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

-- # << NEORV32 - CPU Co-Processor: Integer Multiplier/Divider Unit (RISC-V "M" Extension) >>      #

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

-- # Multiplier and Divider unit. Implements the RISC-V M & Zmmul CPU extensions.                  #

-- #                                                                                               #

-- # Multiplier core (signed/unsigned) uses classical serial algorithm. Latency = 31+3 cycles.     #

-- # Multiplications can be mapped to DSP blocks (faster!) when FAST_MUL_EN = true.                #

-- # Divider core (unsigned-only) uses classical serial algorithm. latency = 32+4 cycles.          #

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

-- # BSD 3-Clause License                                                                          #

-- #                                                                                               #

-- # Copyright (c) 2022, 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.                                                   #

-- #                                                                                               #

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

-- # 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_cp_muldiv is

  generic (

    FAST_MUL_EN : boolean; -- use DSPs for faster multiplication

    DIVISION_EN : boolean  -- implement divider hardware

  );

  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

    start_i : in  std_ulogic; -- trigger operation

    -- 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

    -- result and status --

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

    valid_o : out std_ulogic -- data output valid

  );

end neorv32_cpu_cp_muldiv;

architecture neorv32_cpu_cp_muldiv_rtl of neorv32_cpu_cp_muldiv is

  -- operations --

  constant cp_op_mul_c    : std_ulogic_vector(2 downto 0) := "000"; -- mul

  constant cp_op_mulh_c   : std_ulogic_vector(2 downto 0) := "001"; -- mulh

  constant cp_op_mulhsu_c : std_ulogic_vector(2 downto 0) := "010"; -- mulhsu

  constant cp_op_mulhu_c  : std_ulogic_vector(2 downto 0) := "011"; -- mulhu

  constant cp_op_div_c    : std_ulogic_vector(2 downto 0) := "100"; -- div

  constant cp_op_divu_c   : std_ulogic_vector(2 downto 0) := "101"; -- divu

  constant cp_op_rem_c    : std_ulogic_vector(2 downto 0) := "110"; -- rem

  constant cp_op_remu_c   : std_ulogic_vector(2 downto 0) := "111"; -- remu

  -- controller --

  type state_t is (IDLE, DIV_PREPROCESS, PROCESSING, FINALIZE);

  signal state         : state_t;

  signal cnt           : std_ulogic_vector(4 downto 0);

  signal cp_op         : std_ulogic_vector(2 downto 0); -- operation to execute

  signal cp_op_ff      : std_ulogic_vector(2 downto 0); -- operation that was executed

  signal start_div     : std_ulogic;

  signal start_mul     : std_ulogic;

  signal operation     : std_ulogic;

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

  signal rs1_is_signed : std_ulogic;

  signal rs2_is_signed : std_ulogic;

  signal div_res_corr  : std_ulogic;

  signal out_en        : std_ulogic;

  -- divider core --

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

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

  signal div_sub          : std_ulogic_vector(data_width_c   downto 0);

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

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

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

  -- multiplier core --

  signal mul_product    : std_ulogic_vector(63 downto 0);

  signal mul_do_add     : std_ulogic_vector(data_width_c downto 0);

  signal mul_sign_cycle : std_ulogic;

  signal mul_p_sext     : std_ulogic;

  signal mul_op_x       : signed(32 downto 0); -- for using DSPs

  signal mul_op_y       : signed(32 downto 0); -- for using DSPs

begin

  -- Co-Processor Controller ----------------------------------------------------------------

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

  coprocessor_ctrl: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      state        <= IDLE;

      div_opy      <= (others => def_rst_val_c);

      cnt          <= (others => def_rst_val_c);

      cp_op_ff     <= (others => def_rst_val_c);

      start_div    <= '0';

      out_en       <= '0';

      valid_o      <= '0';

      div_res_corr <= def_rst_val_c;

    elsif rising_edge(clk_i) then

      -- defaults --

      start_div <= '0';

      out_en    <= '0';

      valid_o   <= '0';

      -- FSM --

      case state is

        when IDLE =>

          cp_op_ff <= cp_op;

          cnt      <= "11110";

          if (start_i = '1') then

            if (operation = '1') and (DIVISION_EN = true) then -- division

              start_div <= '1';

              state     <= DIV_PREPROCESS;

            else -- multiplication

              if (FAST_MUL_EN = true) then

                valid_o <= '1';

                state   <= FINALIZE;

              else

                state <= PROCESSING;

              end if;

            end if;

          end if;

        when DIV_PREPROCESS =>

          -- check relevant input signs for result sign compensation --

          if (cp_op = cp_op_div_c) then -- signed div operation

            div_res_corr <= (rs1_i(rs1_i'left) xor rs2_i(rs2_i'left)) and or_reduce_f(rs2_i); -- different signs AND rs2 not zero

          elsif (cp_op = cp_op_rem_c) then -- signed rem operation

            div_res_corr <= rs1_i(rs1_i'left);

          else

            div_res_corr <= '0';

          end if;

          -- abs(rs2) --

          if ((rs2_i(rs2_i'left) and rs2_is_signed) = '1') then -- signed division?

            div_opy <= std_ulogic_vector(0 - unsigned(rs2_i)); -- make positive

          else

            div_opy <= rs2_i;

          end if;

          state <= PROCESSING;

        when PROCESSING =>

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

          if (cnt = "00000") or (ctrl_i(ctrl_trap_c) = '1') then -- abort on trap

            valid_o <= '1';

            state   <= FINALIZE;

          end if;

        when FINALIZE =>

          out_en <= '1';

          state  <= IDLE;

        when others =>

          state <= IDLE;

      end case;

    end if;

  end process coprocessor_ctrl;

  -- co-processor command --

  cp_op <= ctrl_i(ctrl_ir_funct3_2_c downto ctrl_ir_funct3_0_c);

  -- operation: 0=mul, 1=div --

  operation <= '1' when (cp_op(2) = '1') else '0';

  -- opx (rs1) signed? --

  rs1_is_signed <= '1' when (cp_op = cp_op_mulh_c) or (cp_op = cp_op_mulhsu_c) or (cp_op = cp_op_div_c) or (cp_op = cp_op_rem_c) else '0';

  -- opy (rs2) signed? --

  rs2_is_signed <= '1' when (cp_op = cp_op_mulh_c) or (cp_op = cp_op_div_c) or (cp_op = cp_op_rem_c) else '0';

  -- start MUL operation (do it fast!) --

  start_mul <= '1' when (state = IDLE) and (start_i = '1') and (operation = '0') else '0';

  -- Multiplier Core (signed/unsigned) ------------------------------------------------------

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

  -- iterative multiplication (bit-serial) --

  multiplier_core_serial:

  if (FAST_MUL_EN = false) generate

    multiplier_core: process(rstn_i, clk_i)

    begin

      if (rstn_i = '0') then

        mul_product <= (others => def_rst_val_c);

      elsif rising_edge(clk_i) then

        if (start_mul = '1') then -- start new multiplication

          mul_product(63 downto 32) <= (others => '0');

          mul_product(31 downto 00) <= rs2_i;

        elsif (state = PROCESSING) or (state = FINALIZE) then -- processing step or sign-finalization step

          mul_product(63 downto 31) <= mul_do_add(32 downto 0);

          mul_product(30 downto 00) <= mul_product(31 downto 1);

        end if;

      end if;

    end process multiplier_core;

  end generate;

  -- parallel multiplication (using DSP blocks) --

  multiplier_core_dsp:

  if (FAST_MUL_EN = true) generate

    multiplier_core: process(clk_i)

      variable tmp_v : signed(65 downto 0);

    begin

      if rising_edge(clk_i) then

        if (start_mul = '1') then

          mul_op_x <= signed((rs1_i(rs1_i'left) and rs1_is_signed) & rs1_i);

          mul_op_y <= signed((rs2_i(rs2_i'left) and rs2_is_signed) & rs2_i);

        end if;

        tmp_v := mul_op_x * mul_op_y;

        mul_product <= std_ulogic_vector(tmp_v(63 downto 0));

        --mul_buf_ff  <= mul_op_x * mul_op_y;

        --mul_product <= std_ulogic_vector(mul_buf_ff(63 downto 0)); -- let the register balancing do the magic here

      end if;

    end process multiplier_core;

  end generate;

  -- do another addition (bit-serial) --

  mul_update: process(mul_product, mul_sign_cycle, mul_p_sext, rs1_is_signed, rs1_i)

  begin

    -- current bit of rs2_i to take care of --

    if (mul_product(0) = '1') then -- multiply with 1

      if (mul_sign_cycle = '1') then -- for signed operations only: take care of negative weighted MSB -> multiply with -1

        mul_do_add <= std_ulogic_vector(unsigned(mul_p_sext & mul_product(63 downto 32)) - unsigned((rs1_i(rs1_i'left) and rs1_is_signed) & rs1_i));

      else -- multiply with +1

        mul_do_add <= std_ulogic_vector(unsigned(mul_p_sext & mul_product(63 downto 32)) + unsigned((rs1_i(rs1_i'left) and rs1_is_signed) & rs1_i));

      end if;

    else -- multiply with 0

      mul_do_add <= mul_p_sext & mul_product(63 downto 32);

    end if;

  end process mul_update;

  -- sign control --

  mul_sign_cycle <= rs2_is_signed when (state = FINALIZE) else '0';

  mul_p_sext     <= mul_product(mul_product'left) and rs1_is_signed;

  -- Divider Core (unsigned) ----------------------------------------------------------------

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

  divider_core_serial:

  if (DIVISION_EN = true) generate

    divider_core: process(rstn_i, clk_i)

    begin

      if (rstn_i = '0') then

        quotient  <= (others => def_rst_val_c);

        remainder <= (others => def_rst_val_c);

      elsif rising_edge(clk_i) then

        if (start_div = '1') then -- start new division

          if ((rs1_i(rs1_i'left) and rs1_is_signed) = '1') then -- signed division?

            quotient <= std_ulogic_vector(0 - unsigned(rs1_i)); -- make positive

          else

            quotient <= rs1_i;

          end if;

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

        elsif (state = PROCESSING) or (state = FINALIZE) then -- running?

          quotient <= quotient(30 downto 0) & (not div_sub(32));

          if (div_sub(32) = '0') then -- still overflowing

            remainder <= div_sub(31 downto 0);

          else -- underflow

            remainder <= remainder(30 downto 0) & quotient(31);

          end if;

        end if;

      end if;

    end process divider_core;

    -- try another subtraction --

    div_sub <= std_ulogic_vector(unsigned('0' & remainder(30 downto 0) & quotient(31)) - unsigned('0' & div_opy));

    -- result sign compensation --

    div_sign_comp_in <= quotient when (cp_op = cp_op_div_c) or (cp_op = cp_op_divu_c) else remainder;

    div_sign_comp    <= std_ulogic_vector(0 - unsigned(div_sign_comp_in));

    div_res          <= div_sign_comp when (div_res_corr = '1') else div_sign_comp_in;

  end generate;

  -- no divider --

  divider_core_serial_none:

  if (DIVISION_EN = false) generate

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

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

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

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

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

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

  end generate;

  -- Data Output ----------------------------------------------------------------------------

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

  operation_result: process(out_en, cp_op_ff, mul_product, div_res, div_sign_comp_in)

  begin

    if (out_en = '1') then

      case cp_op_ff is

        when cp_op_mul_c =>

          res_o <= mul_product(31 downto 00);

        when cp_op_mulh_c | cp_op_mulhsu_c | cp_op_mulhu_c =>

          res_o <= mul_product(63 downto 32);

        when cp_op_div_c | cp_op_rem_c =>

          res_o <= div_res;

        when others => -- cp_op_divu_c | cp_op_remu_c

          res_o <= div_sign_comp_in;

      end case;

    else

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

    end if;

  end process operation_result;

end neorv32_cpu_cp_muldiv_rtl;