RISC-V CPU, simple 3-stage pipeline, for low-end applications (e.g., embedded, IoT)
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Open-source RISC-V CPUs from Bluespec, Inc.

This is one of a family of free, open-source RISC-V CPUs created by Bluespec, Inc.

  • Piccolo: 3-stage, in-order pipeline

    Piccolo is intended for low-end applications (Embedded Systems, IoT, microcontrollers, etc.).

  • Flute: 5-stage, in-order pipeline

    Flute is intended for low-end to medium applications that require 64-bit operation, an MMU (Virtual Memory) and more performance than Piccolo-class processors.

  • Bassoon: deep, out-of-order pipeline [Coming!]

The three repo structures are nearly identical, and the ways to build and run are identical. This README is identical--please substitute "Piccolo" or "Flute" or "Basoon" below wherever you see .

About the source codes (in BSV and Verilog)

The BSV source code in this repository, from which the synthesizable Verilog RTL in this repository is generated, is highly parameterized to allow generating many possible configurations, some of which are adequate to boot a Linux kernel.

The pre-generated synthesizable Verilog RTL source files in this repository are for a few specific configurations:

  1. RV32ACIMU: (DARPA SSITH users: this is the "P1" processor)

    • RV32I: base RV32 integer instructions
    • 'A' extension: atomic memory ops
    • 'C' extension: compressed instructions
    • 'M' extension: integer multiply/divide instructions
    • Privilege levels M (machine) and U (user)
    • Supports external, timer, software and non-maskable interrupts
    • Passes all riscv-isa tests for RV32ACIMU
    • Boots FreeRTOS

    • RV64I: base RV64 integer instructions
    • 'A' extension: atomic memory ops
    • 'C' extension: compressed instructions
    • 'D' extension: double-precision floating point instructions
    • 'F' extension: single-precision floating point instructions
    • 'M' extension: integer multiply/divide instructions
    • Privilege levels M (machine), S (Supervisor) and U (user)
    • Supports external, timer, software and non-maskable interrupts
    • Passes all riscv-isa tests for RV64ACDFIMSU
    • Boots the Linux kernel

If you want to generate other Verilog variants, you'll need a Bluespec bsc compiler [Note: Bluespec, Inc. provides free licenses to academia and for non-profit research].

The BSV source code supports:

  • RV32I or RV64I

  • Optional 'A', 'C', 'D', 'F' and 'M' extensions

  • Privilege level options M, MU and MSU

  • For privilege S, virtual memory schemes Sv32 (RV32) and Sv39 (RV64)

  • Hardware implementation option: serial shifter (smaller hardware, slower) or barrel shifter (more HW, faster) for shift instructions

  • Hardware implementation option: serial integer multiplier (smaller hardware, slower) or synthesized (more HW, faster)

  • AXI4-Lite Fabric interfaces, with optional 32-bit or 64-bit datapaths (independent of RV32/RV64 choice)

  • and several other localized options

Testbench included

This repository contains a simple testbench (a small SoC) with which one can run RISC-V binaries in simulation by loading standard mem hex files and executing in Bluespec's Bluesim, Verilator simulation or iVerilog simulation. The testbench contains an AXI4-Lite interconnect fabric that connects the CPU to models of a boot ROM, a memory, a timer and a UART for console I/O.

[Note: iverilog functionality is currently limited because we are still working out robust mechanisms to import C code, which is used in parts of the testbench.]

This repository contains several sample build directories, to build RV32ACIMU or RV64ACDFIMSU simulators, using Bluespec Bluesim simulation, Verilator Verilog simulation, or Icarus Verilog ("iverilog") simulation.

The generated Verilog is synthesizable. Bluespec tests all this code on Xilinx FPGAs.


  • Ongoing continuous micro-architectural improvements for performance and hardware area.

Source codes

This repository contains two levels of source code: Verilog and BSV.

Verilog RTL can be found in directories with names suffixed in '_verilator' or '_iverilog' in the 'builds' directory:

    builds/..._<verilator or iverilog>/Verilog_RTL/

[There is no difference between Verilog in a Verilator directory vs. the corresponding iverilog directory. ]

The Verilog RTL is synthesizable (and hence acceptable to Verilator). It can be simulated in any Verilog simulator (we provide Makefiles to build simulation executables for Verilator and for Icarus Verilog (iverilog)).

The RTL represents RISC-V CPU RTL, plus a rudimentary surrounding SoC enabling immediate simulation here, and which is rich enough to enable booting a Linux kernel. Users are free to use the CPU RTL in their own Verilog system designs. The top-level module for the CPU RTL is Verilog_RTL/mkBRVF_Core.v. The top-level module for the surrounding SoC is Verilog_RTL/mkTop_HW_Side.v. The SoC has an AXI4-Lite fabric, a timer, a software-interrupt device, and a UART. Additional library RTL can be found in the directory src_bsc_lib_RTL.

Bluespec BSV source code (which was used to generate the Verilog RTL) can be found in:

  • src_Core/, for the CPU core, with sub-directories:

    • ISA/: generic types/constants/functions for the RISC-V ISA (not CPU-implementation-specific)
    • RegFiles/: generic register files for the GPRs (General-Purpose Registers) and CSRs (Control and Status Registers)
    • Core/: the CPU Core
    • Near_Mem_VM/: for the MMU and first-level cache. In the CPU, this is instantiated twice to provide completely separate channels (MMU and Cache) for instructions and data.
    • BSV_Additional_Libs/: generic utilities (not CPU-specific)
    • Debug_Module/: RISC-V Debug Module to debug the CPU from GDB or other debuggers
  • src_Testbench/, for the surrounding testbench, with sub-directories:

    • Top/: The system top-level (Top_HW_Side.bsv), a memory model that loads from a memory hex file, and some imported C functions for polled reads from the console tty (not currently available for Icarus Verilog).

    • SoC/: An interconnect, a boot ROM, a memory controller, a timer and software-interrupt device, and a UART for console tty I/O.

    • Fabrics/: Generic AXI4-Lite code for the SoC fabric.

The BSV source code has a rich set of parameters, mentioned above. The provided RTL source has been generated from the BSV source automatically using Bluespec's bsc compiler, with certain particular sets of choices for the various parameters. The generated RTL is not parameterized.

To generate Verilog variants with other parameter choices, the user will need Bluespec's bsc compiler. See the next section for examples of how the build is configured for different ISA features.

In fact the CPU also supports a "Tandem Verifier" that produces an instruction-by-instruction trace that can be checked for correctness against a RISC-V Golden Reference Model. Please contact Bluespec, Inc. for more information.

Building and running from the Verilog sources, out of the box

In any of the Verilog-build directories:

  • $ make simulator will create a Verilog simulation executable using Verilator or iverilog, respectively

  • $ make test will run the executable on the standard RISC-V ISA test rv32ui-p-add or rv64ui-p-add, which is one of the tests in the Tests/isa/ directory. Examining the test: target in Makefile, we see that it first runs the program Tests/elf_to_hex/elf_to_hex on the rv32ui-p-add or rv64ui-p-add ELF file to create a Mem.hex file, and then runs the simulation executable which loads this Mem.hex file into its memory.

  • Following the pattern of $ make test, the user can run any of the other tests in the Tests/isa/ directory by pointing at the chosen ELF file.

  • $ make isa_tests will run the executable on all the standard RISC-V ISA tests relevant for ARCH (regression testing). This uses the Python script Tests/Run_regression.py. Please see the documentation at the top of that program for details.

Note: an RV32ACIMU simulator will only successfully run ELF files compiled for RV32ACIMU, privilege U and M; running it on any other ELF file will result in illegal instruction traps. An RV64ACDFIMSU simulator will successfully run ELF files compiled for RV64ACDFIMSU, privilege U, S and M.

Tool dependencies:

We test our builds with the following versions of iVerilog and Verilator. Later versions are probably ok; we have observed some problems with earlier versions of both tools.

    $ iverilog -v
    Icarus Verilog version 10.1 (stable) ()

    $ verilator --version
    Verilator 3.922 2018-03-17 rev verilator_3_920-32-gdf3d1a4

What you can build and run if you have Bluespec's bsc compiler

[Note: Bluespec, Inc. provides free licenses to academia and for non-profit research].

Note: even without Bluespec's bsc compiler, you can use the Verilog sources in any of the builds/<ARCH>_<CPU>_verilator/Verilog_RTL directories-- build and run Verilog simulations, incorporate the Verilog CPU into your own SoC, etc. This section describes additional things you can do with a bsc compiler.

Building a Bluesim simulator

In any of the following directories:

  • $ make compile simulator

will compile and link a Bluesim executable. Then, you can make test or make isa_tests as described above to run an individual ISA test or run regressions on the full suite of relevant ISA tests.

Re-generating Verilog RTL

You can regenerate the Verilog RTL in any of the build/<ARCH>_<CPU>_verilator/ or build/<ARCH>_<CPU>_iverilog/ directories. Example:

    $ cd  builds/RV32ACIMU_<CPU>_verilator
    $ make compile

Creating a new architecture configuration

In the builds/ directory, you can create a new sub-directory to build a new configuration of interest. For example:

    $ cd  builds
$ Resources/mkBuild_Dir.py  ..  RV32CI  bluesim

will create a new directory: builds\RV32CIU_<CPU>_bluesim populated with a Makefile to compile and link a bluesim simulation for an RV32 CPU with 'I' and 'C' ISA options. You can build and run that simulator as usual:

    $ cd  builds/RV32CIU_<CPU>_bluesim
    $ make compile simulator test isa_tests