A FPGA friendly 32 bit RISC-V CPU implementation
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README.md

Index

Description

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

  • RV32I[M][C] instruction set
  • Pipelined with 5 stages (Fetch, Decode, Execute, Memory, WriteBack)
  • 1.44 DMIPS/Mhz when all features are enabled
  • Optimized for FPGA, fully portable
  • AXI4 and Avalon ready
  • Optional MUL/DIV extensions
  • Optional instruction and data caches
  • Optional MMU
  • Optional debug extension allowing Eclipse debugging via a GDB >> openOCD >> JTAG connection
  • Optional interrupts and exception handling with Machine and User modes as defined in the RISC-V Privileged ISA Specification v1.9.
  • Two implementations of shift instructions: Single cycle and shiftNumber cycles
  • Each stage can have optional bypass or interlock hazard logic
  • FreeRTOS port
  • The data cache supports atomic LR/SC
  • Optional RV32 compressed instruction support in the reworkFetch branch for configurations without instruction cache (will be merge in master, WIP)

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 :
Gitter

For commercial support, please contact spinalhdl@gmail.com.

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    -> 346 Mhz 481 LUT 539 FF
  Cyclone V  -> 201 Mhz 347 ALMs
  Cyclone IV -> 190 Mhz 673 LUT 529 FF
  iCE40      -> 81 Mhz 1130 LC

VexRiscv smallest (RV32I, 0.52 DMIPS/Mhz, no datapath bypass) ->
  Artix 7    -> 340 Mhz 562 LUT 589 FF
  Cyclone V  -> 202 Mhz 387 ALMs
  Cyclone IV -> 180 Mhz 780 LUT 579 FF
  iCE40      -> 71 Mhz 1278 LC

VexRiscv small and productive (RV32I, 0.82 DMIPS/Mhz)  ->
  Artix 7    -> 327 Mhz 698 LUT 558 FF
  Cyclone V  -> 158 Mhz 524 ALMs
  Cyclone IV -> 146 Mhz 1,061 LUT 552 FF
  iCE40      -> 55 Mhz 1541 LC

VexRiscv small and productive with I$ (RV32I, 0.72 DMIPS/Mhz, 4KB-I$)  ->
  Artix 7    -> 331 Mhz 727 LUT 600 FF
  Cyclone V  -> 152 Mhz 536 ALMs
  Cyclone IV -> 156 Mhz 1,075 LUT 565 FF
  iCE40      -> 54 Mhz 1686 LC

VexRiscv full no cache (RV32IM, 1.22 DMIPS/Mhz, single cycle barrel shifter, debug module, catch exceptions, static branch) ->
  Artix 7    -> 295 Mhz 1399 LUT 971 FF
  Cyclone V  -> 151 Mhz 922 ALMs
  Cyclone IV -> 136 Mhz 1,859 LUT 992 FF

VexRiscv full (RV32IM, 1.21 DMIPS/Mhz with cache trashing, 4KB-I$,4KB-D$, single cycle barrel shifter, debug module, catch exceptions, static branch) ->
  Artix 7    -> 253 Mhz 1840 LUT 1394 FF
  Cyclone V  -> 126 Mhz 1,172 ALMs
  Cyclone IV -> 117 Mhz 2,548 LUT 1,703 FF

VexRiscv full max perf -> (RV32IM, 1.44 DMIPS/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    -> 183 Mhz 1813 LUT 1424 FF
  Cyclone V  -> 93 Mhz 1,253 ALMs
  Cyclone IV -> 84 Mhz 2,642 LUT 1,711 FF

VexRiscv full with MMU (RV32IM, 1.26 DMIPS/Mhz with cache trashing, 4KB-I$, 4KB-D$, single cycle barrel shifter, debug module, catch exceptions, dynamic branch, MMU) ->
  Artix 7    -> 214 Mhz 2070 LUT 1913 FF
  Cyclone V  -> 108 Mhz 1,430 ALMs
  Cyclone IV -> 100 Mhz 2,976 LUT 2,201 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)

Dependencies

On Ubuntu 14 :

# JAVA JDK 8. Do not try with JDK >= 9
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/GenFull.scala
  • src/main/scala/vexriscv/GenSmallest.scala

To generate the corresponding RTL as a VexRiscv.v file, run:

sbt "run-main vexriscv.demo.GenFull"

or

sbt "run-main 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:

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 "run-main 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 Zylin plugin

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.

By using FreedomStudio

You can get FreedomStudio (which is package with Eclipse and some plugins) here: https://www.sifive.com/products/tools/

See https://drive.google.com/drive/folders/1a7FyMOYgFc9UDhfsWUSCjyqDCvOrts2J?usp=sharing to import a makefile project and create a debug configuration.

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:

Alt text

To generate the Briey SoC Hardware:

sbt "run-main 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    -> 239 Mhz 3227 LUT 3410 FF
  Cyclone V  -> 125 Mhz 2,207 ALMs
  Cyclone IV -> 112 Mhz 4,594 LUT 3,620

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 "run-main vexriscv.demo.Murax"

# To generate the SoC with a demo program already in ram
sbt "run-main 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    -> 299 Mhz 984 LUT 1186 FF 
  Cyclone V  -> 175 Mhz 710 ALMs
  Cyclone IV -> 137 Mhz 1,436 LUT 1,193 FF 
  iCE40      -> 48 Mhz 2337 LC (icestorm)
  iCE40Ultra -> 20 Mhz 2337 LC (icestorm)

MuraxFast bypassed stages (0.65 DMIPS/Mhz, 8 bits GPIO) ->
  Artix 7    -> 294 Mhz 1128 LUT 1219 FF 
  Cyclone V  -> 165 Mhz 840 ALMs
  Cyclone IV -> 141 Mhz 1,680 LUT 1,227 FF 
  iCE40      -> 48 Mhz 2702 LC (icestorm)
  iCE40Ultra -> 22 Mhz 2702 LC (icestorm)

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"

Build the RISC-V GCC

A prebuild GCC toolsuite can be found here:

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,
        relaxedPcCalculation = 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 "run-main 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.

PcManagerSimplePlugin

This plugin implements the program counter and a jump service to all plugins.

Parameters type description
resetVector BigInt Address of the program counter after the reset
relaxedPcCalculation Boolean By default jump have an asynchronous immediate effect on the program counter, which allow to reduce the branch penalties by one cycle but could reduce the FMax as it will combinatorialy drive the instruction bus address signal. To avoid this you can set this parameter to true, which will make the jump affecting the programm counter in a sequancial way, which will cut the combinatorial path but add one additional cycle of penalty when a jump occur.

This plugin operates on the prefetch stage.

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 If an the read response specify an read error and this parameter is true, it will generate an CPU exception trap
resetVector BigInt Address of the program counter after the reset
relaxedPcCalculation Boolean By default jump have an asynchronous immediate effect on the program counter, which allow to reduce the branch penalties by one cycle but could reduce the FMax as it will combinatorialy drive the instruction bus address signal. To avoid this you can set this parameter to true, which will make the jump affecting the programm counter in a sequancial way, which will cut the combinatorial path but add one additional cycle of penalty when a jump occur.
relaxedBusCmdValid Boolean Same than relaxedPcCalculation, but for the iBus.cmd.valid pin.
compressedGen Boolean Enable RVC support
busLatencyMin Int Specify the minimal latency between the iBus.cmd and iBus.rsp, which will add the corresponding number of stages into the frontend to keep the IPC to 1.
injectorStage Boolean Add a stage between the frontend and the decode stage of the CPU 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 bellow for more descriptions
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 : There should be at least one cycle latency between que cmd and the rsp. The IBus.cmd can remove request when a CPU jump occure or when the CPU is halted by someting in the pipeline. As many arbitration aren't made for this behaviour, it is important to add a buffer to the iBus.cmd to avoid this. Ex : iBus.cmd.s2mPipe, which add a zero latency buffer and cut the iBus.cmd.ready path. You can also do iBus.cmd.s2mPipe.m2sPipe, which will cut all combinatorial path of the bus but then as a latency of 1 cycle. which mean you should probably set the busLatencyMin to 2.

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

The jump interface implemented by this plugin allow all other plugin to request jumps. The stage argument specify from which stage the jump is asked, which will allow the PcManagerSimplePlugin plugin to manage priorities between jump requests.

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

IBusCachedPlugin

Simple and light multi-way instruction cache.

Parameters type description
cacheSize Int Total storage capacity of the cache
bytePerLine Int Number of bytes per cache line
wayCount Int Number of cache ways
twoCycleRam Boolean Check the tags values in the decode stage instead of the fetch stage to relax timings
asyncTagMemory Boolean Read the cache tags in a asyncronus manner instead of syncronous one
addressWidth Int Address width, should be 32
cpuDataWidth Int Cpu data width, should be 32
memDataWidth Int Memory data width, could potentialy be something else than 32, but only 32 is currently tested
catchIllegalAccess Boolean Catch when a memory access is done on non valid memory address (MMU)
catchAccessFault Boolean Catch when the memeory bus is responding with an error
catchMemoryTranslationMiss Boolean Catch when the MMU miss a TLB
resetVector BigInt Address of the program counter after the reset
relaxedPcCalculation Boolean By default jump have an asynchronous immediate effect on the program counter, which allow to reduce the branch penalties by one cycle but could reduce the FMax as it will combinatorialy drive the instruction bus address signal. To avoid this you can set this parameter to true, which will make the jump affecting the programm counter in a sequancial way, which will cut the combinatorial path but add one additional cycle of penalty when a jump occur.
compressedGen Boolean Enable RVC support
prediction BranchPrediction Can be set to NONE/STATIC/DYNAMIC/DYNAMIC_TARGET to specify the branch predictor implementation, see bellow for more descriptions
historyRamSizeLog2 Int Specify the number of entries in the direct mapped prediction cache of DYNAMIC/DYNAMIC_TARGET implementation. 2 pow historyRamSizeLog2 entries

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 a 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 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 If set to true, instruction which have no 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,
        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.
      )
    )
  }

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 and neutral 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

Single way cache implementation with a victim buffer. (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.

MemoryTranslatorPlugin

Simple software 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 from a larger software filled TLB cache via a query which looks up one entry per cycle.

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).