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

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

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

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

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

-- # BSD 3-Clause License                                                                          #

-- #                                                                                               #

-- # Copyright (c) 2021, Stephan Nolting. All rights reserved.                                     #

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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 (

    -- RISC-V CPU Extensions --

    CPU_EXTENSION_RISCV_M     : boolean := false; -- implement mul/div extension?

    CPU_EXTENSION_RISCV_Zmmul : boolean := false; -- implement multiply-only M sub-extension?

    CPU_EXTENSION_RISCV_Zfinx : boolean := false; -- implement 32-bit floating-point extension (using INT reg!)

    -- Extension Options --

    FAST_MUL_EN               : boolean := false; -- use DSPs for M extension's multiplier

    FAST_SHIFT_EN             : boolean := false  -- use barrel shifter for shift operations

  );

  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

    cmp_i       : in  std_ulogic_vector(1 downto 0); -- comparator status

    -- data output --

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

    add_o       : out std_ulogic_vector(data_width_c-1 downto 0); -- address computation result

    fpu_flags_o : out std_ulogic_vector(4 downto 0); -- FPU exception flags

    -- status --

    idone_o     : out std_ulogic -- iterative processing units done?

  );

end neorv32_cpu_alu;

architecture neorv32_cpu_cpu_rtl of neorv32_cpu_alu is

  -- operands --

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

  -- results --

  signal addsub_res : std_ulogic_vector(data_width_c downto 0);

  --

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

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

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

  -- co-processor arbiter and interface --

  type cp_ctrl_t is record

    cmd     : std_ulogic;

    cmd_ff  : std_ulogic;

    start   : std_ulogic;

    busy    : std_ulogic;

    timeout : std_ulogic_vector(9 downto 0);

  end record;

  signal cp_ctrl : cp_ctrl_t;

  -- co-processor interface --

  signal cp_start  : std_ulogic_vector(3 downto 0); -- trigger co-processor i

  signal cp_valid  : std_ulogic_vector(3 downto 0); -- co-processor i done

  signal cp_result : cp_data_if_t; -- co-processor result

begin

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

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

  opa <= pc2_i when (ctrl_i(ctrl_alu_opa_mux_c) = '1') else rs1_i; -- operand a (first ALU input operand), only required for arithmetic ops

  opb <= imm_i when (ctrl_i(ctrl_alu_opb_mux_c) = '1') else rs2_i; -- operand b (second ALU input operand)

  -- Binary Adder/Subtracter ----------------------------------------------------------------

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

  binary_arithmetic_core: process(ctrl_i, opa, opb)

    variable cin_v  : std_ulogic_vector(0 downto 0);

    variable op_a_v : std_ulogic_vector(data_width_c downto 0);

    variable op_b_v : std_ulogic_vector(data_width_c downto 0);

    variable op_y_v : std_ulogic_vector(data_width_c downto 0);

    variable res_v  : std_ulogic_vector(data_width_c downto 0);

  begin

    -- operand sign-extension --

    op_a_v := (opa(opa'left) and (not ctrl_i(ctrl_alu_unsigned_c))) & opa;

    op_b_v := (opb(opb'left) and (not ctrl_i(ctrl_alu_unsigned_c))) & opb;

    -- add/sub(slt) select --

    if (ctrl_i(ctrl_alu_addsub_c) = '1') then -- subtraction

      op_y_v   := not op_b_v;

      cin_v(0) := '1';

    else -- addition

      op_y_v   := op_b_v;

      cin_v(0) := '0';

    end if;

    -- adder core (result + carry/borrow) --

    addsub_res <= std_ulogic_vector(unsigned(op_a_v) + unsigned(op_y_v) + unsigned(cin_v(0 downto 0)));

  end process binary_arithmetic_core;

  -- direct output of address result --

  add_o <= addsub_res(data_width_c-1 downto 0);

  -- ALU arithmetic logic core --

  arithmetic_core: process(ctrl_i, addsub_res)

  begin

    if (ctrl_i(ctrl_alu_arith_c) = alu_arith_cmd_addsub_c) then -- ADD/SUB

      arith_res <= addsub_res(data_width_c-1 downto 0);

    else -- SLT

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

      arith_res(0) <= addsub_res(addsub_res'left); -- => carry/borrow

    end if;

  end process arithmetic_core;

  -- Co-Processor Arbiter -------------------------------------------------------------------

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

  -- Interface:

  -- Co-processor "valid" signal has to be asserted (for one cycle) one cycle before asserting output data

  -- Co-processor "output data" has to be always zero unless co-processor was explicitly triggered

  cp_arbiter: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      cp_ctrl.cmd_ff  <= '0';

      cp_ctrl.busy    <= '0';

      cp_ctrl.timeout <= (others => '0');

    elsif rising_edge(clk_i) then

      cp_ctrl.cmd_ff <= cp_ctrl.cmd;

      -- timeout counter --

      if (cp_ctrl.start = '1') then

        cp_ctrl.busy <= '1';

      elsif (or_reduce_f(cp_valid) = '1') then

        cp_ctrl.busy <= '0';

      end if;

      if (cp_ctrl.busy = '1') and (cp_timeout_en_c = true) then

        cp_ctrl.timeout <= std_ulogic_vector(unsigned(cp_ctrl.timeout) + 1);

      else

        cp_ctrl.timeout <= (others => '0');

      end if;

      if (cp_ctrl.timeout(cp_ctrl.timeout'left) = '1') and (cp_timeout_en_c = true) then -- timeout

        assert false report "NEORV32 CPU CO-PROCESSOR TIMEOUT ERROR!" severity warning;

      end if;

    end if;

  end process cp_arbiter;

  -- is co-processor operation? --

  cp_ctrl.cmd   <= '1' when (ctrl_i(ctrl_alu_func1_c downto ctrl_alu_func0_c) = alu_func_cmd_copro_c) else '0';

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

  -- co-processor select / star trigger --

  cp_start(0) <= '1' when (cp_ctrl.start = '1') and (ctrl_i(ctrl_cp_id_msb_c downto ctrl_cp_id_lsb_c) = "00") else '0';

  cp_start(1) <= '1' when (cp_ctrl.start = '1') and (ctrl_i(ctrl_cp_id_msb_c downto ctrl_cp_id_lsb_c) = "01") else '0';

  cp_start(2) <= '1' when (cp_ctrl.start = '1') and (ctrl_i(ctrl_cp_id_msb_c downto ctrl_cp_id_lsb_c) = "10") else '0';

  cp_start(3) <= '1' when (cp_ctrl.start = '1') and (ctrl_i(ctrl_cp_id_msb_c downto ctrl_cp_id_lsb_c) = "11") else '0';

  -- co-processor operation done? --

  idone_o <= or_reduce_f(cp_valid);

  -- co-processor result - only the *actually selected* co-processor may output data != 0 --

  cp_res <= cp_result(0) or cp_result(1) or cp_result(2) or cp_result(3);

  -- ALU Logic Core -------------------------------------------------------------------------

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

  alu_logic_core: process(ctrl_i, rs1_i, opb)

  begin

    case ctrl_i(ctrl_alu_logic1_c downto ctrl_alu_logic0_c) is

      when alu_logic_cmd_movb_c => logic_res <= opb; -- (default)

      when alu_logic_cmd_xor_c  => logic_res <= rs1_i xor opb; -- only rs1 required for logic ops (opa would also contain pc)

      when alu_logic_cmd_or_c   => logic_res <= rs1_i or  opb;

      when alu_logic_cmd_and_c  => logic_res <= rs1_i and opb;

      when others               => logic_res <= opb; -- undefined

    end case;

  end process alu_logic_core;

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

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

  alu_function_mux: process(ctrl_i, arith_res, logic_res, csr_i, cp_res)

  begin

    case ctrl_i(ctrl_alu_func1_c downto ctrl_alu_func0_c) is

      when alu_func_cmd_arith_c => res_o <= arith_res; -- (default)

      when alu_func_cmd_logic_c => res_o <= logic_res;

      when alu_func_cmd_csrr_c  => res_o <= csr_i;

      when alu_func_cmd_copro_c => res_o <= cp_res;

      when others               => res_o <= arith_res; -- undefined

    end case;

  end process alu_function_mux;

  -- **************************************************************************************************************************

  -- Co-Processors

  -- **************************************************************************************************************************

  -- Co-Processor 0: Shifter (CPU Core ISA) --------------------------------------------------

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

    neorv32_cpu_cp_shifter_inst: neorv32_cpu_cp_shifter

    generic map (

      FAST_SHIFT_EN => FAST_SHIFT_EN -- use barrel shifter for shift operations

    )

    port map (

      -- global control --

      clk_i   => clk_i,           -- global clock, rising edge

      rstn_i  => rstn_i,          -- global reset, low-active, async

      ctrl_i  => ctrl_i,          -- main control bus

      start_i => cp_start(0),     -- trigger operation

      -- data input --

      rs1_i   => rs1_i,           -- rf source 1

      rs2_i   => rs2_i,           -- rf source 2

      imm_i   => imm_i,           -- immediate

      -- result and status --

      res_o   => cp_result(0),    -- operation result

      valid_o => cp_valid(0)      -- data output valid

    );

  -- Co-Processor 1: Integer Multiplication/Division ('M' Extension) ------------------------

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

  neorv32_cpu_cp_muldiv_inst_true:

  if (CPU_EXTENSION_RISCV_M = true) or (CPU_EXTENSION_RISCV_Zmmul = true) generate

    neorv32_cpu_cp_muldiv_inst: neorv32_cpu_cp_muldiv

    generic map (

      FAST_MUL_EN => FAST_MUL_EN,          -- use DSPs for faster multiplication

      DIVISION_EN => CPU_EXTENSION_RISCV_M -- implement divider hardware

    )

    port map (

      -- global control --

      clk_i   => clk_i,           -- global clock, rising edge

      rstn_i  => rstn_i,          -- global reset, low-active, async

      ctrl_i  => ctrl_i,          -- main control bus

      start_i => cp_start(1),     -- trigger operation

      -- data input --

      rs1_i   => rs1_i,           -- rf source 1

      rs2_i   => rs2_i,           -- rf source 2

      -- result and status --

      res_o   => cp_result(1),    -- operation result

      valid_o => cp_valid(1)      -- data output valid

    );

  end generate;

  neorv32_cpu_cp_muldiv_inst_false:

  if (CPU_EXTENSION_RISCV_M = false) and (CPU_EXTENSION_RISCV_Zmmul = false) generate

    cp_result(1) <= (others => '0');

    cp_valid(1)  <= cp_start(1); -- to make sure CPU does not get stalled if there is an accidental access

  end generate;

  -- Co-Processor 2: reserved ---------------------------------------------------------------

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

  cp_result(2) <= (others => '0');

  cp_valid(2)  <= cp_start(2); -- to make sure CPU does not get stalled if there is an accidental access

  -- Co-Processor 3: Single-Precision Floating-Point Unit ('Zfinx' Extension) ---------------

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

  neorv32_cpu_cp_fpu_inst_true:

  if (CPU_EXTENSION_RISCV_Zfinx = true) generate

    neorv32_cpu_cp_fpu_inst: neorv32_cpu_cp_fpu

    port map (

      -- global control --

      clk_i    => clk_i,        -- global clock, rising edge

      rstn_i   => rstn_i,       -- global reset, low-active, async

      ctrl_i   => ctrl_i,       -- main control bus

      start_i  => cp_start(3),  -- trigger operation

      -- data input --

      cmp_i    => cmp_i,        -- comparator status

      rs1_i    => rs1_i,        -- rf source 1

      rs2_i    => rs2_i,        -- rf source 2

      -- result and status --

      res_o    => cp_result(3), -- operation result

      fflags_o => fpu_flags_o,  -- exception flags

      valid_o  => cp_valid(3)   -- data output valid

    );

  end generate;

  neorv32_cpu_cp_fpu_inst_false:

  if (CPU_EXTENSION_RISCV_Zfinx = false) generate

    cp_result(3) <= (others => '0');

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

    cp_valid(3)  <= cp_start(3); -- to make sure CPU does not get stalled if there is an accidental access

  end generate;

end neorv32_cpu_cpu_rtl;