cmp_sub'left); -- carry (borrow) indicates a "less"

  sub_res  <= cmp_sub(data_width_c-1 downto 0); -- use the less-comparator also for SUB operations

  -- equal (for branch check only) --

  cmp_equal <= '1' when (rs1_i = rs2_i) else '0';

  -- output for branch condition evaluation --

  cmp_o(alu_cmp_equal_c) <= cmp_equal;

  cmp_o(alu_cmp_less_c)  <= cmp_less;

  -- Binary Adder ---------------------------------------------------------------------------

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

  add_res <= std_ulogic_vector(unsigned(opa) + unsigned(opb));

  add_o   <= add_res; -- direct output (for PC modification)

  -- Iterative Shifter Unit -----------------------------------------------------------------

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

  shifter_unit: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      shifter.sreg   <= (others => '0');

      shifter.cnt    <= (others => '0');

      shifter.cmd_ff <= '0';

    elsif rising_edge(clk_i) then

      shifter.cmd_ff <= shifter.cmd;

      if (shifter.start = '1') then -- trigger new shift

        shifter.sreg <= opa; -- shift operand

        shifter.cnt  <= opb(index_size_f(data_width_c)-1 downto 0); -- shift amount

      elsif (shifter.run = '1') then -- running shift

        -- coarse shift: multiples of 4 --

        if (or_all_f(shifter.cnt(shifter.cnt'left downto 2)) = '1') then -- shift amount >= 4

          shifter.cnt <= std_ulogic_vector(unsigned(shifter.cnt) - 4);

          if (ctrl_i(ctrl_alu_shift_dir_c) = '0') then -- SLL: shift left logical

            shifter.sreg <= shifter.sreg(shifter.sreg'left-4 downto 0) & "0000";

          else -- SRL: shift right logical / SRA: shift right arithmetical

            shifter.sreg <= (shifter.sreg(shifter.sreg'left) and ctrl_i(ctrl_alu_shift_ar_c)) &

                            (shifter.sreg(shifter.sreg'left) and ctrl_i(ctrl_alu_shift_ar_c)) &

                            (shifter.sreg(shifter.sreg'left) and ctrl_i(ctrl_alu_shift_ar_c)) &

                            (shifter.sreg(shifter.sreg'left) and ctrl_i(ctrl_alu_shift_ar_c)) & shifter.sreg(shifter.sreg'left downto 4);

          end if;

        -- fine shift: single shifts, 0..3 times --

        else

          shifter.cnt <= std_ulogic_vector(unsigned(shifter.cnt) - 1);

          if (ctrl_i(ctrl_alu_shift_dir_c) = '0') then -- SLL: shift left logical

            shifter.sreg <= shifter.sreg(shifter.sreg'left-1 downto 0) & '0';

          else -- SRL: shift right logical / SRA: shift right arithmetical

            shifter.sreg <= (shifter.sreg(shifter.sreg'left) and ctrl_i(ctrl_alu_shift_ar_c)) & shifter.sreg(shifter.sreg'left downto 1);

          end if;

        end if;

      end if;

    end if;

  end process shifter_unit;

  -- is shift operation? --

  shifter.cmd   <= '1' when (ctrl_i(ctrl_alu_cmd2_c downto ctrl_alu_cmd0_c) = alu_cmd_shift_c) and (ctrl_i(ctrl_cp_use_c) = '0') else '0';

  shifter.start <= '1' when (shifter.cmd = '1') and (shifter.cmd_ff = '0') else '0';

  -- shift operation running? --

  shifter.run  <= '1' when (or_all_f(shifter.cnt) = '1') or (shifter.start = '1') else '0';

  shifter.halt <= '1' when (or_all_f(shifter.cnt(shifter.cnt'left downto 1)) = '1') or (shifter.start = '1') else '0';

  -- Coprocessor Arbiter --------------------------------------------------------------------

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

  cp_arbiter: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      cp_ctrl.cmd_ff <= '0';

      cp_ctrl.busy   <= '0';

    elsif rising_edge(clk_i) then

      if (CPU_EXTENSION_RISCV_M = true) then

        cp_ctrl.cmd_ff <= ctrl_i(ctrl_cp_use_c);

        if (cp_ctrl.start = '1') then

          cp_ctrl.busy <= '1';

        elsif ((cp0_valid_i or cp1_valid_i) = '1') then -- cp computation done?

          cp_ctrl.busy <= '0';

        end if;

      else -- no co-processor(s) implemented

        cp_ctrl.cmd_ff <= '0';

        cp_ctrl.busy   <= '0';

      end if;

    end if;

  end process cp_arbiter;

  -- is co-processor operation? --

  cp_ctrl.start <= '1' when (ctrl_i(ctrl_cp_use_c) = '1') and (cp_ctrl.cmd_ff = '0') else '0';

  cp0_start_o   <= '1' when (cp_ctrl.start = '1') and (ctrl_i(ctrl_cp_id_msb_c downto ctrl_cp_id_lsb_c) = cp_sel_muldiv_c) else '0'; -- MULDIV CP

  cp1_start_o   <= '0'; -- not yet implemented

  -- co-processor operation running? --

  cp_ctrl.halt <= cp_ctrl.busy or cp_ctrl.start;

  -- co-processor operands --

  cp_opa_o <= opa;

  cp_opb_o <= opb;

  -- co-processor result --

  cp_res <= cp0_data_i or cp1_data_i; -- only the selcted cp may output data != 0

  -- ALU Function Select --------------------------------------------------------------------

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

  alu_function_mux: process(ctrl_i, opa, opb, add_res, sub_res, cmp_less, shifter.sreg)

  begin

    case ctrl_i(ctrl_alu_cmd2_c downto ctrl_alu_cmd0_c) is

      when alu_cmd_xor_c   => alu_res <= opa xor opb;

      when alu_cmd_or_c    => alu_res <= opa or  opb;

      when alu_cmd_and_c   => alu_res <= opa and opb;

      when alu_cmd_bclr_c  => alu_res <= opa and (not opb);

      when alu_cmd_sub_c   => alu_res <= sub_res;

      when alu_cmd_add_c   => alu_res <= add_res;

      when alu_cmd_shift_c => alu_res <= shifter.sreg;

      when alu_cmd_slt_c   => alu_res <= (others => '0'); alu_res(0) <= cmp_less;

      when others          => alu_res <= (others => '0'); -- undefined

    end case;

  end process alu_function_mux;

  -- ALU Result -----------------------------------------------------------------------------

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

  wait_o <= shifter.halt or cp_ctrl.halt; -- wait until iterative units have completed

  res_o  <= cp_res when (ctrl_i(ctrl_cp_use_c) = '1') else alu_res; -- FIXME?

end neorv32_cpu_cpu_rtl;