= Flute, a free and open-source RISC-V CPU Rishiyur S. Nikhil, Bluespec, Inc. (c) 2021 :revnumber: v1.0 :revdate: 2021-05-19 :sectnums: :toc: :toclevels: 3 :toc-title: Contents :description: Highly-parameterized RISC-V CPU, from small RV32 embedded to Linux-capable RV64 :keywords: RISC-V, Bluespec, BSV, Flute
'''
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== CPU microarchitecture and options
Flute is one of a family of free and open-source RISC-V CPUs from
Bluespec, Inc. (https://bluespec.com[]) For a description of the
family, see Section <
Flute's microarchitecture is a 5-stage, in-order pipeline. The source code is highly parameterized to produce hundreds of variants, ranging from a small, bare-metal RV32 CPU to a medium-size, Linux-capable RV64 CPUs with multiple privilege levels.
There is a sketch of the module hierarchy in Doc/Microarchitecture/Microarchitecture.pdf.
=== RISC-V ISA Spec options
=== CPU microarchitecture options
Barrel-shifter (faster, more expensive hardware) or serial shifter (cheaper, slower hardware) for Shift instructions
Multipliers inferred by RTL Syntheis tool (faster, more expensive hardware), or serial multiplier (cheaper, slower hardware) multiplication for ISA M option
=== "Near-Memory options" (Caches and MMUs)
Flute has separate I- and D-caches.
For all of them, an MMU is included if Privilege S is selected.
=== Development options
Optional standard RISC-V Debug Module that can connect to GDB, to control and observe the CPU.
Optional Tandem Verification trace generator that outputs an instruction-by-instruction trace that can be compared with a golden model RISC-V CPU (e.g., RISC-V Formal Specication, simulator like Spike or Bluespec Cissr)
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== The provided system to run out-of-the-box, including CLINT and PLIC
This repository contains a simple system (a small SoC) that instantiates the CPU. The system can be compiled into a Bluesim or Verilog simulation, and can be synthesized for FPGA.
The immediate layer (the "core") that surrounds the CPU includes a
CLINT (we call it "Near Mem IO") which contains the standard RISC-V
MTIME (Real-time timer) and MTIMECMP (Timer Compare) memory-mapped
registers and an MSIP "Software Interrupt" memory-mapped register.
The immediate layer (the "core") that surrounds the CPU also
includes a standard memory-mapped PLIC (Platform-Level Interrupt
Controller).
The SoC surrounding the core is based on an AMBA AXI4 interconnect, to which is connected a boot ROM model, a DRAM memory model and a UART model for serial communication. The interconnect is parameterized for the number of M (Manager) and S (Subordinate) ports, so one can attach more memories, peripherals, or accelerators (needs recompilation).
The system is run in simulation by loading the DRAM memory model with
a data from a standard Verilog "Mem Hex" file (hexadecimal memory
contents), which can be produced from a RISC-V ELF binary file.
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== Directory structure
All the hardware designs in this repository are written in BSV, an
HLHDL (High-Level Hardware Description Language). BSV sources files
have extension .bsv. These are compiled to standard synthesizable
Verilog RTL using the free and open source bsc compiler
(https://github.com/B-Lang-org/bsc[]).
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=== CPU and Core, in src_Core/
The src_Core/ directory contains the sources for the CPU and immediate surrounding core(s).
The CPU itself is in sub-directories CPU, ISA, and RegFiles.
The CLINT (unit with memory-mapped MTIME, MTIMECMP and MSIP registers) are in Near_Mem_IO.
The PLIC (Platform-level Interrupt Controller) is in PLIC.
The alternative "near-memory" subsystems (caches, MMUs) are in
Near_Mem_VM_WT_L1,
Near_Mem_VM_WB_L1, and
Near_Mem_VM_WB_L1_L2.
The optional RISC-V Debug Module is in Debug_Module.
Two alterative "cores" are in Core (used with WT_L1 and WB_L1) and
Core_v2 (used with WB_L1_L2). These directories also contain the
optional Tandem Verification generators. These cores instantiate the
CPU, chosen near-memory subsystem, CLINT, PLIC, optional Debug Module
and optional Tandem Verification generator.
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=== SoC and simulation top-level, in src_Testbench/
The SoC is in src_Testbench/SoC. This includes a boot ROM model, an
AXI4 interconnect fabric, a memory controller for DRAM, and a UART
model for serial communications.
The file SoC/SoC_Map.bsv specifies the system's address map
(addresses for memory, boot ROM, CLINT, PLIC, UART, etc.)
The subdirectory src_Testbench/Fabrics/ contains the code for AXI4 interfaces,
transactors and fabrics.
Everything in the SoC and below is synthesizable, and can be synthesized for FPGA.
The src_Testbench/Top/ subdirectory is the only part that is meant
for simulaton only (not synthesizable). It contains a thin layer
around the SoC to provide a simulation clock and reset, a memory model
for the DRAM, and connections from the UART to the terminal.
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=== ISA tests (in Tests/)
The directory Tests/isa is a copy of the "official" RISC-V ISA
tests (original is at https://github.com/riscv/riscv-tests[]). It
also contains compiled versions of all the tests (each has a RISC-V
ELF file and an "objdump" file that shows its disassemby).
The directory Tests/elf_to_hex contains a small C program to convert
an ELF file to a "Mem Hex" memory hexadecimal contents file.
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=== Provided example builds (including generated Verilogs)
As mentioned earlier, one can generate hundreds of variants of Flute
depending on the choice of configuration parameters. This repository
contains "build" directories for a few particular configurations,
both to provide an out-of-the-box experience and to serve as example
templates which you can modify to create your own variant:
The following are for RV32I + C (Compressed instructions), bare-metal (M and U privilege levels). One builds for the Bluesim simulator, the other for Icarus Verilog (iverilog):
The following are for RV64GC (RV64IMAFDC), privilege levels M, S and U, virtual memory Sv39. One builds for the Bluesim simulator, the other for a Verilator simulator. These have booted FreeRTOS, Linux and FreeBSD, in simulation and on FPGA.
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== Building and running simulations (Bluesim or Verilog simulation)
For some of the actions below, you need to have installed the free and open-source bsc compiler, which you can find at https://github.com/B-Lang-org/bsc[].
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=== Building for Bluesim simulation (bsc needed)
In one of the Bluesim build directories,
e.g,. builds/Flute_RV32CI_MU_WT_L1_bluesim_tohost/ the command
will compile and build a Bluesim simulator.
See section below for how to run the simulation.
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=== Building for Verilator simulation (bsc not needed)
Each of the Verilator build directories,
e.g,. builds/Flute_RV64GC_MSU_WB_L1_L2_verilator_tohost/
contains a Verilog_RTL directory where we have already generated the
Verilog RTL sources for you from the BSV sources.
You do not need the free and open-source bsc compiler to just build the Verilog simulator from the Verilog sources.
The following command will build a Verilator simulation executable.
See section below for how to run the simulation.
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==== Regenerating the Verilog RTL (bsc needed)
will regenerate the Verilog files in the Verilog_RTL directory.
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=== Running a simulation (Bluesim or Verilog simulation)
Once you have built a Bluesim, IVerilog or Verilator simulator as described in the previous sections, you can run it as follows (bsc is not needed for this).
To run a single ISA test:
This runs the default ISA test (Tests/isa/rv32ui-p-add for RV32,
Tests/isa/rv64ui-p-add for RV64), and prints an instruction trace
during execution. (First, it uses an elf_to_hex program, provided
in the Tests/ directory, to convert the relevant ELF file into a
"Mem Hex" memory-contents file, which is loaded into the memory
model at the start of simulation).
You can choose a different ISA test from the Tests/isa/ directory by
specifying it on the command line, like this:
Note: if you specify a test that contains an instruction outside the set of instructions for your build (e.g., an ELF that uses C (compressed) instructions for a build that does not support C) this will result in an illegal instruction trap, as expected.
You can run all relevant ISA tests (i.e., all those tests that are relevant for the build's chosen ISA options) with:
This will spawn multiple parallel processes to run run through all the
relevant tests. The Logs subdirectory contains a log for each ISA
test that was run.
make isa_tests actually invokes the Python program
Tests/Run_regression.py, which you can run directly if you wish.
Running it with --help will describe its command-line arguments,
including the ISA architecture string, using which it selects the
"relevant" ISA tests.
The following will "clean" your build directory. The first command
just deletes intermediate files and directories created during
creation of the simulator. The latter will also deleted the simulator
itself and restore the directory to its pristine state.
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==== Running your own ELF file on Flute
If you look at the actions taken by the Makefile in the above examples, you can see how you can substitute your own ELF file as a program to run.
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== 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:
will create a new directory: Flute_RV32ACIM_MU_WT_L1_bluesim_tohost/
populated with a Makefile to compile and link a bluesim simulation
for an RV32I CPU with M,A, and C ISA options, M and U privilege
levels, L1 I-Cache and D-Cache with write-through policy (no L2
cache), for building a Bluesim simulator, and which observes the
tohost memory location for test completion (which is the standard
method in ISA tests to signal completion).
You can build and run that simulator as usual:
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''' [#CPU_Family] == Other 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 (https://github.com/bluespec/Piccolo[]): 3-stage, in-order pipeline + For low-end applications (Embedded Systems, IoT, microcontrollers, etc.).
Flute (https://github.com/bluespec/Flute[]): 5-stage, in-order pipeline. + For low-end to medium applications that require 32-bit or 64-bit operation, an MMU (Virtual Memory) and more performance.
Toooba (https://github.com/bluespec/Toooba[]): superscalar, deep, out-of-order RV64 pipeline, using MIT's RISCY-OOO core.
All of them are written in entirely in BSV, an HLHDL (High-Level Hardware Description Language).
The three repo structures are nearly identical, and the ways to build and run are nearly identical.