.. _core-integration:

Core Integration
================

The main module is named ``ibex_top`` and can be found in ``ibex_top.sv``.
Note that the core logic is split-out from the register file and RAMs under ``ibex_top``.
This is to facilitate a dual-core lockstep implementation (see :ref:`security`).

Register File
-------------

Ibex comes with three different register file implementations that can be selected using the enumerated parameter ``RegFile`` defined in :file:`rtl/ibex_pkg.sv`.
Depending on the target technology, either the flip-flop-based ("ibex_pkg::RegFileFF", default), the latch-based ("ibex_pkg::RegFileLatch") or an FPGA-targeted ("ibex_pkg::RegFileFPGA") implementation should be selected.
For more information about the three register file implementations and their trade-offs, check out :ref:`register-file`.

Identification CSRs
-------------------

The RISC-V Privileged Architecture specifies several read-only CSRs that identify the vendor and micro-architecture of a CPU.
These are ``mvendorid``, ``marchid`` and ``mimpid``.
The fixed, read-only values for these CSRs are defined in :file:`rtl/ibex_pkg.sv`.
Implementers should carefully consider appropriate values for these registers.
Ibex, as an open source implementation, has an assigned architecture ID (``marchid``) of 22.
(Allocations are specified in `marchid.md of the riscv-isa-manual repository <https://github.com/riscv/riscv-isa-manual/blob/master/marchid.md>`_.)
If significant changes are made to the micro-architecture a different architecture ID should be used.
The vendor ID and implementation ID (``mvendorid`` and ``mimpid``) both read as 0 by default, meaning non-implemented.
Implementers may wish to use other values here.
Please see the RISC-V Privileged Architecture specification for more details on what these IDs represent and how they should be chosen.

.. _integration-prims:

Primitives
----------

Ibex uses a number of primitive modules (that are held outside the :file:`rtl/` which contains the Ibex RTL).
Full implementations of these primitives are provided in the Ibex repository but implementors may wish to provide their own implementations.
Some of the primitives are only used for specific Ibex configurations so can be ignored/removed if you're not using one of those configurations.

The mandatory primitives (used by all configurations) are:
 * ``prim_buf`` - A buffer, used to ensure security critical logic isn't optimized out in synthesis (by applying suitable constraints to prim_buf).
   In configurations where ``SecureIbex == 0`` it must exist but can be implemented as a straight passthrough.
 * ``prim_clock_gating`` - A clock gate.

The configuration dependent primitives are:
 * ``prim_clock_mux2`` - A clock mux, used by the lockstep duplicate core.
   Required where ``SecureIbex == 1``.
 * ``prim_flop`` - A flip flop, used to ensure security critical logic isn't optimized out in synthesis (by applying suitable constraints to prim_flop).
   Required where ``SecureIbex == 1``.
 * ``prim_ram_1p`` - A single ported RAM.
   Required where ``ICache == 1``.
 * ``prim_ram_1p_scr`` - A single ported RAM which scrambles its contents with cryptographic primitives.
   Required where ``ICache == 1`` and ``SecureIbex == 1``.
 * ``prim_lfsr`` - Linear feedback shift register, used for pseudo random number generation for dummy instruction insertion.
   Required where ``SecureIbex == 1``.
 * ``prim_onehot_check`` - Checks a onehot signal is correct, for detecting fault injection attacks.
   Required where ``SecureIbex == 1``.
 * ``prim_secded_X`` - Various primitives to encode and decode SECDED (single error correct, double error detect) error detection and correction codes.
   Required where ``SecureIbex == 1``.

Primitives exclusively used by other primitives:
 * ``prim_present`` / ``prim_prince`` / ``prim_subst_perm`` - Cryptographic primitives used by ``prim_ram_1p_scr``.
 * ``prim_ram_1p_adv`` - Wrapper around ``prim_ram_1p`` that adds support for ECC, used by ``prim_ram_1p_scr``.

.. _integration-fusesoc-files:

RTL File List
-------------

Ibex flows use `FuseSoC <https://github.com/olofk/fusesoc>`_ to gather needed RTL files and run builds.
If you want to use Ibex without FuseSoC the following FuseSoC command will copy all the needed files into a build directory.

.. code-block:: bash

  fusesoc --cores-root . run --target=lint --setup --build-root ./build/ibex_out lowrisc:ibex:ibex_top

FuseSoC uses Python and it can be installed using pip.

.. code-block:: bash

  pip3 install -U -r python-requirements.txt

Ibex uses a `custom fork of FuseSoC <https://github.com/lowRISC/fusesoc/tree/ot>`_, so you must install it via this method rather than installing FuseSoC separately.

The RTL will be in :file:`./build/ibex_out/src` which is further divided into different sub-directories.
A file list containing paths to all of the RTL files can be found in :file:`./build/ibex_out/ibex-verilator/lowrisc_ibex_ibex_top_0.1.vc`.

Instantiation Template
----------------------

.. code-block:: verilog

  ibex_top #(
      .PMPEnable        ( 0                                ),
      .PMPGranularity   ( 0                                ),
      .PMPNumRegions    ( 4                                ),
      .MHPMCounterNum   ( 0                                ),
      .MHPMCounterWidth ( 40                               ),
      .RV32E            ( 0                                ),
      .RV32M            ( ibex_pkg::RV32MFast              ),
      .RV32B            ( ibex_pkg::RV32BNone              ),
      .RegFile          ( ibex_pkg::RegFileFF              ),
      .ICache           ( 0                                ),
      .ICacheECC        ( 0                                ),
      .ICacheScramble   ( 0                                ),
      .BranchPrediction ( 0                                ),
      .SecureIbex       ( 0                                ),
      .RndCnstLfsrSeed  ( ibex_pkg::RndCnstLfsrSeedDefault ),
      .RndCnstLfsrPerm  ( ibex_pkg::RndCnstLfsrPermDefault ),
      .DbgTriggerEn     ( 0                                ),
      .DmHaltAddr       ( 32'h1A110800                     ),
      .DmExceptionAddr  ( 32'h1A110808                     )
  ) u_top (
      // Clock and reset
      .clk_i                  (),
      .rst_ni                 (),
      .test_en_i              (),
      .scan_rst_ni            (),
      .ram_cfg_i              (),

      // Configuration
      .hart_id_i              (),
      .boot_addr_i            (),

      // Instruction memory interface
      .instr_req_o            (),
      .instr_gnt_i            (),
      .instr_rvalid_i         (),
      .instr_addr_o           (),
      .instr_rdata_i          (),
      .instr_rdata_intg_i     (),
      .instr_err_i            (),

      // Data memory interface
      .data_req_o             (),
      .data_gnt_i             (),
      .data_rvalid_i          (),
      .data_we_o              (),
      .data_be_o              (),
      .data_addr_o            (),
      .data_wdata_o           (),
      .data_wdata_intg_o      (),
      .data_rdata_i           (),
      .data_rdata_intg_i      (),
      .data_err_i             (),

      // Interrupt inputs
      .irq_software_i         (),
      .irq_timer_i            (),
      .irq_external_i         (),
      .irq_fast_i             (),
      .irq_nm_i               (),

      // Debug interface
      .debug_req_i            (),
      .crash_dump_o           (),

      // Special control signals
      .fetch_enable_i         (),
      .alert_minor_o          (),
      .alert_major_internal_o (),
      .alert_major_bus_o      (),
      .core_sleep_o           ()
  );

Parameters
----------

+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| Name                         | Type/Range          | Default    | Description                                                           |
+==============================+=====================+============+=======================================================================+
| ``PMPEnable``                | bit                 | 0          | Enable PMP support                                                    |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``PMPGranularity``           | int (0..31)         | 0          | Minimum granularity of PMP address matching                           |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``PMPNumRegions``            | int (1..16)         | 4          | Number implemented PMP regions (ignored if PMPEnable == 0)            |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``MHPMCounterNum``           | int (0..10)         | 0          | Number of performance monitor event counters                          |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``MHPMCounterWidth``         | int (64..1)         | 40         | Bit width of performance monitor event counters                       |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``RV32E``                    | bit                 | 0          | RV32E mode enable (16 integer registers only)                         |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``RV32M``                    | ibex_pkg::rv32m_e   | RV32MFast  | M(ultiply) extension select:                                          |
|                              |                     |            | "ibex_pkg::RV32MNone": No M-extension                                 |
|                              |                     |            | "ibex_pkg::RV32MSlow": Slow multi-cycle multiplier, iterative divider |
|                              |                     |            | "ibex_pkg::RV32MFast": 3-4 cycle multiplier, iterative divider        |
|                              |                     |            | "ibex_pkg::RV32MSingleCycle": 1-2 cycle multiplier, iterative divider |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``RV32B``                    | ibex_pkg::rv32b_e   | RV32BNone  | B(itmanipulation) extension select:                                   |
|                              |                     |            | "ibex_pkg::RV32BNone": No B-extension                                 |
|                              |                     |            | "ibex_pkg::RV32BBalanced": Sub-extensions Zba, Zbb, Zbs, Zbf and Zbt  |
|                              |                     |            | "ibex_pkg::RV32BOTEarlGrey": All sub-extensions except Zbe            |
|                              |                     |            | "ibex_pkg::RV32BFull": All sub-extensions                             |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``RegFile``                  | ibex_pkg::regfile_e | RegFileFF  | Register file implementation select:                                  |
|                              |                     |            | "ibex_pkg::RegFileFF": Generic flip-flop-based register file          |
|                              |                     |            | "ibex_pkg::RegFileFPGA": Register file for FPGA targets               |
|                              |                     |            | "ibex_pkg::RegFileLatch": Latch-based register file for ASIC targets  |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``BranchTargetALU``          | bit                 | 0          | Enables branch target ALU removing a stall cycle from taken branches  |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``WritebackStage``           | bit                 | 0          | Enables third pipeline stage (writeback) improving performance of     |
|                              |                     |            | loads and stores                                                      |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``ICache``                   | bit                 | 0          | Enable instruction cache instead of prefetch buffer                   |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``ICacheECC``                | bit                 | 0          | Enable SECDED ECC protection in ICache (if  ICache == 1)              |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``ICacheScramble``           | bit                 | 0          | Enabling this parameter replaces tag and data RAMs of ICache with     |
|                              |                     |            | scrambling RAM primitives.                                            |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``BranchPrediction``         | bit                 | 0          | *EXPERIMENTAL* Enable Static branch prediction                        |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``SecureIbex``               | bit                 | 0          | Enable various additional features targeting secure code execution.   |
|                              |                     |            | Note: SecureIbex == 1'b1 and  RV32M == ibex_pkg::RV32MNone is an      |
|                              |                     |            | illegal combination.                                                  |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``RndCnstLfsrSeed``          | lfsr_seed_t         | see above  | Set the starting seed of the LFSR used to generate dummy instructions |
|                              |                     |            | (only relevant when SecureIbex == 1'b1)                               |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``RndCnstLfsrPerm``          | lfsr_perm_t         | see above  | Set the permutation applied to the output of the LFSR used to         |
|                              |                     |            | generate dummy instructions (only relevant when SecureIbex == 1'b1)   |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``DbgTriggerEn``             | bit                 | 0          | Enable debug trigger support (one trigger only)                       |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``DmHaltAddr``               | int                 | 0x1A110800 | Address to jump to when entering Debug Mode                           |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+
| ``DmExceptionAddr``          | int                 | 0x1A110808 | Address to jump to when an exception occurs while in Debug Mode       |
+------------------------------+---------------------+------------+-----------------------------------------------------------------------+

Any parameter marked *EXPERIMENTAL* when enabled is not verified to the same standard as the rest of the Ibex core.

Note that Ibex uses SystemVerilog enum parameters e.g. for ``RV32M`` and ``RV32B``.
This is well supported by most tools but some care is needed when overriding these parameters at the top level:

* Synopsys VCS does not support overriding enum and string parameters at the top level via command line.
  As a workaround, SystemVerilog defines are used in Ibex top level files simulated with VCS.
  These defines can be set via command line.

* Yosys does not support overriding enum parameters at the top level by setting enum names.
  Instead, the enum values need to be used.

Interfaces
----------

+----------------------------+-------------------------+-----+----------------------------------------+
| Signal(s)                  | Width                   | Dir | Description                            |
+============================+=========================+=====+========================================+
| ``clk_i``                  | 1                       | in  | Clock signal                           |
+----------------------------+-------------------------+-----+----------------------------------------+
| ``rst_ni``                 | 1                       | in  | Active-low asynchronous reset          |
+----------------------------+-------------------------+-----+----------------------------------------+
| ``test_en_i``              | 1                       | in  | Test input, enables clock and allows   |
|                            |                         |     | test control of reset.                 |
+----------------------------+-------------------------+-----+----------------------------------------+
| ``scan_rst_ni``            | 1                       | in  | Test controlled reset.  If DFT not     |
|                            |                         |     | used, tie off to 1.                    |
+----------------------------+-------------------------+-----+----------------------------------------+
| ``ram_cfg_i``              | 10                      | in  | RAM configuration inputs, routed to    |
|                            |                         |     | the icache RAMs                        |
+----------------------------+-------------------------+-----+----------------------------------------+
| ``hart_id_i``              | 32                      | in  | Hart ID, usually static, can be read   |
|                            |                         |     | from :ref:`csr-mhartid` CSR            |
+----------------------------+-------------------------+-----+----------------------------------------+
| ``boot_addr_i``            | 32                      | in  | First program counter after reset      |
|                            |                         |     | = ``boot_addr_i`` + 0x80,              |
|                            |                         |     | see :ref:`exceptions-interrupts`       |
+----------------------------+-------------------------+-----+----------------------------------------+
| ``instr_*``                | Instruction fetch interface, see :ref:`instruction-fetch`              |
+----------------------------+------------------------------------------------------------------------+
| ``data_*``                 | Load-store unit interface, see :ref:`load-store-unit`                  |
+----------------------------+------------------------------------------------------------------------+
| ``irq_*``                  | Interrupt inputs, see :ref:`exceptions-interrupts`                     |
+----------------------------+-------------------------+-----+----------------------------------------+
| ``scramble_*``             | Scrambling key interface, see :ref:`icache`                            |
+----------------------------+------------------------------------------------------------------------+
| ``debug_*``                | Debug interface, see :ref:`debug-support`                              |
+----------------------------+------------------------------------------------------------------------+
| ``crash_dump_o``           | A set of signals that can be captured on reset to aid crash debugging. |
+----------------------------+------------------------------------------------------------------------+
| ``double_fault_seen_o``    | A double fault was observed, see :ref:`double-fault-detect`            |
+----------------------------+-------------------------+-----+----------------------------------------+
| ``fetch_enable_i``         | 4                       | in  | Allow the core to fetch instructions.  |
|                            |                         |     | If this bit is set low, the core will  |
|                            |                         |     | pause fetching new instructions and    |
|                            |                         |     | immediately halt once any in-flight    |
|                            |                         |     | instructions in the ID/EX and WB       |
|                            |                         |     | stages have finished. A multi-bit      |
|                            |                         |     | encoding scheme is used. See           |
|                            |                         |     | `IbexMuBiOn` / `IbexMuBiOff` in        |
|                            |                         |     | :file:`rtl/ibex_pkg.sv`                |
+----------------------------+-------------------------+-----+----------------------------------------+
| ``core_sleep_o``           | 1                       | out | Core in WFI with no outstanding data   |
|                            |                         |     | or instruction accesses. Deasserts     |
|                            |                         |     | if an external event (interrupt or     |
|                            |                         |     | debug req) wakes the core up           |
+----------------------------+-------------------------+-----+----------------------------------------+
| ``alert_minor_o``          | 1                       | out | Core has detected a fault which it can |
|                            |                         |     | safely recover from. Can be used by a  |
|                            |                         |     | system to log errors over time and     |
|                            |                         |     | detect tampering / attack. This signal |
|                            |                         |     | is a pulse, one cycle per alert.       |
+----------------------------+-------------------------+-----+----------------------------------------+
| ``alert_major_internal_o`` | 1                       | out | Core has detected an internal fault    |
|                            |                         |     | which cannot be recovered from. Can be |
|                            |                         |     | used by a system to reset the core and |
|                            |                         |     | possibly  take other remedial action.  |
|                            |                         |     | This signal is a pulse, but might be   |
|                            |                         |     | set for multiple cycles per alert.     |
+----------------------------+-------------------------+-----+----------------------------------------+
| ``alert_major_bus_o``      | 1                       | out | Core has detected a bus fault          |
|                            |                         |     | which cannot be recovered from. Can be |
|                            |                         |     | used by a system to reset the core and |
|                            |                         |     | possibly  take other remedial action.  |
|                            |                         |     | This signal is a pulse, but might be   |
|                            |                         |     | set for multiple cycles per alert.     |
+----------------------------+-------------------------+-----+----------------------------------------+
