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

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

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

-- # Main data and address ALU. Include comparator unit.                                           #

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

-- # BSD 3-Clause License                                                                          #

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

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

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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

  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 --

    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_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);

  -- 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 --

  signal shift_cmd    : std_ulogic;

  signal shift_cmd_ff : std_ulogic;

  signal shift_start  : std_ulogic;

  signal shift_run    : std_ulogic;

  signal shift_cnt    : std_ulogic_vector(4 downto 0);

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

  -- co-processor interface --

  signal cp_cmd_ff : std_ulogic;

  signal cp_run    : std_ulogic;

  signal cp_start  : std_ulogic;

  signal cp_busy   : std_ulogic;

  signal cp_rb_ff0 : std_ulogic;

  signal cp_rb_ff1 : std_ulogic;

begin

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

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

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

  begin

    -- opa (first ALU input operand) --

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

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

        opa <= rs1_i;

      else

        opa <= pc2_i;

      end if;

    else

      opa <= csr_i;

    end if;

    -- opb (second ALU input operand) --

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

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

        opb <= rs2_i;

      else

        opb <= imm_i;

      end if;

    else

      opb <= rs1_i;

    end if;

    -- opc (second operand for comparison (and SUB)) --

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

      opc <= imm_i;

    else

      opc <= rs2_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 (x = y) --

  cmp_equal <= '1' when (rs1_i = opc) 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

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

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

  shifter_unit: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

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

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

      shift_cmd_ff <= '0';

    elsif rising_edge(clk_i) then

      shift_cmd_ff <= shift_cmd;

      if (shift_start = '1') then -- trigger new shift

        shift_sreg <= opa; -- shift operand

        shift_cnt  <= opb(4 downto 0); -- shift amount

      elsif (shift_run = '1') then -- running shift

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

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

          shift_sreg <= shift_sreg(shift_sreg'left-1 downto 0) & '0';

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

          shift_sreg <= (shift_sreg(shift_sreg'left) and ctrl_i(ctrl_alu_shift_ar_c)) & shift_sreg(shift_sreg'left downto 1);

        end if;

      end if;

    end if;

  end process shifter_unit;

  -- is shift operation? --

  shift_cmd   <= '1' when (ctrl_i(ctrl_alu_cmd2_c downto ctrl_alu_cmd0_c) = alu_cmd_shift_c) else '0';

  shift_start <= '1' when (shift_cmd = '1') and (shift_cmd_ff = '0') else '0';

  -- shift operation running? --

  shift_run <= '1' when (shift_cnt /= "00000") or (shift_start = '1') else '0';

  -- Coprocessor Interface ------------------------------------------------------------------

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

  cp_interface: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      cp_cmd_ff <= '0';

      cp_busy   <= '0';

      cp_rb_ff0 <= '0';

      cp_rb_ff1 <= '0';

    elsif rising_edge(clk_i) then

      if (ctrl_i(ctrl_sys_m_ext_en_c) = '1') then

        cp_cmd_ff <= ctrl_i(ctrl_cp_use_c);

        cp_rb_ff0 <= '0';

        cp_rb_ff1 <= cp_rb_ff0;

        if (cp_start = '1') then

          cp_busy <= '1';

        elsif ((cp0_valid_i or cp1_valid_i) = '1') then

          cp_busy   <= '0';

          cp_rb_ff0 <= '1';

        end if;

      else -- no co-processors implemented

        cp_cmd_ff <= '0';

        cp_busy   <= '0';

        cp_rb_ff0 <= '0';

        cp_rb_ff1 <= '0';

      end if;

    end if;

  end process cp_interface;

  -- is co-processor operation? --

  cp_start <= '1' when (ctrl_i(ctrl_cp_use_c) = '1') and (cp_cmd_ff = '0') else '0';

  -- co-processor operation running? --

  cp_run <= cp_busy or cp_start;

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

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

  alu_function_mux: process(ctrl_i, opa, opb, add_res, sub_res, cmp_less, shift_sreg)

  begin

    case ctrl_i(ctrl_alu_cmd2_c downto ctrl_alu_cmd0_c) is

      when alu_cmd_bitc_c  => alu_res <= opa and (not opb); -- bit clear (for CSR modification only)

      when alu_cmd_sub_c   => alu_res <= sub_res;

      when alu_cmd_add_c   => alu_res <= add_res;

      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_shift_c => alu_res <= shift_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 <= shift_run or cp_run; -- wait until iterative units have completed

  res_o  <= (cp0_data_i or cp1_data_i) when (cp_rb_ff1 = '1') else alu_res; -- FIXME

end neorv32_cpu_cpu_rtl;