Introduction
============

.. figure:: images/blockdiagram.svg
   :name: blockdiagram

   Block Diagram

Ibex is a 2-stage in-order 32b RISC-V processor core.
Ibex has been designed to be small and efficient.
Via two parameters, the core is configurable to support four ISA configurations.
:numref:`blockdiagram` shows a block diagram of the core.

Standards Compliance
--------------------

Ibex is a standards-compliant 32b RISC-V processor.
It follows these specifications:

* `RISC-V Instruction Set Manual, Volume I: User-Level ISA, document version 20190608-Base-Ratified (June 8, 2019) <https://github.com/riscv/riscv-isa-manual/releases/download/Ratified-IMFDQC-and-Priv-v1.11/riscv-spec-20190608.pdf>`_
* `RISC-V Instruction Set Manual, Volume II: Privileged Architecture, document version 20190608-Base-Ratified (June 8, 2019) <https://github.com/riscv/riscv-isa-manual/releases/download/Ratified-IMFDQC-and-Priv-v1.11/riscv-privileged-20190608.pdf>`_.
  Ibex implements the Machine ISA version 1.11.
* `RISC-V External Debug Support, version 0.13.2 <https://content.riscv.org/wp-content/uploads/2019/03/riscv-debug-release.pdf>`_

Many features in the RISC-V specification are optional, and Ibex can be parametrized to enable or disable some of them.

Ibex can be parametrized to support either of the following two instruction sets.

* The RV32I Base Integer Instruction Set, version 2.1
* The RV32E Base Integer Instruction Set, version 1.9 (draft from June 8, 2019)

In addition, the following instruction set extensions are available.

.. list-table:: Ibex Instruction Set Extensions
   :header-rows: 1

   * - Extension
     - Version
     - Configurability

   * - **C**: Standard Extension for Compressed Instructions
     - 2.0
     - always enabled

   * - **M**: Standard Extension for Integer Multiplication and Division
     - 2.0
     - optional

   * - **Zicsr**: Control and Status Register Instructions
     - 2.0
     - always enabled

   * - **Zifencei**: Instruction-Fetch Fence
     - 2.0
     - always enabled

Most content of the RISC-V privileged specification is optional.
Ibex currently supports the following features according to the RISC-V Privileged Specification, version 1.11.

* M-Mode and U-Mode
* All CSRs listed in :ref:`cs-registers`
* Performance counters as described in :ref:`performance-counters`
* Vectorized trap handling as described at :ref:`exceptions-interrupts`

ASIC Synthesis
--------------

ASIC synthesis is supported for Ibex.
The whole design is completely synchronous and uses positive-edge triggered flip-flops, except for the register file, which can be implemented either with latches or with flip-flops.
See :ref:`register-file` for more details.
The core occupies an area of roughly 18.9 kGE when using the latch-based register file and implementing the RV32IMC ISA, or 11.6 kGE when implementing the RV32EC ISA.

FPGA Synthesis
--------------

FPGA synthesis is supported for Ibex when the flip-flop based register file is used.
Since latches are not well supported on FPGAs, it is crucial to select the flip-flop based register file.

Contents
--------

 * :ref:`getting-started` discusses the requirements and initial steps to start using Ibex.
 * :ref:`core-integration` provides the instantiation template and gives descriptions of the design parameters as well as the input and output ports.
 * :ref:`pipeline-details` described the overal pipeline structure.
 * :ref:`instruction-decode-execute` describes how the Instruction Decode and Execute stage works.
 * The instruction and data interfaces of Ibex are explained in :ref:`instruction-fetch` and :ref:`load-store-unit`, respectively.
 * :ref:`icache` describes the optional Instruction Cache.
 * The two register-file flavors are described in :ref:`register-file`.
 * The control and status registers are explained in :ref:`cs-registers`.
 * :ref:`performance-counters` gives an overview of the performance monitors and event counters available in Ibex.
 * :ref:`exceptions-interrupts` deals with the infrastructure for handling exceptions and interrupts,
 * :ref:`pmp` gives a brief overview of PMP support.
 * :ref:`debug-support` gives a brief overview on the debug infrastructure.
 * :ref:`tracer` gives a brief overview of the tracer module.
 * For information regarding formal verification support, check out :ref:`rvfi`.
 * :ref:`examples` gives an overview of how Ibex can be used.


History
-------

Ibex development started in 2015 under the name "Zero-riscy" as part of the `PULP platform <https://pulp-platform.org>`_ for energy-efficient computing.
Much of the code was developed by simplifying the RV32 CPU core "RI5CY" to demonstrate how small a RISC-V CPU core could actually be `[1] <https://doi.org/10.1109/PATMOS.2017.8106976>`_.
To make it even smaller, support for the "E" extension was added under the code name "Micro-riscy".
In the PULP ecosystem, the core is used as the control core for PULP, PULPino and PULPissimo.

In December 2018 lowRISC took over the development of Zero-riscy and renamed it to Ibex.

References
----------

1. `Schiavone, Pasquale Davide, et al. "Slow and steady wins the race? A comparison of ultra-low-power RISC-V cores for Internet-of-Things applications." 27th International Symposium on Power and Timing Modeling, Optimization and Simulation (PATMOS 2017) <https://doi.org/10.1109/PATMOS.2017.8106976>`_
