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-- #################################################################################################
-- # << NEORV32 - Bus Interface Unit >>                                                            #
-- # ********************************************************************************************* #
-- # Instruction and data bus interfaces and physical memory protection (PMP).                     #
-- # ********************************************************************************************* #
-- # BSD 3-Clause License                                                                          #
-- #                                                                                               #
-- # Copyright (c) 2020, 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?
    BUS_TIMEOUT           : natural := 15;   -- cycles after which a valid bus access will timeout
    -- Physical memory protection (PMP) --
    PMP_USE               : boolean := false; -- implement physical memory protection?
    PMP_NUM_REGIONS       : natural := 4; -- number of regions (1..4)
    PMP_GRANULARITY       : natural := 16 -- granularity (1=8B, 2=16B, 3=32B, ...)
  );
  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
    -- 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
    priv_mode_i    : in  std_ulogic_vector(1 downto 0); -- current CPU privilege level
    -- 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
    -- 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
  );
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
 
  -- 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_addr34_t is array (0 to PMP_NUM_REGIONS-1) of std_ulogic_vector(data_width_c+1 downto 0);
  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_addr34_t; -- 34-bit physical address
    region_base   : pmp_addr_t; -- masked region base address for comparator
    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;
 
  -- pmp faults anybody? --
  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
 
  -- Data Interface: Access Address ---------------------------------------------------------
  -- -------------------------------------------------------------------------------------------
  mem_adr_reg: process(rstn_i, clk_i)
  begin
    if rising_edge(clk_i) then
      if (ctrl_i(ctrl_bus_mar_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_mdo_we_c) = '1') then
        mdo <= wdata_i;
      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);
        d_bus_ben <= (others => '0');
        d_bus_ben(to_integer(unsigned(mar(1 downto 0)))) <= '1';
      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
      -- memory data in register (MDI) --
      if (ctrl_i(ctrl_bus_mdi_we_c) = '1') then
        mdi <= d_bus_rdata;
      end if;
    end if;
  end process mem_out_buf;
 
  -- input data alignment and sign extension --
  read_align: process(mdi, mar, ctrl_i)
    variable signed_v : std_ulogic;
  begin
    signed_v := not ctrl_i(ctrl_bus_unsigned_c);
    case ctrl_i(ctrl_bus_size_msb_c downto ctrl_bus_size_lsb_c) is -- data size
      when "00" => -- byte
        case mar(1 downto 0) is
          when "00" =>
            rdata_o(31 downto 08) <= (others => (signed_v and mdi(07)));
            rdata_o(07 downto 00) <= mdi(07 downto 00); -- byte 0
          when "01" =>
            rdata_o(31 downto 08) <= (others => (signed_v and mdi(15)));
            rdata_o(07 downto 00) <= mdi(15 downto 08); -- byte 1
          when "10" =>
            rdata_o(31 downto 08) <= (others => (signed_v and mdi(23)));
            rdata_o(07 downto 00) <= mdi(23 downto 16); -- byte 2
          when others =>
            rdata_o(31 downto 08) <= (others => (signed_v and mdi(31)));
            rdata_o(07 downto 00) <= mdi(31 downto 24); -- byte 3
        end case;
      when "01" => -- half-word
        if (mar(1) = '0') then
          rdata_o(31 downto 16) <= (others => (signed_v and mdi(15)));
          rdata_o(15 downto 00) <= mdi(15 downto 00); -- low half-word
        else
          rdata_o(31 downto 16) <= (others => (signed_v and mdi(31)));
          rdata_o(15 downto 00) <= mdi(31 downto 16); -- high half-word
        end if;
      when others => -- word
        rdata_o <= mdi; -- full word
    end case;
  end process read_align;
 
 
  -- Instruction Interface: Check for Misaligned Access -------------------------------------
  -- -------------------------------------------------------------------------------------------
  misaligned_i_check: process(ctrl_i, fetch_pc_i)
  begin
    -- check instruction access --
    i_misaligned <= '0'; -- default
    if (CPU_EXTENSION_RISCV_C = true) then -- 16-bit and 32-bit instruction accesses
      i_misaligned <= '0'; -- no alignment exceptions possible
    else -- 32-bit instruction accesses only
      if (fetch_pc_i(1) = '1') then -- PC(0) is always zero
        i_misaligned <= '1';
      end if; 
    end if;
  end process misaligned_i_check;
 
 
  -- Instruction Fetch Arbiter --------------------------------------------------------------
  -- -------------------------------------------------------------------------------------------
  ifetch_arbiter: process(rstn_i, clk_i)
  begin
    if (rstn_i = '0') then
      i_arbiter.rd_req    <= '0';
      i_arbiter.wr_req    <= '0';
      i_arbiter.err_align <= '0';
      i_arbiter.err_bus   <= '0';
      i_arbiter.timeout   <= (others => '0');
    elsif rising_edge(clk_i) then
      i_arbiter.wr_req <= '0'; -- instruction fetch is read-only
 
      -- 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) and (not ctrl_i(ctrl_bus_ierr_ack_c));
        --if (i_arbiter.err_align = '1') or (i_arbiter.err_bus = '1') then -- any error?
        --  if (ctrl_i(ctrl_bus_ierr_ack_c) = '1') then -- wait for controller to acknowledge error
        --    i_arbiter.rd_req <= '0';
        --  end if;
        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;
 
  -- 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;
  i_bus_wdata_o <= (others => '0');
  i_bus_ben_o   <= (others => '0');
  i_bus_we_o    <= '0';
  i_bus_re_o    <= 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_fence_o <= ctrl_i(ctrl_bus_fencei_c);
  instr_o       <= i_bus_rdata_i;
 
 
  -- Data Access Arbiter --------------------------------------------------------------------
  -- -------------------------------------------------------------------------------------------
  data_access_arbiter: process(rstn_i, clk_i)
  begin
    if (rstn_i = '0') then
      d_arbiter.rd_req    <= '0';
      d_arbiter.wr_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) and (not ctrl_i(ctrl_bus_derr_ack_c));
        --if (d_arbiter.err_align = '1') or (d_arbiter.err_bus = '1') then -- any error?
        --  if (ctrl_i(ctrl_bus_derr_ack_c) = '1') then -- wait for controller to acknowledge error
        --    d_arbiter.wr_req <= '0';
        --    d_arbiter.rd_req <= '0';
        --  end if;
        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_o    <= 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_o    <= 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_fence_o <= ctrl_i(ctrl_bus_fence_c);
  d_bus_rdata   <= d_bus_rdata_i;
 
 
  -- Physical Memory Protection (PMP) -------------------------------------------------------
  -- -------------------------------------------------------------------------------------------
  -- compute address masks --
  pmp_masks: process(clk_i)
  begin
    if rising_edge(clk_i) then -- address configuration (not the actual address check!) has a latency of +1 cycles
      for r in 0 to PMP_NUM_REGIONS-1 loop -- iterate over all regions
        pmp.addr_mask(r) <= (others => '0'); -- default
        for i in PMP_GRANULARITY+1 to 33 loop
          if (i = PMP_GRANULARITY+1) then
            pmp.addr_mask(r)(i) <= '0';
          else -- current bit = not AND(all previous bits)
            pmp.addr_mask(r)(i) <= not (and_all_f(pmp_addr_i(r)(i-1 downto PMP_GRANULARITY)));
          end if;
        end loop; -- i
      end loop; -- r
    end if;
  end process pmp_masks;
 
 
  -- compute operands for comparator --
  pmp_prepare_check:
  for r in 0 to PMP_NUM_REGIONS-1 generate -- iterate over all regions
    -- ignore lowest 3 bits of access addresses -> minimal region size = 8 bytes
    pmp.region_i_addr(r) <= (fetch_pc_i(31 downto 3) & "000") and pmp.addr_mask(r)(33 downto 2);
    pmp.region_d_addr(r) <= (mar(31 downto 3) & "000")        and pmp.addr_mask(r)(33 downto 2);
    pmp.region_base(r)   <= pmp_addr_i(r)(33 downto 2)        and pmp.addr_mask(r)(33 downto 2);
  end generate; -- r
 
 
  -- check for access address match --
  pmp_addr_check: process (pmp)
  begin
    for r in 0 to PMP_NUM_REGIONS-1 loop -- iterate over all regions
      -- instruction interface --
      pmp.i_match(r) <= '0';
      if (pmp.region_i_addr(r)(31 downto PMP_GRANULARITY+2) = pmp.region_base(r)(31 downto PMP_GRANULARITY+2)) then
        pmp.i_match(r) <= '1';
      end if;
      -- data interface --
      pmp.d_match(r) <= '0';
      if (pmp.region_d_addr(r)(31 downto PMP_GRANULARITY+2) = pmp.region_base(r)(31 downto PMP_GRANULARITY+2)) then
        pmp.d_match(r) <= '1';
      end if;
    end loop; -- r
  end process pmp_addr_check;
 
 
  -- check access type and regions's permissions --
  pmp_check_permission: process(pmp, pmp_ctrl_i, priv_mode_i)
  begin
    for r in 0 to PMP_NUM_REGIONS-1 loop -- iterate over all regions
      if ((priv_mode_i = 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_USE = true) else '0';
  ld_pmp_fault <= or_all_f(pmp.ld_fault) when (PMP_USE = true) else '0';
  st_pmp_fault <= or_all_f(pmp.st_fault) when (PMP_USE = true) else '0';
 
 
end neorv32_cpu_bus_rtl;