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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) 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_bus is

  generic (

    CPU_EXTENSION_RISCV_C : boolean := true;  -- implement compressed extension?

    -- Physical memory protection (PMP) --

    PMP_NUM_REGIONS       : natural := 0;     -- number of regions (0..64)

    PMP_MIN_GRANULARITY   : natural := 64*1024; -- minimal region granularity in bytes, has to be a power of 2, min 8 bytes

    -- Bus Timeout --

    BUS_TIMEOUT           : natural := 63     -- cycles after an UNACKNOWLEDGED bus access triggers a bus fault exception

  );

  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

    --

    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_cancel_o : out std_ulogic; -- cancel current bus transaction

    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

    i_bus_lock_o   : out std_ulogic; -- locked/exclusive access

    -- 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_cancel_o : out std_ulogic; -- cancel current bus transaction

    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

    d_bus_lock_o   : out std_ulogic  -- locked/exclusive access

  );

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 granularity --

  constant pmp_g_c : natural := index_size_f(PMP_MIN_GRANULARITY);

  -- 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

  -- data interface registers --

  signal mar, mdo, mdi : 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 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

    timeout   : std_ulogic_vector(index_size_f(BUS_TIMEOUT)-1 downto 0);

  end record;

  signal i_arbiter, d_arbiter : bus_arbiter_t;

  -- physical memory protection --

  type pmp_addr_t is array (0 to PMP_NUM_REGIONS-1) of std_ulogic_vector(data_width_c-1 downto 0);

  type pmp_t is record

    addr_mask     : pmp_addr_t;

    region_base   : pmp_addr_t; -- region config base address

    region_i_addr : pmp_addr_t; -- masked instruction access base address for comparator

    region_d_addr : pmp_addr_t; -- masked data access base address for comparator

    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(clk_i)

  begin

    if 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;

  -- read-back for exception controller --

  mar_o <= mar;

  -- alignment check --

  misaligned_d_check: process(mar, ctrl_i)

  begin

    -- check data access --

    d_misaligned <= '0'; -- default

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

      when "00" => -- byte

        d_misaligned <= '0';

      when "01" => -- half-word

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

          d_misaligned <= '1';

        end if;

      when others => -- word

        if (mar(1 downto 0) /= "00") then

          d_misaligned <= '1';

        end if;

    end case;

  end process misaligned_d_check;

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

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

  mem_do_reg: process(clk_i)

  begin

    if rising_edge(clk_i) then

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

        mdo <= wdata_i; -- memory data output register (MDO)

      end if;

    end if;

  end process mem_do_reg;

  -- byte enable and output data alignment --

  byte_enable: process(mar, mdo, ctrl_i)

  begin

    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) <= mdo(07 downto 00);

        d_bus_wdata(15 downto 08) <= mdo(07 downto 00);

        d_bus_wdata(23 downto 16) <= mdo(07 downto 00);

        d_bus_wdata(31 downto 24) <= mdo(07 downto 00);

        case mar(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) <= mdo(15 downto 00);

        d_bus_wdata(15 downto 00) <= mdo(15 downto 00);

        if (mar(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 <= mdo;

        d_bus_ben   <= "1111"; -- full word

    end case;

  end process byte_enable;

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

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

  mem_out_buf: process(clk_i)

  begin

    if rising_edge(clk_i) then

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

        mdi <= d_bus_rdata; -- memory data input register (MDI)

      end if;

    end if;

  end process mem_out_buf;

  -- input data alignment and sign extension --

  read_align: process(mdi, mar, ctrl_i)

    variable byte_in_v  : std_ulogic_vector(07 downto 0);

    variable hword_in_v : std_ulogic_vector(15 downto 0);

  begin

    -- sub-word input --

    case mar(1 downto 0) is

      when "00"   => byte_in_v := mdi(07 downto 00); hword_in_v := mdi(15 downto 00); -- byte 0 / half-word 0

      when "01"   => byte_in_v := mdi(15 downto 08); hword_in_v := mdi(15 downto 00); -- byte 1 / half-word 0

      when "10"   => byte_in_v := mdi(23 downto 16); hword_in_v := mdi(31 downto 16); -- byte 2 / half-word 1

      when others => byte_in_v := mdi(31 downto 24); hword_in_v := mdi(31 downto 16); -- byte 3 / half-word 1

    end case;

    -- actual data size --

    case ctrl_i(ctrl_bus_size_msb_c downto ctrl_bus_size_lsb_c) is

      when "00" => -- byte

        rdata_o(31 downto 08) <= (others => ((not ctrl_i(ctrl_bus_unsigned_c)) and byte_in_v(7))); -- sign extension

        rdata_o(07 downto 00) <= byte_in_v;

      when "01" => -- half-word

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

        rdata_o(15 downto 00) <= hword_in_v; -- high half-word

      when others => -- word

        rdata_o <= mdi; -- full word

    end case;

  end process read_align;

  -- Data Access Arbiter --------------------------------------------------------------------

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

  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';

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

    elsif rising_edge(clk_i) then

      -- data access request --

      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 <= d_misaligned;

        d_arbiter.err_bus   <= '0';

        d_arbiter.timeout   <= std_ulogic_vector(to_unsigned(BUS_TIMEOUT, index_size_f(BUS_TIMEOUT)));

      else -- in progress

        d_arbiter.timeout   <= std_ulogic_vector(unsigned(d_arbiter.timeout) - 1);

        d_arbiter.err_align <= (d_arbiter.err_align or d_misaligned) and (not ctrl_i(ctrl_bus_derr_ack_c));

        d_arbiter.err_bus   <= (d_arbiter.err_bus   or (not or_all_f(d_arbiter.timeout)) or d_bus_err_i or

                                (st_pmp_fault and d_arbiter.wr_req) or (ld_pmp_fault and d_arbiter.rd_req)) and (not ctrl_i(ctrl_bus_derr_ack_c));

        if (d_bus_ack_i = '1') or (ctrl_i(ctrl_bus_derr_ack_c) = '1') then -- wait for normal termination / CPU abort

          d_arbiter.wr_req <= '0';

          d_arbiter.rd_req <= '0';

        end if;

      end if;

    end if;

  end process data_access_arbiter;

  -- cancel bus access --

  d_bus_cancel_o <= (d_arbiter.wr_req or d_arbiter.rd_req) and ctrl_i(ctrl_bus_derr_ack_c);

  -- 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;

  d_bus_lock_o  <= ctrl_i(ctrl_bus_lock_c);

  -- 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;

  -- Instruction Fetch Arbiter --------------------------------------------------------------

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

  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';

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

    elsif rising_edge(clk_i) then

      -- instruction fetch request --

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

        i_arbiter.rd_req    <= ctrl_i(ctrl_bus_if_c);

        i_arbiter.err_align <= i_misaligned;

        i_arbiter.err_bus   <= '0';

        i_arbiter.timeout   <= std_ulogic_vector(to_unsigned(BUS_TIMEOUT, index_size_f(BUS_TIMEOUT)));

      else -- in progress

        i_arbiter.timeout   <= std_ulogic_vector(unsigned(i_arbiter.timeout) - 1);

        i_arbiter.err_align <= (i_arbiter.err_align or i_misaligned)                                                     and (not ctrl_i(ctrl_bus_ierr_ack_c));

        i_arbiter.err_bus   <= (i_arbiter.err_bus   or (not or_all_f(i_arbiter.timeout)) or i_bus_err_i or if_pmp_fault) and (not ctrl_i(ctrl_bus_ierr_ack_c));

        if (i_bus_ack_i = '1') or (ctrl_i(ctrl_bus_ierr_ack_c) = '1') then -- wait for normal termination / CPU abort

          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

  -- cancel bus access --

  i_bus_cancel_o <= i_arbiter.rd_req and ctrl_i(ctrl_bus_ierr_ack_c);

  -- 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;

  i_bus_lock_o  <= '0'; -- instruction fetch cannot be atomic

  -- check instruction access --

  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) -------------------------------------------------------

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

  -- compute address masks (ITERATIVE!!!) --

  pmp_masks: process(clk_i)

  begin

    if rising_edge(clk_i) then -- address mask computation (not the actual address check!) has a latency of max +32 cycles

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

        pmp.addr_mask(r) <= (others => '0');

        for i in pmp_g_c to data_width_c-1 loop

          pmp.addr_mask(r)(i) <= pmp.addr_mask(r)(i-1) or (not pmp_addr_i(r)(i-1));

        end loop; -- i

      end loop; -- r

    end if;

  end process pmp_masks;

  -- address access check --

  pmp_address_check:

  for r in 0 to PMP_NUM_REGIONS-1 generate -- iterate over all regions

    pmp.region_i_addr(r) <= fetch_pc_i                             and pmp.addr_mask(r);

    pmp.region_d_addr(r) <= mar                                    and pmp.addr_mask(r);

    pmp.region_base(r)   <= pmp_addr_i(r)(data_width_c+1 downto 2) and pmp.addr_mask(r);

    --

    pmp.i_match(r) <= '1' when (pmp.region_i_addr(r)(data_width_c-1 downto pmp_g_c) = pmp.region_base(r)(data_width_c-1 downto pmp_g_c)) else '0';

    pmp.d_match(r) <= '1' when (pmp.region_d_addr(r)(data_width_c-1 downto pmp_g_c) = pmp.region_base(r)(data_width_c-1 downto pmp_g_c)) else '0';

  end generate; -- r

  -- check access type and regions's permissions --

  pmp_check_permission: process(pmp, pmp_ctrl_i, ctrl_i)

  begin

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

      if ((ctrl_i(ctrl_priv_lvl_msb_c downto ctrl_priv_lvl_lsb_c) = priv_mode_u_c) or (pmp_ctrl_i(r)(pmp_cfg_l_c) = '1')) and -- user privilege level or locked pmp entry -> enforce permissions also for machine mode

         (pmp_ctrl_i(r)(pmp_cfg_ah_c downto pmp_cfg_al_c) /= pmp_off_mode_c) then -- active entry

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

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

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

      else

        pmp.if_fault(r) <= '0';

        pmp.ld_fault(r) <= '0';

        pmp.st_fault(r) <= '0';

      end if;

    end loop; -- r

  end process pmp_check_permission;

  -- final PMP access fault signals --

  if_pmp_fault <= or_all_f(pmp.if_fault) when (PMP_NUM_REGIONS > 0) else '0';

  ld_pmp_fault <= or_all_f(pmp.ld_fault) when (PMP_NUM_REGIONS > 0) else '0';

  st_pmp_fault <= or_all_f(pmp.st_fault) when (PMP_NUM_REGIONS > 0) else '0';

end neorv32_cpu_bus_rtl;