Flute

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

• Piccolo: 3-stage, in-order pipeline
• Flute: 5-stage, in-order pipeline
• Bassoon: deep, out-of-order pipeline [Coming!]

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.

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 two specific configurations:

1. RV32IMU with 'M' extension (integer multiply/divide)

   • Privilege levels M (machine) and U (user)
   • Supports external, timer and software interrupts
   • Passes all riscv-isa tests for RV32IMU

2. RV64AIMSU with 'A' extension (atomic memory ops) and 'M' extension (integer multiply/divide)

   • Privilege levels M (machine), S (Supervisor) and U (user)
   • Supports external, timer and software interrupts
   • Sv32 and Sv39 Virtual Memory schemes
   • Passes all riscv-isa tests for RV64AIMSU
   • Boots the Linux kernel [ 2018-10-02: almost, but not yet; hopefully in a few days. ]

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 serial shifter (smaller hardware, slower) or barrel shifter (more HW, faster) for shift instructions
• Optional 'M' (integer multiply/divide)
• Optional 'A' (Atomic Memory Ops)
• Privilege M (machine), S (supervisor) and U (user)
• For privilege S, MMUs for Sv32 Virtual Memory for RV32 and Sv39 Virtual Memory for RV64
• 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: UART input is not currently available using iverilog].

This repository contains six sample build directories, to build RV32IMU or RV64AIMSU simulators, using Bluespec Bluesim simulation, a Verilator Verilog simulation, or an Icarus Verilog ("iverilog") simulation.

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

Plans

• We will be adding the RISC-V 'C' option (compressed instructions). [Expected October 2018]
• We will be adding the RISC-V 'F' and 'D' options (single and double precision floating point). [Expected November 2018]
• Continuous micro-architectural improvements for performance and hardware area. [Ongoing]

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Source codes

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

Verilog RTL can be found in the following directories:

    builds/RV32IMU_verilator/Verilog_RTL/
    builds/RV32IMU_iverilog/Verilog_RTL/
    builds/RV64AIMSU_verilator/Verilog_RTL/
    builds/RV64AIMSU_iverilog/Verilog_RTL/

This is synthesizable RTL (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)

• 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 BSC_FLAGS in following Makefiles for examples of how the build is configured for different ISA features:

    builds/Resources/Include_RV32IMU.mk
    builds/Resources/Include_RV64AIMSU.mk

In fact the CPU can also support a standard RISC-V Debug Module, and a "Tandem Verifier" to check it for correctness on an instruction-by-instruction basis. Please contact Bluespec, Inc. if you are interested in such variants.

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

• In any of the following directories:

        builds/RV32IMU_verilator/
        builds/RV64AIMSU_verilator/
        builds/RV32IMU_iverilog/
        builds/RV64AIMSU_iverilog/
  

  • $ make mkSim 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.

Note: an RV32IMU simulator will only successfully run ELF files compiled for RV32IM, privilege U and M; running it on any other ELF file will result in illegal instruction traps. An RV64AIMSU simulator will only successfully run ELF files compiled for RV64AIMSU, privilege U, S and M; running it on any other ELF file will result in illegal instruction traps.

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

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Running regressions of all standard RISC-V ISA tests

In the Tests/ directory there is a Python program using which you can run the simulation executables (Bluesim, verilator or iverilog) on all the relevant standard RISC-V ISA tests (a few hundred tests). Please see Tests/README.txt for more details.

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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].

In either of the following directories:

    builds/RV32IMU_Bluesim
    builds/RV64AIMSU_Bluesim

• $ make compile link

will compile and link a Bluesim executable. Then,

• $ make test

will run it on the rv32ui-p-add or rv64ui-p-add test. This 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.

Note: an RV32IMU simulator will only successfully run ELF files compiled for RV32IM, privilege U and M; running it on any other ELF file will result in illegal instruction traps. An RV64AIMSU simulator will only successfully run ELF files compiled for RV64AIMSU, privilege U, S and M; running it on any other ELF file will result in illegal instruction traps.

You can also regenerate the Verilog RTL in the build/RV32IMU_verilator/, build/RV32IMU_iverilog/ build/RV64AIMSU_verilator/or build/RV64AIMSU_iverilog/ directories. Example:

• $ cd builds/RV32IMU_verilator
• $ make gen_RTL

In each of the builds/RV... directories, you can edit the Makefile to pass different flags and macros to bsc which will generate different variants of the CPU as described above (RV32I/64I, with/without M, with/without A, with/without privilege S and MMUs, etc.)

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