Subversion Repositories neorv32

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

-- # << NEORV32 - CPU Control >>                                                                   #

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

-- # CPU operation is split into a fetch engine (responsible for fetching instruction data), an    #

-- # issue engine (for recoding compressed instructions and for constructing 32-bit instruction    #

-- # words) and an execute engine (responsible for actually executing the instructions), a trap    #

-- # handling controller and the RISC-V status and control register set (CSRs).                    #

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

-- # BSD 3-Clause License                                                                          #

-- #                                                                                               #

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

-- #                                                                                               #

-- # Redistribution and use in source and binary forms, with or without modification, are          #

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

-- #                                                                                               #

-- # 1. Redistributions of source code must retain the above copyright notice, this list of        #

-- #    conditions and the following disclaimer.                                                   #

-- #                                                                                               #

-- # 2. Redistributions in binary form must reproduce the above copyright notice, this list of     #

-- #    conditions and the following disclaimer in the documentation and/or other materials        #

-- #    provided with the distribution.                                                            #

-- #                                                                                               #

-- # 3. Neither the name of the copyright holder nor the names of its contributors may be used to  #

-- #    endorse or promote products derived from this software without specific prior written      #

-- #    permission.                                                                                #

-- #                                                                                               #

-- # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS   #

-- # OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF               #

-- # MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE    #

-- # COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,     #

-- # EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE #

-- # GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED    #

-- # AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING     #

-- # NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED  #

-- # OF THE POSSIBILITY OF SUCH DAMAGE.                                                            #

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

-- # 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_control is

  generic (

    -- General --

    HW_THREAD_ID                 : std_ulogic_vector(31 downto 0):= x"00000000"; -- hardware thread id

    CPU_BOOT_ADDR                : std_ulogic_vector(31 downto 0):= x"00000000"; -- cpu boot address

    -- RISC-V CPU Extensions --

    CPU_EXTENSION_RISCV_A        : boolean := false; -- implement atomic extension?

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

    CPU_EXTENSION_RISCV_E        : boolean := false; -- implement embedded RF extension?

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

    CPU_EXTENSION_RISCV_U        : boolean := false; -- implement user mode extension?

    CPU_EXTENSION_RISCV_Zicsr    : boolean := true;  -- implement CSR system?

    CPU_EXTENSION_RISCV_Zifencei : boolean := true;  -- implement instruction stream sync.?

    -- 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

    -- Hardware Performance Monitors (HPM) --

    HPM_NUM_CNTS                 : natural := 0      -- number of inmplemnted HPM counters (0..29)

  );

  port (

    -- global control --

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

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

    ctrl_o        : out std_ulogic_vector(ctrl_width_c-1 downto 0); -- main control bus

    -- status input --

    alu_wait_i    : in  std_ulogic; -- wait for ALU

    bus_i_wait_i  : in  std_ulogic; -- wait for bus

    bus_d_wait_i  : in  std_ulogic; -- wait for bus

    -- data input --

    instr_i       : in  std_ulogic_vector(data_width_c-1 downto 0); -- instruction

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

    alu_add_i     : in  std_ulogic_vector(data_width_c-1 downto 0); -- ALU address result

    rs1_i         : in  std_ulogic_vector(data_width_c-1 downto 0); -- rf source 1

    -- data output --

    imm_o         : out std_ulogic_vector(data_width_c-1 downto 0); -- immediate

    fetch_pc_o    : out std_ulogic_vector(data_width_c-1 downto 0); -- PC for instruction fetch

    curr_pc_o     : out std_ulogic_vector(data_width_c-1 downto 0); -- current PC (corresponding to current instruction)

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

    -- interrupts (risc-v compliant) --

    msw_irq_i     : in  std_ulogic; -- machine software interrupt

    mext_irq_i    : in  std_ulogic; -- machine external interrupt

    mtime_irq_i   : in  std_ulogic; -- machine timer interrupt

    -- fast interrupts (custom) --

    firq_i        : in  std_ulogic_vector(3 downto 0);

    -- system time input from MTIME --

    time_i        : in  std_ulogic_vector(63 downto 0); -- current system time

    -- physical memory protection --

    pmp_addr_o    : out pmp_addr_if_t; -- addresses

    pmp_ctrl_o    : out pmp_ctrl_if_t; -- configs

    -- bus access exceptions --

    mar_i         : in  std_ulogic_vector(data_width_c-1 downto 0);  -- memory address register

    ma_instr_i    : in  std_ulogic; -- misaligned instruction address

    ma_load_i     : in  std_ulogic; -- misaligned load data address

    ma_store_i    : in  std_ulogic; -- misaligned store data address

    be_instr_i    : in  std_ulogic; -- bus error on instruction access

    be_load_i     : in  std_ulogic; -- bus error on load data access

    be_store_i    : in  std_ulogic  -- bus error on store data access

  );

end neorv32_cpu_control;

architecture neorv32_cpu_control_rtl of neorv32_cpu_control is

  -- instruction fetch enginge --

  type fetch_engine_state_t is (IFETCH_RESET, IFETCH_REQUEST, IFETCH_ISSUE);

  type fetch_engine_t is record

    state       : fetch_engine_state_t;

    state_nxt   : fetch_engine_state_t;

    state_prev  : fetch_engine_state_t;

    pc          : std_ulogic_vector(data_width_c-1 downto 0);

    pc_nxt      : std_ulogic_vector(data_width_c-1 downto 0);

    reset       : std_ulogic;

    bus_err_ack : std_ulogic;

  end record;

  signal fetch_engine : fetch_engine_t;

  -- instrucion prefetch buffer (IPB, real FIFO) --

  type ipb_data_fifo_t is array (0 to ipb_entries_c-1) of std_ulogic_vector(2+31 downto 0);

  type ipb_t is record

    wdata : std_ulogic_vector(2+31 downto 0); -- write status (bus_error, align_error) + 32-bit instruction data

    we    : std_ulogic; -- trigger write

    free  : std_ulogic; -- free entry available?

    clear : std_ulogic; -- clear all entries

    --

    rdata : std_ulogic_vector(2+31 downto 0); -- read data: status (bus_error, align_error) + 32-bit instruction data

    re    : std_ulogic; -- read enable

    avail : std_ulogic; -- data available?

    --

    w_pnt : std_ulogic_vector(index_size_f(ipb_entries_c) downto 0); -- write pointer

    r_pnt : std_ulogic_vector(index_size_f(ipb_entries_c) downto 0); -- read pointer

    match : std_ulogic;

    empty : std_ulogic;

    full  : std_ulogic;

    --

    data  : ipb_data_fifo_t; -- fifo memory

  end record;

  signal ipb : ipb_t;

  -- pre-decoder --

  signal ci_instr16 : std_ulogic_vector(15 downto 0);

  signal ci_instr32 : std_ulogic_vector(31 downto 0);

  signal ci_illegal : std_ulogic;

  -- instruction issue enginge --

  type issue_engine_state_t is (ISSUE_ACTIVE, ISSUE_REALIGN);

  type issue_engine_t is record

    state     : issue_engine_state_t;

    state_nxt : issue_engine_state_t;

    align     : std_ulogic;

    align_nxt : std_ulogic;

    buf       : std_ulogic_vector(2+15 downto 0);

    buf_nxt   : std_ulogic_vector(2+15 downto 0);

  end record;

  signal issue_engine : issue_engine_t;

  -- instruction issue interface --

  type cmd_issue_t is record

    data  : std_ulogic_vector(35 downto 0); -- 4-bit status + 32-bit instruction

    valid : std_ulogic; -- data word is valid when set

  end record;

  signal cmd_issue : cmd_issue_t;

  -- instruction execution engine --

  type execute_engine_state_t is (SYS_WAIT, DISPATCH, TRAP, EXECUTE, ALU_WAIT, BRANCH, FENCE_OP, LOADSTORE_0, LOADSTORE_1, LOADSTORE_2, SYS_ENV, CSR_ACCESS);

  type execute_engine_t is record

    state        : execute_engine_state_t;

    state_nxt    : execute_engine_state_t;

    state_prev   : execute_engine_state_t;

    --

    i_reg        : std_ulogic_vector(31 downto 0);

    i_reg_nxt    : std_ulogic_vector(31 downto 0);

    i_reg_last   : std_ulogic_vector(31 downto 0); -- last executed instruction

    --

    is_ci        : std_ulogic; -- current instruction is de-compressed instruction

    is_ci_nxt    : std_ulogic;

    is_cp_op     : std_ulogic; -- current instruction is a co-processor operation

    is_cp_op_nxt : std_ulogic;

    --

    branch_taken : std_ulogic; -- branch condition fullfilled

    pc           : std_ulogic_vector(data_width_c-1 downto 0); -- actual PC, corresponding to current executed instruction

    pc_mux_sel   : std_ulogic_vector(1 downto 0); -- source select for PC update

    pc_we        : std_ulogic; -- PC update enabled

    next_pc      : std_ulogic_vector(data_width_c-1 downto 0); -- next PC, corresponding to next instruction to be executed

    last_pc      : std_ulogic_vector(data_width_c-1 downto 0); -- PC of last executed instruction

    --

    sleep        : std_ulogic; -- CPU in sleep mode

    sleep_nxt    : std_ulogic;

    if_rst       : std_ulogic; -- instruction fetch was reset

    if_rst_nxt   : std_ulogic;

  end record;

  signal execute_engine : execute_engine_t;

  -- trap controller --

  type trap_ctrl_t is record

    exc_buf       : std_ulogic_vector(exception_width_c-1 downto 0);

    exc_fire      : std_ulogic; -- set if there is a valid source in the exception buffer

    irq_buf       : std_ulogic_vector(interrupt_width_c-1 downto 0);

    irq_fire      : std_ulogic; -- set if there is a valid source in the interrupt buffer

    exc_ack       : std_ulogic; -- acknowledge all exceptions

    irq_ack       : std_ulogic_vector(interrupt_width_c-1 downto 0); -- acknowledge specific interrupt

    irq_ack_nxt   : std_ulogic_vector(interrupt_width_c-1 downto 0);

    cause         : std_ulogic_vector(5 downto 0); -- trap ID for mcause CSR

    cause_nxt     : std_ulogic_vector(5 downto 0);

    --

    env_start     : std_ulogic; -- start trap handler env

    env_start_ack : std_ulogic; -- start of trap handler acknowledged

    env_end       : std_ulogic; -- end trap handler env

    --

    instr_be      : std_ulogic; -- instruction fetch bus error

    instr_ma      : std_ulogic; -- instruction fetch misaligned address

    instr_il      : std_ulogic; -- illegal instruction

    env_call      : std_ulogic;

    break_point   : std_ulogic;

  end record;

  signal trap_ctrl : trap_ctrl_t;

  -- atomic operations controller --

  type atomic_ctrl_t is record

    env_start  : std_ulogic; -- begin atomic operations

    env_end    : std_ulogic; -- end atomic operations

    env_end_ff : std_ulogic; -- end atomic operations dealyed

    env_abort  : std_ulogic; -- atomic operations abort (results in failure)

    lock       : std_ulogic; -- lock status

  end record;

  signal atomic_ctrl : atomic_ctrl_t;

  -- CPU main control bus --

  signal ctrl_nxt, ctrl : std_ulogic_vector(ctrl_width_c-1 downto 0);

  -- fast instruction fetch access --

  signal bus_fast_ir : std_ulogic;

  -- RISC-V control and status registers (CSRs) --

  type pmp_ctrl_t     is array (0 to PMP_NUM_REGIONS-1) of std_ulogic_vector(7 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_ctrl_rd_t  is array (0 to 63) of std_ulogic_vector(7 downto 0);

  type pmp_addr_rd_t  is array (0 to 63) of std_ulogic_vector(data_width_c-1 downto 0);

  type mhpmevent_t    is array (0 to HPM_NUM_CNTS-1) of std_ulogic_vector(hpmcnt_event_size_c-1 downto 0);

  type mhpmcnt_t      is array (0 to HPM_NUM_CNTS-1) of std_ulogic_vector(32 downto 0);

  type mhpmcnth_t     is array (0 to HPM_NUM_CNTS-1) of std_ulogic_vector(31 downto 0);

  type mhpmevent_rd_t is array (0 to 29) of std_ulogic_vector(hpmcnt_event_size_c-1 downto 0);

  type mhpmcnt_rd_t   is array (0 to 29) of std_ulogic_vector(32 downto 0);

  type mhpmcnth_rd_t  is array (0 to 29) of std_ulogic_vector(31 downto 0);

  type csr_t is record

    addr              : std_ulogic_vector(11 downto 0); -- csr address

    we                : std_ulogic; -- csr write enable

    we_nxt            : std_ulogic;

    re                : std_ulogic; -- csr read enable

    re_nxt            : std_ulogic;

    wdata             : std_ulogic_vector(data_width_c-1 downto 0); -- csr write data

    rdata             : std_ulogic_vector(data_width_c-1 downto 0); -- csr read data

    --

    mstatus_mie       : std_ulogic; -- mstatus.MIE: global IRQ enable (R/W)

    mstatus_mpie      : std_ulogic; -- mstatus.MPIE: previous global IRQ enable (R/W)

    mstatus_mpp       : std_ulogic_vector(1 downto 0); -- mstatus.MPP: machine previous privilege mode

    --

    mie_msie          : std_ulogic; -- mie.MSIE: machine software interrupt enable (R/W)

    mie_meie          : std_ulogic; -- mie.MEIE: machine external interrupt enable (R/W)

    mie_mtie          : std_ulogic; -- mie.MEIE: machine timer interrupt enable (R/W)

    mie_firqe         : std_ulogic_vector(3 downto 0); -- mie.firq*e: fast interrupt enabled (R/W)

    --

    mcounteren_cy     : std_ulogic; -- mcounteren.cy: allow cycle[h] access from user-mode

    mcounteren_tm     : std_ulogic; -- mcounteren.tm: allow time[h] access from user-mode

    mcounteren_ir     : std_ulogic; -- mcounteren.ir: allow instret[h] access from user-mode

    mcounteren_hpm    : std_ulogic_vector(HPM_NUM_CNTS-1 downto 0); -- mcounteren.hpmx: allow mhpmcounterx[h] access from user-mode

    --

    mcountinhibit_cy  : std_ulogic; -- mcounterinhibit.cy: enable auto-increment for [m]cycle[h]

    mcountinhibit_ir  : std_ulogic; -- mcounterinhibit.ir: enable auto-increment for [m]instret[h]

    mcountinhibit_hpm : std_ulogic_vector(HPM_NUM_CNTS-1 downto 0); -- mcounterinhibit.hpm3: enable auto-increment for mhpmcounterx[h]

    --

    mip_status        : std_ulogic_vector(interrupt_width_c-1  downto 0); -- current buffered IRQs

    mip_clear         : std_ulogic_vector(interrupt_width_c-1  downto 0); -- set bits clear the according buffered IRQ

    --

    privilege         : std_ulogic_vector(1 downto 0); -- hart's current privilege mode

    priv_m_mode       : std_ulogic; -- CPU in M-mode

    priv_u_mode       : std_ulogic; -- CPU in u-mode

    --

    mepc              : std_ulogic_vector(data_width_c-1 downto 0); -- mepc: machine exception pc (R/W)

    mcause            : std_ulogic_vector(data_width_c-1 downto 0); -- mcause: machine trap cause (R/W)

    mtvec             : std_ulogic_vector(data_width_c-1 downto 0); -- mtvec: machine trap-handler base address (R/W), bit 1:0 == 00

    mtval             : std_ulogic_vector(data_width_c-1 downto 0); -- mtval: machine bad address or isntruction (R/W)

    --

    mhpmevent         : mhpmevent_t; -- mhpmevent*: machine performance-monitoring event selector (R/W)

    mhpmevent_rd      : mhpmevent_rd_t; -- mhpmevent*: actual read data

    --

    mscratch          : std_ulogic_vector(data_width_c-1 downto 0); -- mscratch: scratch register (R/W)

    mcycle            : std_ulogic_vector(32 downto 0); -- mcycle (R/W), plus carry bit

    minstret          : std_ulogic_vector(32 downto 0); -- minstret (R/W), plus carry bit

    --

    mcycleh           : std_ulogic_vector(31 downto 0); -- mcycleh (R/W)

    minstreth         : std_ulogic_vector(31 downto 0); -- minstreth (R/W)

    --

    mhpmcounter       : mhpmcnt_t; -- mhpmcounter* (R/W), plus carry bit

    mhpmcounterh      : mhpmcnth_t; -- mhpmcounter*h (R/W)

    mhpmcounter_rd    : mhpmcnt_rd_t; -- mhpmcounter* (R/W): actual read data

    mhpmcounterh_rd   : mhpmcnth_rd_t; -- mhpmcounter*h (R/W): actual read data

    --

    pmpcfg            : pmp_ctrl_t; -- physical memory protection - configuration registers

    pmpcfg_rd         : pmp_ctrl_rd_t; -- physical memory protection - actual read data

    pmpaddr           : pmp_addr_t; -- physical memory protection - address registers

    pmpaddr_rd        : pmp_addr_rd_t; -- physical memory protection - actual read data

  end record;

  signal csr : csr_t;

  -- counter low-to-high-word carry --

  signal mcycle_msb      : std_ulogic;

  signal minstret_msb    : std_ulogic;

  signal mhpmcounter_msb : std_ulogic_vector(HPM_NUM_CNTS-1 downto 0);

  -- (hpm) counter events --

  signal cnt_event, cnt_event_nxt : std_ulogic_vector(hpmcnt_event_size_c-1 downto 0);

  signal hpmcnt_trigger           : std_ulogic_vector(HPM_NUM_CNTS-1 downto 0);

  -- illegal instruction check --

  signal illegal_opcode_lsbs : std_ulogic; -- if opcode != rv32

  signal illegal_instruction : std_ulogic;

  signal illegal_register    : std_ulogic; -- only for E-extension

  signal illegal_compressed  : std_ulogic; -- only fir C-extension

  -- access (privilege) check --

  signal csr_acc_valid : std_ulogic; -- valid CSR access (implemented and valid access rights)

begin

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

-- Instruction Fetch (always fetches aligned 32-bit chunks of data)

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

  -- Fetch Engine FSM Sync ------------------------------------------------------------------

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

  fetch_engine_fsm_sync: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      fetch_engine.state      <= IFETCH_RESET;

      fetch_engine.state_prev <= IFETCH_RESET;

      fetch_engine.pc         <= (others => '0');

    elsif rising_edge(clk_i) then

      if (fetch_engine.reset = '1') then

        fetch_engine.state <= IFETCH_RESET;

      else

        fetch_engine.state <= fetch_engine.state_nxt;

      end if;

      fetch_engine.state_prev <= fetch_engine.state;

      fetch_engine.pc         <= fetch_engine.pc_nxt;

    end if;

  end process fetch_engine_fsm_sync;

  -- PC output --

  fetch_pc_o <= fetch_engine.pc(data_width_c-1 downto 1) & '0'; -- half-word aligned

  -- Fetch Engine FSM Comb ------------------------------------------------------------------

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

  fetch_engine_fsm_comb: process(fetch_engine, execute_engine, ipb, instr_i, bus_i_wait_i, be_instr_i, ma_instr_i)

  begin

    -- arbiter defaults --

    bus_fast_ir              <= '0';

    fetch_engine.state_nxt   <= fetch_engine.state;

    fetch_engine.pc_nxt      <= fetch_engine.pc;

    fetch_engine.bus_err_ack <= '0';

    -- instruction prefetch buffer interface --

    ipb.we    <= '0';

    ipb.wdata <= be_instr_i & ma_instr_i & instr_i(31 downto 0); -- store exception info and instruction word

    ipb.clear <= '0';

    -- state machine --

    case fetch_engine.state is

      when IFETCH_RESET => -- reset engine and prefetch buffer, get appilcation PC

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

        fetch_engine.bus_err_ack <= '1'; -- acknowledge any instruction bus errors, the execute engine has to take care of them / terminate current transfer

        fetch_engine.pc_nxt      <= execute_engine.pc(data_width_c-1 downto 1) & '0'; -- initialize with "real" application PC

        ipb.clear                <= '1'; -- clear prefetch buffer

        fetch_engine.state_nxt   <= IFETCH_REQUEST;

      when IFETCH_REQUEST => -- output current PC to bus system and request 32-bit (aligned!) instruction data

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

        if (ipb.free = '1') then -- free entry in buffer?

          bus_fast_ir            <= '1'; -- fast instruction fetch request

          fetch_engine.state_nxt <= IFETCH_ISSUE;

        end if;

      when IFETCH_ISSUE => -- store instruction data to prefetch buffer

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

        fetch_engine.bus_err_ack <= be_instr_i or ma_instr_i; -- ACK bus/alignment errors

        if (bus_i_wait_i = '0') or (be_instr_i = '1') or (ma_instr_i = '1') then -- wait for bus response

          fetch_engine.pc_nxt    <= std_ulogic_vector(unsigned(fetch_engine.pc) + 4);

          ipb.we                 <= '1';

          fetch_engine.state_nxt <= IFETCH_REQUEST;

        end if;

      when others => -- undefined

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

        fetch_engine.state_nxt <= IFETCH_RESET;

    end case;

  end process fetch_engine_fsm_comb;

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

-- Instruction Prefetch Buffer

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

  -- Instruction Prefetch Buffer (FIFO) -----------------------------------------------------

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

  instr_prefetch_buffer: process(clk_i)

  begin

    if rising_edge(clk_i) then

      -- write port --

      if (ipb.clear = '1') then

        ipb.w_pnt <= (others => '0');

      elsif (ipb.we = '1') then

        ipb.w_pnt <= std_ulogic_vector(unsigned(ipb.w_pnt) + 1);

      end if;

      if (ipb.we = '1') then -- write data

        ipb.data(to_integer(unsigned(ipb.w_pnt(ipb.w_pnt'left-1 downto 0)))) <= ipb.wdata;

      end if;

      -- read port --

      if (ipb.clear = '1') then

        ipb.r_pnt <= (others => '0');

      elsif (ipb.re = '1') then

        ipb.r_pnt <= std_ulogic_vector(unsigned(ipb.r_pnt) + 1);

      end if;

    end if;

  end process instr_prefetch_buffer;

  -- async read --

  ipb.rdata <= ipb.data(to_integer(unsigned(ipb.r_pnt(ipb.r_pnt'left-1 downto 0))));

  -- status --

  ipb.match <= '1' when (ipb.r_pnt(ipb.r_pnt'left-1 downto 0) = ipb.w_pnt(ipb.w_pnt'left-1 downto 0))  else '0';

  ipb.full  <= '1' when (ipb.r_pnt(ipb.r_pnt'left) /= ipb.w_pnt(ipb.w_pnt'left)) and (ipb.match = '1') else '0';

  ipb.empty <= '1' when (ipb.r_pnt(ipb.r_pnt'left)  = ipb.w_pnt(ipb.w_pnt'left)) and (ipb.match = '1') else '0';

  ipb.free  <= not ipb.full;

  ipb.avail <= not ipb.empty;

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

-- Instruction Issue (recoding of compressed instructions and 32-bit instruction word construction)

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

  -- Issue Engine FSM Sync ------------------------------------------------------------------

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

  issue_engine_fsm_sync: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      issue_engine.state <= ISSUE_ACTIVE;

      issue_engine.align <= CPU_BOOT_ADDR(1); -- 32- or 16-bit boundary

      issue_engine.buf   <= (others => '0');

    elsif rising_edge(clk_i) then

      if (ipb.clear = '1') then

        if (CPU_EXTENSION_RISCV_C = true) then

          if (execute_engine.pc(1) = '1') then -- branch to unaligned address?

            issue_engine.state <= ISSUE_REALIGN;

            issue_engine.align <= '1'; -- aligned on 16-bit boundary

          else

            issue_engine.state <= issue_engine.state_nxt;

            issue_engine.align <= '0'; -- aligned on 32-bit boundary

          end if;

        else

          issue_engine.state <= issue_engine.state_nxt;

          issue_engine.align <= '0'; -- always aligned on 32-bit boundaries

        end if;

      else

        issue_engine.state <= issue_engine.state_nxt;

        issue_engine.align <= issue_engine.align_nxt;

      end if;

      issue_engine.buf <= issue_engine.buf_nxt;

    end if;

  end process issue_engine_fsm_sync;

  -- Issue Engine FSM Comb ------------------------------------------------------------------

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

  issue_engine_fsm_comb: process(issue_engine, ipb, execute_engine, ci_illegal, ci_instr32)

  begin

    -- arbiter defaults --

    issue_engine.state_nxt <= issue_engine.state;

    issue_engine.align_nxt <= issue_engine.align;

    issue_engine.buf_nxt   <= issue_engine.buf;

    -- instruction prefetch buffer interface defaults --

    ipb.re <= '0';

    -- instruction issue interface defaults --

    -- cmd_issue.data = <illegal_compressed_instruction> & <bus_error & alignment_error> & <is_compressed_instrucion> & <32-bit_instruction_word>

    cmd_issue.data  <= '0' & ipb.rdata(33 downto 32) & '0' & ipb.rdata(31 downto 0);

    cmd_issue.valid <= '0';

    -- state machine --

    case issue_engine.state is

      when ISSUE_ACTIVE => -- issue instruction if available

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

        if (ipb.avail = '1') then -- instructions available?

          if (issue_engine.align = '0') or (CPU_EXTENSION_RISCV_C = false) then -- begin check in LOW instruction half-word

            if (execute_engine.state = DISPATCH) then -- ready to issue new command?

              cmd_issue.valid      <= '1';

              issue_engine.buf_nxt <= ipb.rdata(33 downto 32) & ipb.rdata(31 downto 16); -- store high half-word - we might need it for an unaligned uncompressed instruction

              if (ipb.rdata(1 downto 0) = "11") or (CPU_EXTENSION_RISCV_C = false) then -- uncompressed and "aligned"

                ipb.re <= '1';

                cmd_issue.data <= '0' & ipb.rdata(33 downto 32) & '0' & ipb.rdata(31 downto 0);

              else -- compressed

                ipb.re <= '1';

                cmd_issue.data <= ci_illegal & ipb.rdata(33 downto 32) & '1' & ci_instr32;

                issue_engine.align_nxt <= '1';

              end if;

            end if;

          else -- begin check in HIGH instruction half-word

            if (execute_engine.state = DISPATCH) then -- ready to issue new command?

              cmd_issue.valid      <= '1';

              issue_engine.buf_nxt <= ipb.rdata(33 downto 32) & ipb.rdata(31 downto 16); -- store high half-word - we might need it for an unaligned uncompressed instruction

              if (issue_engine.buf(1 downto 0) = "11") then -- uncompressed and "unaligned"

                ipb.re <= '1';

                cmd_issue.data <= '0' & issue_engine.buf(17 downto 16) & '0' & (ipb.rdata(15 downto 0) & issue_engine.buf(15 downto 0));

              else -- compressed

                -- do not read from ipb here!

                cmd_issue.data <= ci_illegal & ipb.rdata(33 downto 32) & '1' & ci_instr32;

                issue_engine.align_nxt <= '0';

              end if;

            end if;

          end if;

        end if;

      when ISSUE_REALIGN => -- re-align input fifos after a branch to an unaligned address

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

        issue_engine.buf_nxt <= ipb.rdata(33 downto 32) & ipb.rdata(31 downto 16);

        if (ipb.avail = '1') then -- instructions available?

          ipb.re <= '1';

          issue_engine.state_nxt <= ISSUE_ACTIVE;

        end if;

      when others => -- undefined

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

        issue_engine.state_nxt <= ISSUE_ACTIVE;

    end case;

  end process issue_engine_fsm_comb;

  -- 16-bit instructions: half-word select --

  ci_instr16 <= ipb.rdata(15 downto 0) when (issue_engine.align = '0') else issue_engine.buf(15 downto 0);

  -- Compressed Instructions Recoding -------------------------------------------------------

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

  neorv32_cpu_decompressor_inst_true:

  if (CPU_EXTENSION_RISCV_C = true) generate

    neorv32_cpu_decompressor_inst: neorv32_cpu_decompressor

    port map (

      -- instruction input --

      ci_instr16_i => ci_instr16, -- compressed instruction input

      -- instruction output --

      ci_illegal_o => ci_illegal, -- is an illegal compressed instruction

      ci_instr32_o => ci_instr32  -- 32-bit decompressed instruction

    );

  end generate;

  neorv32_cpu_decompressor_inst_false:

  if (CPU_EXTENSION_RISCV_C = false) generate

    ci_instr32 <= (others => '0');

    ci_illegal <= '0';

  end generate;

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

-- Instruction Execution

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

  -- Immediate Generator --------------------------------------------------------------------

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

  imm_gen: process(execute_engine.i_reg, clk_i)

    variable opcode_v : std_ulogic_vector(6 downto 0);

  begin

    opcode_v := execute_engine.i_reg(instr_opcode_msb_c downto instr_opcode_lsb_c+2) & "11";

    if rising_edge(clk_i) then

      if (execute_engine.state = BRANCH) then -- next_PC as immediate for jump-and-link operations (=return address)

        imm_o <= execute_engine.next_pc;

      else -- "normal" immediate from instruction

        case opcode_v is -- save some bits here, LSBs are always 11 for rv32

          when opcode_store_c => -- S-immediate