Index

• Index
• Description
• Area usage and maximal frequency
• Dependencies
• CPU generation
• Regression tests
• Interactive debug of the simulated CPU via GDB OpenOCD and Verilator
• Using Eclipse to run the software and debug it
  • By using Zylin plugin
  • By using FreedomStudio
• Briey SoC
• Murax SoC
• Running Linux
• Build the RISC-V GCC
• CPU parametrization and instantiation example
• Add a custom instruction to the CPU via the plugin system
• Adding a new CSR via the plugin system
• CPU clock and resets
• VexRiscv Architecture
  • Plugins

Description

This repository hosts a RISC-V implementation written in SpinalHDL. Here are some specs :

• RV32I[M][C][A] instruction set (Atomic only inside a single core)
• Pipelined from 2 to 5+ stages ([Fetch*X], Decode, Execute, [Memory], [WriteBack])
• 1.44 DMIPS/Mhz --no-inline when nearly all features are enabled (1.57 DMIPS/Mhz when the divider lookup table is enabled)
• Optimized for FPGA, do not use any vendor specific IP block / primitive
• AXI4, Avalon, wishbone ready
• Optional MUL/DIV extensions
• Optional instruction and data caches
• Optional hardware refilled MMU
• Optional debug extension allowing Eclipse debugging via a GDB >> openOCD >> JTAG connection
• Optional interrupts and exception handling with Machine, [Supervisor] and [User] modes as defined in the RISC-V Privileged ISA Specification v1.10.
• Two implementations of shift instructions: Single cycle and shiftNumber cycles
• Each stage can have optional bypass or interlock hazard logic
• Linux compatible
• Zephyr compatible
• FreeRTOS port

The hardware description of this CPU is done by using a very software oriented approach (without any overhead in the generated hardware). Here is a list of software concepts used:

• There are very few fixed things. Nearly everything is plugin based. The PC manager is a plugin, the register file is a plugin, the hazard controller is a plugin, ...
• There is an automatic a tool which allows plugins to insert data in the pipeline at a given stage, and allows other plugins to read it in another stage through automatic pipelining.
• There is a service system which provides a very dynamic framework. For instance, a plugin could provide an exception service which can then be used by other plugins to emit exceptions from the pipeline.

There is a gitter channel for all questions about VexRiscv :

For commercial support, please contact [email protected].

Area usage and maximal frequency

The following numbers were obtained by synthesizing the CPU as toplevel without any specific synthesis options to save area or to get better maximal frequency (neutral).
The clock constraint is set to an unattainable value, which tends to increase the design area.
The dhrystone benchmark was compiled with the -O3 -fno-inline option.
All the cached configurations have some cache trashing during the dhrystone benchmark except the VexRiscv full max perf one. This of course reduces the performance. It is possible to produce dhrystone binaries which fit inside a 4KB I$ and 4KB D$ (I already had this case once) but currently it isn't the case.
The CPU configurations used below can be found in the src/scala/vexriscv/demo directory.

VexRiscv smallest (RV32I, 0.52 DMIPS/Mhz, no datapath bypass, no interrupt) ->
  Artix 7    -> 366 Mhz 488 LUT 505 FF
  Cyclone V  -> 181 Mhz 350 ALMs
  Cyclone IV -> 177 Mhz 732 LUT 494 FF
  iCE40       -> 85 Mhz 1131 LC

VexRiscv smallest (RV32I, 0.52 DMIPS/Mhz, no datapath bypass) ->
  Artix 7    -> 317 Mhz 539 LUT 559 FF
  Cyclone V  -> 191 Mhz 393 ALMs
  Cyclone IV -> 171 Mhz 826 LUT 547 FF
  iCE40      -> 72 Mhz 1284 LC

VexRiscv small and productive (RV32I, 0.82 DMIPS/Mhz)  ->
  Artix 7    -> 338 Mhz 697 LUT 527 FF
  Cyclone V  -> 149 Mhz 495 ALMs
  Cyclone IV -> 137 Mhz 1,103 LUT 522 FF
  iCE40      -> 65 Mhz 1593 LC

VexRiscv small and productive with I$ (RV32I, 0.70 DMIPS/Mhz, 4KB-I$)  ->
  Artix 7    -> 314 Mhz 721 LUT 562 FF
  Cyclone V  -> 152 Mhz 504 ALMs
  Cyclone IV -> 142 Mhz 1,146 LUT 528 FF
  iCE40      -> 69 Mhz 1661 LC

VexRiscv full no cache (RV32IM, 1.21 DMIPS/Mhz 2.30 Coremark/Mhz, single cycle barrel shifter, debug module, catch exceptions, static branch) ->
  Artix 7    -> 325 Mhz 1448 LUT 976 FF
  Cyclone V  -> 141 Mhz 957 ALMs
  Cyclone IV -> 139 Mhz 2,001 LUT 966 FF

VexRiscv full (RV32IM, 1.21 DMIPS/Mhz 2.30 Coremark/Mhz with cache trashing, 4KB-I$,4KB-D$, single cycle barrel shifter, debug module, catch exceptions, static branch) ->
  Artix 7    -> 241 Mhz 1692 LUT 1202 FF
  Cyclone V  -> 132 Mhz 1,127 ALMs
  Cyclone IV -> 124 Mhz 2,296 LUT 1,115 FF

VexRiscv full max dmips/mhz -> (RV32IM, 1.44 DMIPS/Mhz 2.70 Coremark/Mhz,, 16KB-I$,16KB-D$, single cycle barrel shifter, debug module, catch exceptions, dynamic branch prediction in the fetch stage, branch and shift operations done in the Execute stage) ->
  Artix 7    -> 195 Mhz 1824 LUT 1110 FF
  Cyclone V  -> 83 Mhz 1,067 ALMs
  Cyclone IV -> 78 Mhz 2,335 LUT 1,045 FF

VexRiscv full with MMU (RV32IM, 1.24 DMIPS/Mhz 2.35 Coremark/Mhz, with cache trashing, 4KB-I$, 4KB-D$, single cycle barrel shifter, debug module, catch exceptions, dynamic branch, MMU) ->
  Artix 7    -> 218 Mhz 1966 LUT 1551 FF
  Cyclone V  -> 123 Mhz 1,298 ALMs
  Cyclone IV -> 109 Mhz 2,703 LUT 1,498 FF

VexRiscv linux balanced (RV32IMA, 1.21 DMIPS/Mhz 2.27 Coremark/Mhz, with cache trashing, 4KB-I$, 4KB-D$, single cycle barrel shifter, catch exceptions, static branch, MMU, Supervisor, Compatible with mainstream linux) ->
  Artix 7    -> 239 Mhz 2483 LUT 2134 FF
  Cyclone V  -> 130 Mhz 1,636 ALMs
  Cyclone IV -> 116 Mhz 3,324 LUT 2,010 FF

The following configuration results in 1.44 DMIPS/MHz:

• 5 stage : F -> D -> E -> M -> WB
• single cycle ADD/SUB/Bitwise/Shift ALU
• branch/jump done in the E stage
• memory load values are bypassed in the WB stage (late result)
• 33 cycle division with bypassing in the M stage (late result)
• single cycle multiplication with bypassing in the WB stage (late result)
• dynamic branch prediction done in the F stage with a direct mapped target buffer cache (no penalties on correct predictions)

Note that recently, the capability to remove the Fetch/Memory/WriteBack stage was added to reduce the area of the CPU, which end up with a smaller CPU and a better DMIPS/Mhz for the small configurations.

Dependencies

On Ubuntu 14 :

# JAVA JDK 8
sudo add-apt-repository -y ppa:openjdk-r/ppa
sudo apt-get update
sudo apt-get install openjdk-8-jdk -y
sudo update-alternatives --config java
sudo update-alternatives --config javac

# Install SBT - https://www.scala-sbt.org/
echo "deb https://dl.bintray.com/sbt/debian /" | sudo tee -a /etc/apt/sources.list.d/sbt.list
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 2EE0EA64E40A89B84B2DF73499E82A75642AC823
sudo apt-get update
sudo apt-get install sbt

# Verilator (for sim only, realy need 3.9+, in general apt-get will give you 3.8)
sudo apt-get install git make autoconf g++ flex bison
git clone http://git.veripool.org/git/verilator   # Only first time
unsetenv VERILATOR_ROOT  # For csh; ignore error if on bash
unset VERILATOR_ROOT  # For bash
cd verilator
git pull        # Make sure we're up-to-date
git checkout verilator_3_918
autoconf        # Create ./configure script
./configure
make
sudo make install

CPU generation

You can find two example CPU instances in:

• src/main/scala/vexriscv/demo/GenFull.scala
• src/main/scala/vexriscv/demo/GenSmallest.scala

To generate the corresponding RTL as a VexRiscv.v file, run the following commands in the root directory of this repository:

sbt "runMain vexriscv.demo.GenFull"

or

sbt "runMain vexriscv.demo.GenSmallest"

NOTES:

• It could take time the first time you run it.
• The VexRiscv project may need an unreleased master-head of the SpinalHDL repo. If it fails to compile, just get the SpinalHDL repository and do a "sbt clean compile publish-local" in it as described in the dependencies chapter.

Regression tests

To run tests (need the verilator simulator), go in the src/test/cpp/regression folder and run :

# To test the GenFull CPU
# (Don't worry about the CSR test not passing, basicaly the GenFull isn't the truly full version of the CPU, some CSR features are disable in it)
make clean run

# To test the GenSmallest CPU
make clean run IBUS=SIMPLE DBUS=SIMPLE CSR=no MMU=no DEBUG_PLUGIN=no MUL=no DIV=no

The self-test includes:

• ISA tests from https://github.com/riscv/riscv-tests/tree/master/isa
• Dhrystone benchmark
• 24 FreeRTOS tests
• Some handwritten tests to check the CSR, debug module and MMU plugins

You can enable FreeRTOS tests by adding FREERTOS=yes to the command line, but it will take time to run. Also, it uses THREAD_COUNT host CPU threads to run multiple regression in parallel.

Interactive debug of the simulated CPU via GDB OpenOCD and Verilator

It's as described to run tests, but you just have to add DEBUG_PLUGIN_EXTERNAL=yes in the make arguments. Work for the GenFull, but not for the GenSmallest as this configuration has no debug module.

Then you can use the https://github.com/SpinalHDL/openocd_riscv tool to create a GDB server connected to the target (the simulated CPU)

#in the VexRiscv repository, to run the simulation on which one OpenOCD can connect itself =>
sbt "runMain vexriscv.demo.GenFull"
cd src/test/cpp/regression
make run DEBUG_PLUGIN_EXTERNAL=yes

#In the openocd git, after building it =>
src/openocd -c "set VEXRISCV_YAML PATH_TO_THE_GENERATED_CPU0_YAML_FILE" -f tcl/target/vexriscv_sim.cfg

#Run a GDB session with an elf RISCV executable (GenFull CPU)
YourRiscvToolsPath/bin/riscv32-unknown-elf-gdb VexRiscvRepo/src/test/resources/elf/uart.elf
target remote localhost:3333
monitor reset halt
load
continue

# Now it should print messages in the Verilator simulation of the CPU

Using Eclipse to run the software and debug it

By using gnu-mcu-eclipse

You can download releases of the IDE here : https://github.com/gnu-mcu-eclipse/org.eclipse.epp.packages/releases

In the IDE, you can import a makefile project by :

• file -> import -> C/C++ -> existing Code as Makefile Project
• Select the folder which contain the makefile, select "Cross GCC" (not "RISC-V Cross GCC")

To create a new debug configuration :

• run -> Debug Configurations -> GDB OpenOCD Debugging double click
• Look at https://drive.google.com/open?id=1c46tyEV0xLwOsk76b0y2qqs8CYy7Zq3f for a configuration example

By using Zylin plugin (old)

You can use the Eclipse + Zylin embedded CDT plugin to do it (http://opensource.zylin.com/embeddedcdt.html). Tested with Helios Service Release 2 (http://www.Eclipse.org/downloads/download.php?file=/technology/epp/downloads/release/helios/SR2/Eclipse-cpp-helios-SR2-linux-gtk-x86_64.tar.gz) and the corresponding zylin plugin.

To following commands will download Eclipse and install the plugin.

wget http://www.eclipse.org/downloads/download.php?file=/technology/epp/downloads/release/helios/SR2/eclipse-cpp-helios-SR2-linux-gtk-x86_64.tar.gz
tar -xvzf download.php?file=%2Ftechnology%2Fepp%2Fdownloads%2Frelease%2Fhelios%2FSR2%2Feclipse-cpp-helios-SR2-linux-gtk-x86_64.tar.gz
cd eclipse
./eclipse -application org.eclipse.equinox.p2.director -repository http://opensource.zylin.com/zylincdt -installIU com.zylin.cdt.feature.feature.group/

See https://drive.google.com/drive/folders/1NseNHH05B6lmIXqQFVwK8xRjWE4ydeG-?usp=sharing to import a makefile project and create a debug configuration.

Note that sometime this Eclipse need to be restarted in order to be able to place new breakpoints.

Briey SoC

As a demonstrator, a SoC named Briey is implemented in src/main/scala/vexriscv/demo/Briey.scala. This SoC is very similar to the Pinsec SOC:

To generate the Briey SoC Hardware:

sbt "runMain vexriscv.demo.Briey"

To run the verilator simulation of the Briey SoC which can then be connected to OpenOCD/GDB, first get those dependencies:

sudo apt-get install build-essential xorg-dev libudev-dev libts-dev libgl1-mesa-dev libglu1-mesa-dev libasound2-dev libpulse-dev libopenal-dev libogg-dev libvorbis-dev libaudiofile-dev libpng12-dev libfreetype6-dev libusb-dev libdbus-1-dev zlib1g-dev libdirectfb-dev libsdl2-dev

Then go in src/test/cpp/briey and run the simulation with (UART TX is printed in the terminal, VGA is displayed in a GUI):

make clean run

To connect OpenOCD (https://github.com/SpinalHDL/openocd_riscv) to the simulation :

src/openocd -f tcl/interface/jtag_tcp.cfg -c "set BRIEY_CPU0_YAML /home/spinalvm/Spinal/VexRiscv/cpu0.yaml" -f tcl/target/briey.cfg

You can find multiple software examples and demos here: https://github.com/SpinalHDL/VexRiscvSocSoftware/tree/master/projects/briey

You can find some FPGA projects which instantiate the Briey SoC here (DE1-SoC, DE0-Nano): https://drive.google.com/drive/folders/0B-CqLXDTaMbKZGdJZlZ5THAxRTQ?usp=sharing

Here are some measurements of Briey SoC timings and area :

  Artix 7    -> 232 Mhz 3042 LUT 3281 FF
  Cyclone V  -> 138 Mhz 2,179 ALMs
  Cyclone IV -> 120 Mhz 4,333 LUT 3,167 FF

Murax SoC

Murax is a very light SoC (it fits in an ICE40 FPGA) which can work without any external components:

• VexRiscv RV32I[M]
• JTAG debugger (Eclipse/GDB/openocd ready)
• 8 kB of on-chip ram
• Interrupt support
• APB bus for peripherals
• 32 GPIO pin
• one 16 bits prescaler, two 16 bits timers
• one UART with tx/rx fifo

Depending the CPU configuration, on the ICE40-hx8k FPGA with icestorm for synthesis, the full SoC has the following area/performance :

• RV32I interlocked stages => 51 Mhz, 2387 LC 0.45 DMIPS/Mhz
• RV32I bypassed stages => 45 Mhz, 2718 LC 0.65 DMIPS/Mhz

Its implementation can be found here: src/main/scala/vexriscv/demo/Murax.scala.

To generate the Murax SoC Hardware :

# To generate the SoC without any content in the ram
sbt "runMain vexriscv.demo.Murax"

# To generate the SoC with a demo program already in ram
sbt "runMain vexriscv.demo.MuraxWithRamInit"

The demo program included by default with MuraxWithRamInit will blink the LEDs and echo characters received on the UART back to the user. To see this when running the Verilator sim, type some text and press enter.

Then go in src/test/cpp/murax and run the simulation with :

make clean run

To connect OpenOCD (https://github.com/SpinalHDL/openocd_riscv) to the simulation :

src/openocd -f tcl/interface/jtag_tcp.cfg -c "set MURAX_CPU0_YAML /home/spinalvm/Spinal/VexRiscv/cpu0.yaml" -f tcl/target/murax.cfg

You can find multiple software examples and demos here: https://github.com/SpinalHDL/VexRiscvSocSoftware/tree/master/projects/murax

Here are some timing and area measurements of the Murax SoC:

Murax interlocked stages (0.45 DMIPS/Mhz, 8 bits GPIO) ->
  Artix 7 -   > 301 Mhz 1032 LUT 1199 FF
  Cyclone V  -> 183 Mhz 736 ALMs
  Cyclone IV -> 148 Mhz 1,481 LUT 1,204 FF
  iCE40      ->  69 Mhz 2403 LC (nextpnr)

MuraxFast bypassed stages (0.65 DMIPS/Mhz, 8 bits GPIO) ->
  Artix 7    -> 321 Mhz 1198 LUT 1298 FF
  Cyclone V  -> 165 Mhz 873 ALMs
  Cyclone IV -> 145 Mhz 1,691 LUT 1,239 FF
  iCE40      ->  61 Mhz 2778 LC (nextpnr)

Some scripts to generate the SoC and call the icestorm toolchain can be found here: scripts/Murax/

A toplevel simulation testbench with the same features + a GUI are implemented with SpinalSim. You can find it in src/test/scala/vexriscv/MuraxSim.scala.

To run it :

# This will generate the Murax RTL + run its testbench. You need Verilator 3.9xx installated.
sbt "test:runMain vexriscv.MuraxSim"

Running Linux

A default configuration is located in src/main/scala/vexriscv/demo/Linux.scala

This file also contains

• The commands to compile the buildroot image
• How to run the Verilator simulation in interative mode

There is currently no SoC to run it on hardware, it is WIP. But the CPU simulation can already boot linux and run user space application (even python).

Note that VexRiscv can run Linux on both cache full and cache less design.

Build the RISC-V GCC

A prebuild GCC toolsuite can be found here:

• https://www.sifive.com/products/tools/ => SiFive GNU Embedded Toolchain

The VexRiscvSocSoftware makefiles are expecting to find this prebuild version in /opt/riscv/contentOfThisPreBuild

wget https://static.dev.sifive.com/dev-tools/riscv64-unknown-elf-gcc-20171231-x86_64-linux-centos6.tar.gz
tar -xzvf riscv64-unknown-elf-gcc-20171231-x86_64-linux-centos6.tar.gz
sudo mv riscv64-unknown-elf-gcc-20171231-x86_64-linux-centos6 /opt/riscv64-unknown-elf-gcc-20171231-x86_64-linux-centos6
sudo mv /opt/riscv64-unknown-elf-gcc-20171231-x86_64-linux-centos6 /opt/riscv
echo 'export PATH=/opt/riscv/bin:$PATH' >> ~/.bashrc

If you want to compile the rv32i and rv32im GCC toolchain from source code and install them in /opt/, do the following (will take one hour):

# Be carefull, sometime the git clone has issue to successfully clone riscv-gnu-toolchain.
sudo apt-get install autoconf automake autotools-dev curl libmpc-dev libmpfr-dev libgmp-dev gawk build-essential bison flex texinfo gperf libtool patchutils bc zlib1g-dev -y

git clone --recursive https://github.com/riscv/riscv-gnu-toolchain riscv-gnu-toolchain
cd riscv-gnu-toolchain

echo "Starting RISC-V Toolchain build process"

ARCH=rv32im
rmdir -rf $ARCH
mkdir $ARCH; cd $ARCH
../configure  --prefix=/opt/$ARCH --with-arch=$ARCH --with-abi=ilp32
sudo make -j4
cd ..


ARCH=rv32i
rmdir -rf $ARCH
mkdir $ARCH; cd $ARCH
../configure  --prefix=/opt/$ARCH --with-arch=$ARCH --with-abi=ilp32
sudo make -j4
cd ..

echo -e "\\nRISC-V Toolchain installation completed!"

CPU parametrization and instantiation example

You can find many examples of different configurations in the https://github.com/SpinalHDL/VexRiscv/tree/master/src/main/scala/vexriscv/demo folder.

Here is one such example:

import vexriscv._
import vexriscv.plugin._

//Instanciate one VexRiscv
val cpu = new VexRiscv(
  //Provide a configuration instance
  config = VexRiscvConfig(
    //Provide a list of plugins which will futher add their logic into the CPU
    plugins = List(
      new IBusSimplePlugin(
        resetVector = 0x00000000l,
        cmdForkOnSecondStage = true,
        cmdForkPersistence  = true
      ),
      new DBusSimplePlugin(
        catchAddressMisaligned = false,
        catchAccessFault = false
      ),
      new DecoderSimplePlugin(
        catchIllegalInstruction = false
      ),
      new RegFilePlugin(
        regFileReadyKind = Plugin.SYNC,
        zeroBoot = true
      ),
      new IntAluPlugin,
      new SrcPlugin(
        separatedAddSub = false,
        executeInsertion = false
      ),
      new LightShifterPlugin,
      new HazardSimplePlugin(
        bypassExecute           = false,
        bypassMemory            = false,
        bypassWriteBack         = false,
        bypassWriteBackBuffer   = false
      ),
      new BranchPlugin(
        earlyBranch = false,
        catchAddressMisaligned = false
      ),
      new YamlPlugin("cpu0.yaml")
    )
  )
)

Add a custom instruction to the CPU via the plugin system

Here is an example of a simple plugin which adds a simple SIMD_ADD instruction:

import spinal.core._
import vexriscv.plugin.Plugin
import vexriscv.{Stageable, DecoderService, VexRiscv}

//This plugin example will add a new instruction named SIMD_ADD which do the following :
//
//RD : Regfile Destination, RS : Regfile Source
//RD( 7 downto  0) = RS1( 7 downto  0) + RS2( 7 downto  0)
//RD(16 downto  8) = RS1(16 downto  8) + RS2(16 downto  8)
//RD(23 downto 16) = RS1(23 downto 16) + RS2(23 downto 16)
//RD(31 downto 24) = RS1(31 downto 24) + RS2(31 downto 24)
//
//Instruction encoding :
//0000011----------000-----0110011
//       |RS2||RS1|   |RD |
//
//Note :  RS1, RS2, RD positions follow the RISC-V spec and are common for all instruction of the ISA

class SimdAddPlugin extends Plugin[VexRiscv]{
  //Define the concept of IS_SIMD_ADD signals, which specify if the current instruction is destined for ths plugin
  object IS_SIMD_ADD extends Stageable(Bool)

  //Callback to setup the plugin and ask for different services
  override def setup(pipeline: VexRiscv): Unit = {
    import pipeline.config._

    //Retrieve the DecoderService instance
    val decoderService = pipeline.service(classOf[DecoderService])

    //Specify the IS_SIMD_ADD default value when instruction are decoded
    decoderService.addDefault(IS_SIMD_ADD, False)

    //Specify the instruction decoding which should be applied when the instruction match the 'key' parttern
    decoderService.add(
      //Bit pattern of the new SIMD_ADD instruction
      key = M"0000011----------000-----0110011",

      //Decoding specification when the 'key' pattern is recognized in the instruction
      List(
        IS_SIMD_ADD              -> True,
        REGFILE_WRITE_VALID      -> True, //Enable the register file write
        BYPASSABLE_EXECUTE_STAGE -> True, //Notify the hazard management unit that the instruction result is already accessible in the EXECUTE stage (Bypass ready)
        BYPASSABLE_MEMORY_STAGE  -> True, //Same as above but for the memory stage
        RS1_USE                  -> True, //Notify the hazard management unit that this instruction use the RS1 value
        RS2_USE                  -> True  //Same than above but for RS2.
      )
    )
  }

  override def build(pipeline: VexRiscv): Unit = {
    import pipeline._
    import pipeline.config._

    //Add a new scope on the execute stage (used to give a name to signals)
    execute plug new Area {
      //Define some signals used internally to the plugin
      val rs1 = execute.input(RS1).asUInt
      //32 bits UInt value of the regfile[RS1]
      val rs2 = execute.input(RS2).asUInt
      val rd = UInt(32 bits)

      //Do some computation
      rd(7 downto 0) := rs1(7 downto 0) + rs2(7 downto 0)
      rd(16 downto 8) := rs1(16 downto 8) + rs2(16 downto 8)
      rd(23 downto 16) := rs1(23 downto 16) + rs2(23 downto 16)
      rd(31 downto 24) := rs1(31 downto 24) + rs2(31 downto 24)

      //When the instruction is a SIMD_ADD one, then write the result into the register file data path.
      when(execute.input(IS_SIMD_ADD)) {
        execute.output(REGFILE_WRITE_DATA) := rd.asBits
      }
    }
  }
}

If you want to add this plugin to a given CPU, you just need to add it to its parameterized plugin list.

This example is a very simple one, but each plugin can really have access to the whole CPU:

• Halt a given stage of the CPU
• Unschedule instructions
• Emit an exception
• Introduce new instruction decoding specification
• Ask to jump the PC somewhere
• Read signals published by other plugins
• override published signals values
• Provide an alternative implementation
• ...

As a demonstrator, this SimdAddPlugin was integrated in the src/main/scala/vexriscv/demo/GenCustomSimdAdd.scala CPU configuration and is self-tested by the src/test/cpp/custom/simd_add application by running the following commands :

# Generate the CPU
sbt "runMain vexriscv.demo.GenCustomSimdAdd"

cd src/test/cpp/regression/

# Optionally add TRACE=yes if you want to get the VCD waveform from the simulation.
# Also you have to know that by default, the testbench introduce instruction/data bus stall.
# Note the CUSTOM_SIMD_ADD flag is set to yes.
make clean run IBUS=SIMPLE DBUS=SIMPLE CSR=no MMU=no DEBUG_PLUGIN=no MUL=no DIV=no DHRYSTONE=no REDO=2 CUSTOM_SIMD_ADD=yes

To retrieve the plugin related signals in your waveform viewer, just filter with simd.

Adding a new CSR via the plugin system

Here are two examples about how to add a custom CSR to the CPU via the plugin system: https://github.com/SpinalHDL/VexRiscv/blob/master/src/main/scala/vexriscv/demo/CustomCsrDemoPlugin.scala

The first one (CustomCsrDemoPlugin) adds an instruction counter and a clock cycle counter into the CSR mapping (and also do tricky stuff as a demonstration).

The second one (CustomCsrDemoGpioPlugin) creates a GPIO peripheral directly mapped into the CSR.

CPU clock and resets

Without the debug plugin, the CPU will have a standard clk input and a reset input. But with the debug plugin the situation is the following :

• clk : As before, the clock which drive the whole CPU design, including the debug logic
• reset : Reset all the CPU states excepted the debug logics
• debugReset : Reset the debug logic of the CPU
• debug_resetOut : a CPU output signal which allows the JTAG to reset the CPU + the memory interconnect + the peripherals

So here is the reset interconnect in case you use the debug plugin :

                                VexRiscv
                            +------------------+
                            |                  |
toplevelReset >----+--------> debugReset       |
                   |        |                  |
                   |  +-----< debug_resetOut   |
                   |  |     |                  |
                   +--or>-+-> reset            |
                          | |                  |
                          | +------------------+
                          |
                          +-> Interconnect / Peripherals

VexRiscv Architecture

VexRiscv is implemented via a 5 stage in-order pipeline on which many optional and complementary plugins add functionalities to provide a functional RISC-V CPU. This approach is completely unconventional and only possible through meta hardware description languages (SpinalHDL in the current case) but has proven its advantages via the VexRiscv implementation:

• You can swap/turn on/turn off parts of the CPU directly via the plugin system
• You can add new functionalities/instruction without having to modify any sources code of the CPU
• It allows the CPU configuration to cover a very large spectrum of implementation without cooking spaghetti code
• It allows your code base to truly produce a parametrized CPU design

If you generate the CPU without any plugin, it will only contain the definition of the 5 pipeline stages and their basic arbitration, but nothing else, as everything else, including the program counter is added into the CPU via plugins.

Plugins

This chapter describes plugins currently implemented.

• IBusSimplePlugin
• IBusCachedPlugin
• DecoderSimplePlugin
• RegFilePlugin
• HazardSimplePlugin
• SrcPlugin
• IntAluPlugin
• LightShifterPlugin
• FullBarrelShifterPlugin
• BranchPlugin
• DBusSimplePlugin
• DBusCachedPlugin
• MulPlugin
• DivPlugin
• MulDivIterativePlugin
• CsrPlugin
• StaticMemoryTranslatorPlugin
• MemoryTranslatorPlugin
• DebugPlugin
• YamlPlugin

IBusSimplePlugin

This plugin implement the CPU frontend (instruction fetch) via a very simple and neutral memory interface going outside the CPU.

Parameters	type	description
catchAccessFault	Boolean	When true, an instruction read response with read error asserted results in a CPU exception trap.
resetVector	BigInt	Address of the program counter after the reset.
cmdForkOnSecondStage	Boolean	When false, branches immediately update the program counter. This minimizes branch penalties but might reduce FMax because the instruction bus address signal is a combinatorial path. When true, this combinatorial path is removed and the program counter is updated one cycle after a branch is detected. While FMax may improve, an additional branch penalty will be incurred as well.
cmdForkPersistence	Boolean	When false, requests on the iBus can disappear/change before they are acknowledged. This reduces area but isn't safe/supported by many arbitration/slaves. When true, once initiated, iBus requests will stay until they are acknowledged.
compressedGen	Boolean	Enable RISC-V compressed instruction (RVC) support.
busLatencyMin	Int	Specifies the minimal latency between the iBus.cmd and iBus.rsp. A corresponding number of stages are added to the frontend to keep the IPC to 1.
injectorStage	Boolean	When true, a stage between the frontend and the decode stage of the CPU is added to improve FMax. (busLatencyMin + injectorStage) should be at least two.
prediction	BranchPrediction	Can be set to NONE/STATIC/DYNAMIC/DYNAMIC_TARGET to specify the branch predictor implementation. See below for more details.
historyRamSizeLog2	Int	Specify the number of entries in the direct mapped prediction cache of DYNAMIC/DYNAMIC_TARGET implementation. 2 pow historyRamSizeLog2 entries.

Here is the SimpleBus interface definition:

case class IBusSimpleCmd() extends Bundle{
  val pc = UInt(32 bits)
}

case class IBusSimpleRsp() extends Bundle with IMasterSlave{
  val error = Bool
  val inst  = Bits(32 bits)

  override def asMaster(): Unit = {
    out(error,inst)
  }
}

case class IBusSimpleBus(interfaceKeepData : Boolean) extends Bundle with IMasterSlave{
  var cmd = Stream(IBusSimpleCmd())
  var rsp = Flow(IBusSimpleRsp())

  override def asMaster(): Unit = {
    master(cmd)
    slave(rsp)
  }
}

Important : Checkout the cmdForkPersistence parameter, because if it's not set, it can break the iBus compatibility with your memory system (unless you externaly add some buffers)

Setting cmdForkPersistence and cmdForkOnSecondStage improves iBus cmd timings.

The iBusSimplePlugin includes bridges to convert from the IBusSimpleBus to AXI4, Avalon, and Wishbone interfaces.

This plugin implements a jump interface that allows all other plugins to issue a jump:

trait JumpService{
  def createJumpInterface(stage : Stage) : Flow[UInt]
}

The stage argument specifies the stage from which the jump is asked. This allows the PcManagerSimplePlugin plugin to manage priorities between jump requests from diffent stages.

IBusCachedPlugin

Simple and light multi-way instruction cache.

Parameters	type	description
resetVector	BigInt	Address of the program counter after the reset.
relaxedPcCalculation	Boolean	When false, branches immediately update the program counter. This minimizes branch penalties but might reduce FMax because the instruction bus address signal is a combinatorial path. When true, this combinatorial path is removed and the program counter is updated one cycle after a branch is detected. While FMax may improve, an additional branch penalty will be incurred as well.
prediction	BranchPrediction	Can be set to NONE/STATIC/DYNAMIC/DYNAMIC_TARGET to specify the branch predictor implementation. See below for more details.
historyRamSizeLog2	Int	Specify the number of entries in the direct mapped prediction cache of DYNAMIC/DYNAMIC_TARGET implementation. 2 pow historyRamSizeLog2 entries
compressedGen	Boolean	Enable RISC-V compressed instruction (RVC) support.
config.cacheSize	Int	Total storage capacity of the cache in bytes.
config.bytePerLine	Int	Number of bytes per cache line
config.wayCount	Int	Number of cache ways
config.twoCycleRam	Boolean	Check the tags values in the decode stage instead of the fetch stage to relax timings
config.asyncTagMemory	Boolean	Read the cache tags in an asynchronous manner instead of syncronous one
config.addressWidth	Int	CPU address width. Should be 32
config.cpuDataWidth	Int	CPU data width. Should be 32
config.memDataWidth	Int	Memory data width. Could potentialy be something else than 32, but only 32 is currently tested
config.catchIllegalAccess	Boolean	Catch when a memory access is done on non-valid memory address (MMU)
config.catchAccessFault	Boolean	Catch when the memeory bus is responding with an error
config.catchMemoryTranslationMiss	Boolean	Catch when the MMU miss a TLB

Note: If you enable the twoCycleRam option and if wayCount is bigger than one, then the register file plugin should be configured to read the regFile in an asynchronous manner.

DecoderSimplePlugin

This plugin provides instruction decoding capabilities to others plugins.

For instance, for a given instruction, the pipeline hazard plugin needs to know if it uses the register file source 1/2 in order to stall the pipeline until the hazard is gone. To provide this kind of information, each plugin which implements an instruction documents this kind of information to the DecoderSimplePlugin plugin.

Parameters	type	description
catchIllegalInstruction	Boolean	When true, instructions that don't match a decoding specification will generate a trap exception

Here is a usage example :

    //Specify the instruction decoding which should be applied when the instruction match the 'key' pattern
    decoderService.add(
      //Bit pattern of the new instruction
      key = M"0000011----------000-----0110011",

      //Decoding specification when the 'key' pattern is recognized in the instruction
      List(
        IS_SIMD_ADD              -> True, //Inform the pipeline that the current instruction is a SIMD_ADD instruction
        REGFILE_WRITE_VALID      -> True, //Notify the hazard management unit that this instruction writes to the register file
        BYPASSABLE_EXECUTE_STAGE -> True, //Notify the hazard management unit that the instruction result is already accessible in the EXECUTE stage (Bypass ready)
        BYPASSABLE_MEMORY_STAGE  -> True, //Same as above but for the memory stage
        RS1_USE                  -> True, //Notify the hazard management unit that this instruction uses the RS1 value
        RS2_USE                  -> True  //Same than above but for RS2.
      )
    )
  }

This plugin operates in the Decode stage.

RegFilePlugin

This plugin implements the register file.

Parameters	type	description
regFileReadyKind	RegFileReadKind	Can bet set to ASYNC or SYNC. Specifies the kind of memory read used to implement the register file. ASYNC means zero cycle latency memory read, while SYNC means one cycle latency memory read which can be mapped into standard FPGA memory blocks
zeroBoot	Boolean	Load all registers with zeroes at the beginning of simulations to keep everything deterministic in logs/traces

This register file use a don't care read-during-write policy, so the bypassing/hazard plugin should take care of this.

HazardSimplePlugin

This plugin checks the pipeline instruction dependencies and, if necessary or possible, will stop the instruction in the decoding stage or bypass the instruction results from the later stages to the decode stage.

Since the register file is implemented with a don't care read-during-write policy, this plugin also manages these kind of hazards.

Parameters	type	description
bypassExecute	Boolean	Enable the bypassing of instruction results coming from the Execute stage
bypassMemory	Boolean	Enable the bypassing of instruction results coming from the Memory stage
bypassWriteBack	Boolean	Enable the bypassing of instruction results coming from the WriteBack stage
bypassWriteBackBuffer	Boolean	Enable the bypassing of the previous cycle register file written value

SrcPlugin

This plugin muxes different input values to produce SRC1/SRC2/SRC_ADD/SRC_SUB/SRC_LESS values which are common values used by many plugins in the execute stage (ALU/Branch/Load/Store).

Parameters	type	description
separatedAddSub	RegFileReadKind	By default SRC_ADD/SRC_SUB are generated from a single controllable adder/substractor, but if this is set to true, it use separate adder/substractors
executeInsertion	Boolean	By default SRC1/SRC2 are generated in the Decode stage, but if this parameter is true, it is done in the Execute stage (It will relax the bypassing network)

Except for SRC1/SRC2, this plugin does everything at the begining of Execute stage.

IntAluPlugin

This plugin implements all ADD/SUB/SLT/SLTU/XOR/OR/AND/LUI/AUIPC instructions in the execute stage by using the SrcPlugin outputs. It is a realy simple plugin.

The result is injected into the pipeline directly at the end of the execute stage.

LightShifterPlugin

Implements SLL/SRL/SRA instructions by using an iterative shifter register, while using one cycle per bit shift.

The result is injected into the pipeline directly at the end of the execute stage.

FullBarrelShifterPlugin

Implements SLL/SRL/SRA instructions by using a full barrel shifter, so it execute all shifts in a single cycle.

Parameters	type	description
earlyInjection	Boolean	By default the result of the shift is injected into the pipeline in the Memory stage to relax timings, but if this option is true it will be done in the Execute stage

BranchPlugin

This plugin implement all branch/jump instructions (JAL/JALR/BEQ/BNE/BLT/BGE/BLTU/BGEU) with primitives used by the cpu frontend plugins to implement branch prediction. The prediction implementation is set in the frontend plugins (IBusX)

Parameters	type	description
earlyBranch	Boolean	By default the branch is done in the Memory stage to relax timings, but if this option is set it's done in the Execute stage
catchAddressMisaligned	Boolean	If a jump/branch is done in an unaligned PC address, it will fire an trap exception

Each miss predicted jumps will produce between 2 and 4 cycles penalty depending the earlyBranch and the PcManagerSimplePlugin.relaxedPcCalculation configurations

Prediction NONE

No prediction: each PC change due to a jump/branch will produce a penalty.

Prediction STATIC

In the decode stage, a conditional branch pointing backwards or a JAL is branched speculatively. If the speculation is right, the branch penalty is reduced to a single cycle, otherwise the standard penalty is applied.

Prediction DYNAMIC

Same as the STATIC prediction, except that to do the prediction, it use a direct mapped 2 bit history cache (BHT) which remembers if the branch is more likely to be taken or not.

Prediction DYNAMIC_TARGET

This predictor uses a direct mapped branch target buffer (BTB) in the Fetch stage which store the PC of the instruction, the target PC of the instruction and a 2 bit history to remember if the branch is more likely to be taken or not. This is the most efficient branch predictor actualy implemented on VexRiscv as when the branch prediction is right, it produce no branch penalty. The down side is that this predictor has a long combinatorial path coming from the prediction cache read port to the programm counter by passing through the jump interface.

DBusSimplePlugin

This plugin implements the load and store instructions (LB/LH/LW/LBU/LHU/LWU/SB/SH/SW) via a simple memory bus going out of the CPU.

Parameters	type	description
catchAddressMisaligned	Boolean	If a memory access is done to an unaligned memory address, it will fire a trap exception
catchAccessFault	Boolean	If a memory read returns an error, it will fire a trap exception
earlyInjection	Boolean	By default, the memory read values are injected into the pipeline in the WriteBack stage to relax the timings. If this parameter is true, it's done in the Memory stage

Here is the DBusSimpleBus

case class DBusSimpleCmd() extends Bundle{
  val wr = Bool
  val address = UInt(32 bits)
  val data = Bits(32 bit)
  val size = UInt(2 bit)
}

case class DBusSimpleRsp() extends Bundle with IMasterSlave{
  val ready = Bool
  val error = Bool
  val data = Bits(32 bit)

  override def asMaster(): Unit = {
    out(ready,error,data)
  }
}


case class DBusSimpleBus() extends Bundle with IMasterSlave{
  val cmd = Stream(DBusSimpleCmd())
  val rsp = DBusSimpleRsp()

  override def asMaster(): Unit = {
    master(cmd)
    slave(rsp)
  }
}

Note that bridges are available to convert this interface into AXI4 and Avalon

There is at least one cycle latency between a cmd and the corresponding rsp. The rsp.ready flag should be false after a read cmd until the rsp is present.

DBusCachedPlugin

Multi way cache implementation with writh-through and allocate on read strategy. (Documentation is WIP)

MulPlugin

Implements the multiplication instruction from the RISC-V M extension. Its implementation was done in a FPGA friendly way by using 4 17*17 bit multiplications. The processing is fully pipelined between the Execute/Memory/Writeback stage. The results of the instructions are always inserted in the WriteBack stage.

DivPlugin

Implements the division/modulo instruction from the RISC-V M extension. It is done in a simple iterative way which always takes 34 cycles. The result is inserted into the Memory stage.

This plugin is now based on the MulDivIterativePlugin one.

MulDivIterativePlugin

This plugin implements the multiplication, division and modulo of the RISC-V M extension in an iterative way, which is friendly for small FPGAs that don't have DSP blocks.

This plugin is able to unroll the iterative calculation process to reduce the number of cycles used to execute mul/div instructions.

Parameters	type	description
genMul	Boolean	Enables multiplication support. Can be set to false if you want to use the MulPlugin instead
genDiv	Boolean	Enables division support
mulUnrollFactor	Int	Number of combinatorial stages used to speed up the multiplication, should be > 0
divUnrollFactor	Int	Number of combinatorial stages used to speed up the division, should be > 0

The number of cycles used to execute a multiplication is '32/mulUnrollFactor' The number of cycles used to execute a division is '32/divUnrollFactor + 1'

Both mul/div are processed into the memory stage (late result).

CsrPlugin

Implements most of the Machine mode and a few of the User mode registers as specified in the RISC-V priviledged spec. The access mode of most of the CSR is parameterizable (NONE/READ_ONLY/WRITE_ONLY/READ_WRITE) to reduce the area usage of unneeded features.

(CsrAccess can be NONE/READ_ONLY/WRITE_ONLY/READ_WRITE)

Parameters	type	description
catchIllegalAccess	Boolean
mvendorid	BigInt
marchid	BigInt
mimpid	BigInt
mhartid	BigInt
misaExtensionsInit	Int
misaAccess	CsrAccess
mtvecAccess	CsrAccess
mtvecInit	BigInt
mepcAccess	CsrAccess
mscratchGen	Boolean
mcauseAccess	CsrAccess
mbadaddrAccess	CsrAccess
mcycleAccess	CsrAccess
minstretAccess	CsrAccess
ucycleAccess	CsrAccess
wfiGen	Boolean
ecallGen	Boolean

If an interrupt occurs, before jumping to mtvec, the plugin will stop the Prefetch stage and wait for all the instructions in the later pipeline stages to complete their execution.

If an exception occur, the plugin will kill the corresponding instruction, flush all previous instructions, and wait until the previously killed instructions reach the WriteBack stage before jumping to mtvec.

StaticMemoryTranslatorPlugin

Static memory translator plugin which allows one to specify which range of the memory addresses is IO mapped and shouldn't be cached.

MmuPlugin

Hardware refilled MMU implementation. Allows others plugins such as DBusCachedPlugin/IBusCachedPlugin to instanciate memory address translation ports. Each port has a small dedicated fully associative TLB cache which is refilled automaticaly via a dbus access sharing.

DebugPlugin

This plugin implements enough CPU debug features to allow comfortable GDB/Eclipse debugging. To access those debug features, it provides a simple memory bus interface. The JTAG interface is provided by another bridge, which makes it possible to efficiently connect multiple CPUs to the same JTAG.

Parameters	type	description
debugClockDomain	ClockDomain	As the debug unit is able to reset the CPU itself, it should use another clock domain to avoid killing itself (only the reset wire should differ)

The internals of the debug plugin are done in a manner which reduces the area usage and the FMax impact of this plugin.

Here is the simple bus to access it, the rsp come one cycle after the request :

case class DebugExtensionCmd() extends Bundle{
  val wr = Bool
  val address = UInt(8 bit)
  val data = Bits(32 bit)
}
case class DebugExtensionRsp() extends Bundle{
  val data = Bits(32 bit)
}

case class DebugExtensionBus() extends Bundle with IMasterSlave{
  val cmd = Stream(DebugExtensionCmd())
  val rsp = DebugExtensionRsp()

  override def asMaster(): Unit = {
    master(cmd)
    in(rsp)
  }
}

Here is the register mapping :

Read address 0x00 ->
  bit 0  : resetIt
  bit 1  : haltIt
  bit 2  : isPipBusy
  bit 3  : haltedByBreak
  bit 4  : stepIt
Write address 0x00 ->
  bit 4  : stepIt
  bit 16 : set resetIt
  bit 17 : set haltIt
  bit 24 : clear resetIt
  bit 25 : clear haltIt and haltedByBreak

Read Address 0x04 ->
  bits (31 downto 0) : Last value written into the register file
Write Address 0x04 ->
  bits (31 downto 0) : Instruction that should be pushed into the CPU pipeline for debug purposes

The OpenOCD port is there : https://github.com/SpinalHDL/openocd_riscv

YamlPlugin

This plugin offers a service to others plugins to generate a usefull Yaml file about the CPU configuration. It contains, for instance, the sequence of instruction required to flush the data cache (information used by openocd).