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

-- # << NEORV32 - Bus Interface Unit >>                                                            #

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

-- # Instruction and data bus interfaces and physical memory protection (PMP).                     #

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

-- # BSD 3-Clause License                                                                          #

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-- # Copyright (c) 2022, 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_bus is

  generic (

    CPU_EXTENSION_RISCV_A : boolean; -- implement atomic extension?

    CPU_EXTENSION_RISCV_C : boolean; -- implement compressed extension?

    -- Physical memory protection (PMP) --

    PMP_NUM_REGIONS       : natural; -- number of regions (0..16)

    PMP_MIN_GRANULARITY   : natural  -- minimal region granularity in bytes, has to be a power of 2, min 4 bytes

  );

  port (

    -- global control --

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

    rstn_i        : in  std_ulogic := '0'; -- global reset, low-active, async

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

    -- cpu instruction fetch interface --

    fetch_pc_i    : in  std_ulogic_vector(data_width_c-1 downto 0); -- PC for instruction fetch

    instr_o       : out std_ulogic_vector(data_width_c-1 downto 0); -- instruction

    i_wait_o      : out std_ulogic; -- wait for fetch to complete

    --

    ma_instr_o    : out std_ulogic; -- misaligned instruction address

    be_instr_o    : out std_ulogic; -- bus error on instruction access

    -- cpu data access interface --

    addr_i        : in  std_ulogic_vector(data_width_c-1 downto 0); -- ALU result -> access address

    wdata_i       : in  std_ulogic_vector(data_width_c-1 downto 0); -- write data

    rdata_o       : out std_ulogic_vector(data_width_c-1 downto 0); -- read data

    mar_o         : out std_ulogic_vector(data_width_c-1 downto 0); -- current memory address register

    d_wait_o      : out std_ulogic; -- wait for access to complete

    --

    excl_state_o  : out std_ulogic; -- atomic/exclusive access status

    ma_load_o     : out std_ulogic; -- misaligned load data address

    ma_store_o    : out std_ulogic; -- misaligned store data address

    be_load_o     : out std_ulogic; -- bus error on load data access

    be_store_o    : out std_ulogic; -- bus error on store data access

    -- physical memory protection --

    pmp_addr_i    : in  pmp_addr_if_t; -- addresses

    pmp_ctrl_i    : in  pmp_ctrl_if_t; -- configs

    -- instruction bus --

    i_bus_addr_o  : out std_ulogic_vector(data_width_c-1 downto 0); -- bus access address

    i_bus_rdata_i : in  std_ulogic_vector(data_width_c-1 downto 0); -- bus read data

    i_bus_wdata_o : out std_ulogic_vector(data_width_c-1 downto 0); -- bus write data

    i_bus_ben_o   : out std_ulogic_vector(03 downto 0); -- byte enable

    i_bus_we_o    : out std_ulogic; -- write enable

    i_bus_re_o    : out std_ulogic; -- read enable

    i_bus_lock_o  : out std_ulogic; -- exclusive access request

    i_bus_ack_i   : in  std_ulogic; -- bus transfer acknowledge

    i_bus_err_i   : in  std_ulogic; -- bus transfer error

    i_bus_fence_o : out std_ulogic; -- fence operation

    -- data bus --

    d_bus_addr_o  : out std_ulogic_vector(data_width_c-1 downto 0); -- bus access address

    d_bus_rdata_i : in  std_ulogic_vector(data_width_c-1 downto 0); -- bus read data

    d_bus_wdata_o : out std_ulogic_vector(data_width_c-1 downto 0); -- bus write data

    d_bus_ben_o   : out std_ulogic_vector(03 downto 0); -- byte enable

    d_bus_we_o    : out std_ulogic; -- write enable

    d_bus_re_o    : out std_ulogic; -- read enable

    d_bus_lock_o  : out std_ulogic; -- exclusive access request

    d_bus_ack_i   : in  std_ulogic; -- bus transfer acknowledge

    d_bus_err_i   : in  std_ulogic; -- bus transfer error

    d_bus_fence_o : out std_ulogic  -- fence operation

  );

end neorv32_cpu_bus;

architecture neorv32_cpu_bus_rtl of neorv32_cpu_bus is

  -- PMP modes --

  constant pmp_off_mode_c   : std_ulogic_vector(1 downto 0) := "00"; -- null region (disabled)

  constant pmp_tor_mode_c   : std_ulogic_vector(1 downto 0) := "01"; -- top of range

--constant pmp_na4_mode_c   : std_ulogic_vector(1 downto 0) := "10"; -- naturally aligned four-byte region

--constant pmp_napot_mode_c : std_ulogic_vector(1 downto 0) := "11"; -- naturally aligned power-of-two region (>= 8 bytes)

  -- PMP configuration register bits --

  constant pmp_cfg_r_c  : natural := 0; -- read permit

  constant pmp_cfg_w_c  : natural := 1; -- write permit

  constant pmp_cfg_x_c  : natural := 2; -- execute permit

  constant pmp_cfg_al_c : natural := 3; -- mode bit low

  constant pmp_cfg_ah_c : natural := 4; -- mode bit high

  constant pmp_cfg_l_c  : natural := 7; -- locked entry

  -- PMP minimal granularity --

  constant pmp_lsb_c : natural := index_size_f(PMP_MIN_GRANULARITY);

  -- data memory address register --

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

  -- data access --

  signal d_bus_wdata : std_ulogic_vector(data_width_c-1 downto 0); -- write data

  signal d_bus_rdata : std_ulogic_vector(data_width_c-1 downto 0); -- read data

  signal rdata_align : std_ulogic_vector(data_width_c-1 downto 0); -- read-data alignment

  signal d_bus_ben   : std_ulogic_vector(3 downto 0); -- write data byte enable

  -- misaligned access? --

  signal d_misaligned, i_misaligned : std_ulogic;

  -- bus arbiter --

  type bus_arbiter_t is record

    rd_req    : std_ulogic; -- read access in progress

    wr_req    : std_ulogic; -- write access in progress

    err_align : std_ulogic; -- alignment error

    err_bus   : std_ulogic; -- bus access error

  end record;

  signal i_arbiter, d_arbiter : bus_arbiter_t;

  -- atomic/exclusive access - reservation controller --

  signal exclusive_lock        : std_ulogic;

  signal exclusive_lock_status : std_ulogic_vector(data_width_c-1 downto 0); -- read data

  -- physical memory protection --

  type pmp_t is record

    i_match  : std_ulogic_vector(PMP_NUM_REGIONS-1 downto 0); -- region match for instruction interface

    d_match  : std_ulogic_vector(PMP_NUM_REGIONS-1 downto 0); -- region match for data interface

    if_fault : std_ulogic_vector(PMP_NUM_REGIONS-1 downto 0); -- region access fault for fetch operation

    ld_fault : std_ulogic_vector(PMP_NUM_REGIONS-1 downto 0); -- region access fault for load operation

    st_fault : std_ulogic_vector(PMP_NUM_REGIONS-1 downto 0); -- region access fault for store operation

  end record;

  signal pmp : pmp_t;

  -- memory control signal buffer (when using PMP) --

  signal d_bus_we, d_bus_we_buf : std_ulogic;

  signal d_bus_re, d_bus_re_buf : std_ulogic;

  signal i_bus_re, i_bus_re_buf : std_ulogic;

  -- pmp faults anyone? --

  signal if_pmp_fault : std_ulogic; -- pmp instruction access fault

  signal ld_pmp_fault : std_ulogic; -- pmp load access fault

  signal st_pmp_fault : std_ulogic; -- pmp store access fault

begin

  -- Sanity Checks --------------------------------------------------------------------------

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

  assert not (PMP_NUM_REGIONS > pmp_num_regions_critical_c) report "NEORV32 CPU CONFIG WARNING! Number of implemented PMP regions (PMP_NUM_REGIONS = " &

  integer'image(PMP_NUM_REGIONS) & ") beyond critical limit (pmp_num_regions_critical_c = " & integer'image(pmp_num_regions_critical_c) &

  "). Inserting another register stage (that will increase memory latency by +1 cycle)." severity warning;

  -- Data Interface: Access Address ---------------------------------------------------------

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

  mem_adr_reg: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      mar <= (others => def_rst_val_c);

    elsif rising_edge(clk_i) then

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

        mar <= addr_i;

      end if;

    end if;

  end process mem_adr_reg;

  -- address read-back for exception controller --

  mar_o <= mar;

  -- Data Interface: Write Data -------------------------------------------------------------

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

  mem_do_reg: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      d_bus_wdata <= (others => def_rst_val_c);

      d_bus_ben   <= (others => def_rst_val_c);

    elsif rising_edge(clk_i) then

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

        -- byte enable and data alignment --

        case ctrl_i(ctrl_bus_size_msb_c downto ctrl_bus_size_lsb_c) is -- data size

          when "00" => -- byte

            d_bus_wdata(07 downto 00) <= wdata_i(7 downto 0);

            d_bus_wdata(15 downto 08) <= wdata_i(7 downto 0);

            d_bus_wdata(23 downto 16) <= wdata_i(7 downto 0);

            d_bus_wdata(31 downto 24) <= wdata_i(7 downto 0);

            case addr_i(1 downto 0) is

              when "00"   => d_bus_ben <= "0001";

              when "01"   => d_bus_ben <= "0010";

              when "10"   => d_bus_ben <= "0100";

              when others => d_bus_ben <= "1000";

            end case;

          when "01" => -- half-word

            d_bus_wdata(31 downto 16) <= wdata_i(15 downto 0);

            d_bus_wdata(15 downto 00) <= wdata_i(15 downto 0);

            if (addr_i(1) = '0') then

              d_bus_ben <= "0011"; -- low half-word

            else

              d_bus_ben <= "1100"; -- high half-word

            end if;

          when others => -- word

            d_bus_wdata <= wdata_i;

            d_bus_ben   <= "1111"; -- full word

        end case;

      end if;

    end if;

  end process mem_do_reg;

  -- Data Interface: Read Data --------------------------------------------------------------

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

  read_align: process(rstn_i, clk_i)

    variable shifted_data_v : std_ulogic_vector(31 downto 0);

  begin

    if (rstn_i = '0') then

      rdata_align <= (others => def_rst_val_c);

    elsif rising_edge(clk_i) then

      -- input data alignment and sign extension --

      case ctrl_i(ctrl_bus_size_msb_c downto ctrl_bus_size_lsb_c) is

        when "00" => -- byte

          case mar(1 downto 0) is

            when "00" => -- byte 0

              rdata_align(07 downto 00) <= d_bus_rdata(07 downto 00);

              rdata_align(31 downto 08) <= (others => ((not ctrl_i(ctrl_bus_unsigned_c)) and d_bus_rdata(07))); -- sign extension

            when "01" => -- byte 1

              rdata_align(07 downto 00) <= d_bus_rdata(15 downto 08);

              rdata_align(31 downto 08) <= (others => ((not ctrl_i(ctrl_bus_unsigned_c)) and d_bus_rdata(15))); -- sign extension

            when "10" => -- byte 2

              rdata_align(07 downto 00) <= d_bus_rdata(23 downto 16);

              rdata_align(31 downto 08) <= (others => ((not ctrl_i(ctrl_bus_unsigned_c)) and d_bus_rdata(23))); -- sign extension

            when others => -- byte 3

              rdata_align(07 downto 00) <= d_bus_rdata(31 downto 24);

              rdata_align(31 downto 08) <= (others => ((not ctrl_i(ctrl_bus_unsigned_c)) and d_bus_rdata(31))); -- sign extension

          end case;

        when "01" => -- half-word

          if (mar(1) = '0') then

            rdata_align(15 downto 00) <= d_bus_rdata(15 downto 00); -- low half-word

            rdata_align(31 downto 16) <= (others => ((not ctrl_i(ctrl_bus_unsigned_c)) and d_bus_rdata(15))); -- sign extension

          else

            rdata_align(15 downto 00) <= d_bus_rdata(31 downto 16); -- high half-word

            rdata_align(31 downto 16) <= (others => ((not ctrl_i(ctrl_bus_unsigned_c)) and d_bus_rdata(31))); -- sign extension

          end if;

        when others => -- word

          rdata_align <= d_bus_rdata; -- full word

      end case;

    end if;

  end process read_align;

  -- insert exclusive lock status for SC operations only --

  rdata_o <= exclusive_lock_status when (CPU_EXTENSION_RISCV_A = true) and (ctrl_i(ctrl_bus_ch_lock_c) = '1') else rdata_align;

  -- Data Interface: Arbiter (controlled by pipeline back-end) ------------------------------

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

  data_access_arbiter: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      d_arbiter.wr_req    <= '0';

      d_arbiter.rd_req    <= '0';

      d_arbiter.err_align <= '0';

      d_arbiter.err_bus   <= '0';

    elsif rising_edge(clk_i) then

      if (d_arbiter.wr_req = '0') and (d_arbiter.rd_req = '0') then -- idle

        d_arbiter.wr_req    <= ctrl_i(ctrl_bus_wr_c);

        d_arbiter.rd_req    <= ctrl_i(ctrl_bus_rd_c);

        d_arbiter.err_align <= '0';

        d_arbiter.err_bus   <= '0';

      else -- in progress, accumulate errors

        d_arbiter.err_align <= d_arbiter.err_align or d_misaligned;

        d_arbiter.err_bus   <= d_arbiter.err_bus or d_bus_err_i or (st_pmp_fault and d_arbiter.wr_req) or (ld_pmp_fault and d_arbiter.rd_req);

        if (d_bus_ack_i = '1') or (ctrl_i(ctrl_trap_c) = '1') then -- wait for ACK or TRAP

          -- > do not abort directly when an error has been detected - wait until the trap environment

          -- > has started (ctrl_i(ctrl_trap_c)) to make sure the error signals are evaluated BEFORE d_wait_o clears

          d_arbiter.wr_req <= '0';

          d_arbiter.rd_req <= '0';

        end if;

      end if;

    end if;

  end process data_access_arbiter;

  -- wait for bus transaction to finish --

  d_wait_o <= (d_arbiter.wr_req or d_arbiter.rd_req) and (not d_bus_ack_i);

  -- output data access error to controller --

  ma_load_o  <= d_arbiter.rd_req and d_arbiter.err_align;

  be_load_o  <= d_arbiter.rd_req and d_arbiter.err_bus;

  ma_store_o <= d_arbiter.wr_req and d_arbiter.err_align;

  be_store_o <= d_arbiter.wr_req and d_arbiter.err_bus;

  -- data bus (read/write)--

  d_bus_addr_o  <= mar;

  d_bus_wdata_o <= d_bus_wdata;

  d_bus_ben_o   <= d_bus_ben;

  d_bus_we      <= ctrl_i(ctrl_bus_wr_c) and (not d_misaligned) and (not st_pmp_fault); -- no actual write when misaligned or PMP fault

  d_bus_re      <= ctrl_i(ctrl_bus_rd_c) and (not d_misaligned) and (not ld_pmp_fault); -- no actual read when misaligned or PMP fault

  d_bus_we_o    <= d_bus_we_buf when (PMP_NUM_REGIONS > pmp_num_regions_critical_c) else d_bus_we;

  d_bus_re_o    <= d_bus_re_buf when (PMP_NUM_REGIONS > pmp_num_regions_critical_c) else d_bus_re;

  d_bus_fence_o <= ctrl_i(ctrl_bus_fence_c);

  d_bus_rdata   <= d_bus_rdata_i;

  -- check data access address alignment --

  misaligned_d_check: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      d_misaligned <= def_rst_val_c;

    elsif rising_edge(clk_i) then

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

        case ctrl_i(ctrl_bus_size_msb_c downto ctrl_bus_size_lsb_c) is -- data size

          when "00"   => d_misaligned <= '0'; -- byte

          when "01"   => d_misaligned <= addr_i(0); -- half-word

          when others => d_misaligned <= addr_i(1) or addr_i(0); -- word

        end case;

      end if;

    end if;

  end process misaligned_d_check;

  -- additional register stage for control signals if using PMP_NUM_REGIONS > pmp_num_regions_critical_c --

  pmp_dbus_buffer: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      d_bus_we_buf <= '0';

      d_bus_re_buf <= '0';

    elsif rising_edge(clk_i) then

      d_bus_we_buf <= d_bus_we;

      d_bus_re_buf <= d_bus_re;

    end if;

  end process pmp_dbus_buffer;

  -- Reservation Controller (LR/SC [A extension]) -------------------------------------------

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

  exclusive_access_controller: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      exclusive_lock <= '0';

    elsif rising_edge(clk_i) then

      if (CPU_EXTENSION_RISCV_A = true) then

        -- remove lock if entering a trap or executing a non-load-reservate memory access --

        if (ctrl_i(ctrl_trap_c) = '1') or (ctrl_i(ctrl_bus_de_lock_c) = '1') then

          exclusive_lock <= '0';

        elsif (ctrl_i(ctrl_bus_lock_c) = '1') then -- set new lock

          exclusive_lock <= '1';

        end if;

      else

        exclusive_lock <= '0';

      end if;

    end if;

  end process exclusive_access_controller;

  -- lock status for SC operation --

  exclusive_lock_status(data_width_c-1 downto 1) <= (others => '0');

  exclusive_lock_status(0) <= not exclusive_lock;

  -- output reservation status to control unit (to check if SC should write at all) --

  excl_state_o <= exclusive_lock;

  -- output to memory system --

  i_bus_lock_o <= '0'; -- instruction fetches cannot be locked

  d_bus_lock_o <= exclusive_lock;

  -- Instruction Interface: Arbiter (controlled by pipeline front-end) ----------------------

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

  ifetch_arbiter: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      i_arbiter.rd_req    <= '0';

      i_arbiter.err_align <= '0';

      i_arbiter.err_bus   <= '0';

    elsif rising_edge(clk_i) then

      if (i_arbiter.rd_req = '0') then -- idle

        i_arbiter.rd_req    <= ctrl_i(ctrl_bus_if_c);

        i_arbiter.err_align <= '0';

        i_arbiter.err_bus   <= '0';

      else -- in progress, accumulate errors

        i_arbiter.err_align <= i_arbiter.err_align or i_misaligned;

        i_arbiter.err_bus   <= i_arbiter.err_bus or i_bus_err_i or if_pmp_fault;

        if (i_bus_ack_i = '1') or (i_arbiter.err_align = '1') or (i_arbiter.err_bus = '1') then -- wait for ACK or ERROR

          i_arbiter.rd_req <= '0';

        end if;

      end if;

    end if;

  end process ifetch_arbiter;

  i_arbiter.wr_req <= '0'; -- instruction fetch is read-only

  -- wait for bus transaction to finish --

  i_wait_o <= i_arbiter.rd_req and (not i_bus_ack_i);

  -- output instruction fetch error to controller --

  ma_instr_o <= i_arbiter.err_align;

  be_instr_o <= i_arbiter.err_bus;

  -- instruction bus (read-only) --

  i_bus_addr_o  <= fetch_pc_i(data_width_c-1 downto 2) & "00"; -- instruction access is always 4-byte aligned (even for compressed instructions)

  i_bus_wdata_o <= (others => '0'); -- instruction fetch is read-only

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

  i_bus_we_o    <= '0';

  i_bus_re      <= ctrl_i(ctrl_bus_if_c) and (not i_misaligned) and (not if_pmp_fault); -- no actual read when misaligned or PMP fault

  i_bus_re_o    <= i_bus_re_buf when (PMP_NUM_REGIONS > pmp_num_regions_critical_c) else i_bus_re;

  i_bus_fence_o <= ctrl_i(ctrl_bus_fencei_c);

  instr_o       <= i_bus_rdata_i;

  -- check instruction access address alignment --

  i_misaligned <= '0' when (CPU_EXTENSION_RISCV_C = true) else -- no alignment exceptions possible when using C-extension

                  '1' when (fetch_pc_i(1) = '1') else '0'; -- 32-bit accesses only

  -- additional register stage for control signals if using PMP_NUM_REGIONS > pmp_num_regions_critical_c --

  pmp_ibus_buffer: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      i_bus_re_buf <= '0';

    elsif rising_edge(clk_i) then

      i_bus_re_buf <= i_bus_re;

    end if;

  end process pmp_ibus_buffer;

  -- Physical Memory Protection (PMP) -------------------------------------------------------

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

  -- check address region --

  pmp_check_address: process(pmp_addr_i, fetch_pc_i, mar)

  begin

    for i in 0 to PMP_NUM_REGIONS-1 loop -- iterate over all regions

      if (i = 0) then -- use ZERO as bottom boundary and current entry as top boundary for first entry

        pmp.i_match(i) <= bool_to_ulogic_f(unsigned(fetch_pc_i(data_width_c-1 downto pmp_lsb_c)) < unsigned(pmp_addr_i(0)(data_width_c-1 downto pmp_lsb_c)));

        pmp.d_match(i) <= bool_to_ulogic_f(unsigned(mar(data_width_c-1 downto pmp_lsb_c))        < unsigned(pmp_addr_i(0)(data_width_c-1 downto pmp_lsb_c)));

      else -- use previous entry as bottom boundary and current entry as top boundary

        pmp.i_match(i) <= bool_to_ulogic_f((unsigned(pmp_addr_i(i-1)(data_width_c-1 downto pmp_lsb_c)) <= unsigned(fetch_pc_i(data_width_c-1 downto pmp_lsb_c))) and

                                           (unsigned(fetch_pc_i(data_width_c-1 downto pmp_lsb_c))      <  unsigned(pmp_addr_i(i)(data_width_c-1 downto pmp_lsb_c))));

        pmp.d_match(i) <= bool_to_ulogic_f((unsigned(pmp_addr_i(i-1)(data_width_c-1 downto pmp_lsb_c)) <= unsigned(mar(data_width_c-1 downto pmp_lsb_c))) and

                                           (unsigned(mar(data_width_c-1 downto pmp_lsb_c))             <  unsigned(pmp_addr_i(i)(data_width_c-1 downto pmp_lsb_c))));

      end if;

    end loop; -- i

  end process pmp_check_address;

  -- check access type and permissions --

  pmp_check_permission: process(pmp, pmp_ctrl_i, ctrl_i)

  begin

    pmp.if_fault <= (others => '0');

    pmp.ld_fault <= (others => '0');

    pmp.st_fault <= (others => '0');

    for i in 0 to PMP_NUM_REGIONS-1 loop -- iterate over all regions

      if ((ctrl_i(ctrl_priv_mode_c) = priv_mode_u_c) or (pmp_ctrl_i(i)(pmp_cfg_l_c) = '1')) and -- enforce if USER-mode or LOCKED

         (pmp_ctrl_i(i)(pmp_cfg_ah_c downto pmp_cfg_al_c) = pmp_tor_mode_c) and -- active entry

         (ctrl_i(ctrl_debug_running_c) = '0') then -- disable PMP checks when in debug mode

        pmp.if_fault(i) <= pmp.i_match(i) and (not pmp_ctrl_i(i)(pmp_cfg_x_c)); -- fetch access match no execute permission

        pmp.ld_fault(i) <= pmp.d_match(i) and (not pmp_ctrl_i(i)(pmp_cfg_r_c)); -- load access match no read permission

        pmp.st_fault(i) <= pmp.d_match(i) and (not pmp_ctrl_i(i)(pmp_cfg_w_c)); -- store access match no write permission

      end if;

    end loop; -- i

  end process pmp_check_permission;

  -- final PMP access fault signals --

  if_pmp_fault <= '1' when (or_reduce_f(pmp.if_fault) = '1') and (PMP_NUM_REGIONS > 0) else '0';

  ld_pmp_fault <= '1' when (or_reduce_f(pmp.ld_fault) = '1') and (PMP_NUM_REGIONS > 0) else '0';

  st_pmp_fault <= '1' when (or_reduce_f(pmp.st_fault) = '1') and (PMP_NUM_REGIONS > 0) else '0';

end neorv32_cpu_bus_rtl;