educational microarchitectures for risc-v isa
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About The Sodor Processor Collection

Diagrams: Sodor Github wiki

More documentation: Librecores Sodor wiki

Downstream development: Librecores Sodor

This repo has been put together to demonstrate a number of simple RISC-V integer pipelines written in Chisel:

  • 1-stage (essentially an ISA simulator)
  • 2-stage (demonstrates pipelining in Chisel)
  • 3-stage (uses sequential memory; supports both Harvard and Princeton versions)
  • 5-stage (can toggle between fully bypassed or fully interlocked)
  • "bus"-based micro-coded implementation

All of the cores implement the RISC-V 32b integer base user-level ISA (RV32I) version 2.0. None of the cores support virtual memory, and thus only implement the Machine-level (M-mode) of the Privileged ISA v1.10 .

All processors talk to a simple scratchpad memory (asynchronous, single-cycle), with no backing outer memory (the 3-stage is the exception - its scratchpad is synchronous). Programs are loaded in via a Debug Transport Module (DTM) described in Debug Spec v0.13 port (while the core is kept in reset), effectively making the scratchpads 3-port memories (instruction, data, debug).

This repository is set up to use the Verilog file generated by Chisel3 which is fed to Verilator along with a test harness in C++ to generate and run the Sodor emulators.

This repo works great as an undergraduate lab (and has been used by Berkeley's CS152 class for 3 semesters and counting). See doc/ for an example, as well as for some processor diagrams. Be careful though - admittedly some of those documents may become dated as things like the Privileged ISA evolve.

Getting the repo

git clone
cd riscv-sodor
git submodule update --init --recursive

Building the processor emulators

Because this repository is designed to be used as RISC-V processor examples written in Chisel3 (and a regressive testsuite for Chisel updates), no external RISC-V tools are used (with the exception of the RISC-V front-end server and optionally, the spike-dasm binary to provide a disassembly of instructions in the generated *.out files). The assumption is that riscv-gnu-toolchain is not available on the local system. Thus, RISC-V unit tests and benchmarks were compiled and committed to the sodor repository in the ./install directory (as are the .dump files).

Install verilator using any of the following possible ways For Ubuntu 17.04

sudo apt install pkg-config verilator
#optionally gtkwave to view waveform dumps

For Ubuntu 16.10 and lower

sudo apt install pkg-config
sudo dpkg -i verilator_3.900-1_amd64.deb

If you don't have enough permissions to use apt on your machine

# make autoconf g++ flex bison should be available
tar -xzf verilator-3.906.tgz
cd verilator-3.906

Install the RISC-V front-end server to talk between the host and RISC-V target processors.

cd riscv-fesvr
mkdir build; cd build
../configure --prefix=/usr/local
make install 

Build the sodor emulators

./configure --with-riscv=/usr/local
# To run the all the stages with the given tests available in ./install
make run-emulator
# To install the executables on the local system
make install
# Clean all generated files
make clean

(Although you can set the prefix to any directory of your choice, they must be the same directory for both riscv-fesvr and riscv-sodor).

(Alternative) Build together with Chisel sources

This repository packages SBT (Scala Built Tool) for convenience. By default SBT will fetch the Chisel package specified in project/build.scala.

If you are a developer of Chisel and are using sodor cores to test your changes to the Chisel repository, it is convenient to rebuild the Chisel package before building the sodor cores. To do that, fetch the Chisel repo from github and pass the path to the local Chisel source directory to the configure script.

$ git clone
$ cd riscv-sodor
$ ./configure --with-riscv=/usr/local --with-chisel=../chisel
$ make

Creating a source release package

$ make dist-src

Running the RISC-V tests

$ make run-emulator

Gathering the results

(all)   $ make reports
(cpi)   $ make reports-cpi
(bp)    $ make reports-bp
(stats) $ make reports-stats

(Optional) Running debug version to produce signal traces

make run-emulator-debug

When run in debug mode, all processors will generate .vcd information (viewable by your favorite waveform viewer). All processors can also spit out cycle-by-cycle log information. Although already done for you by the build system, to generate .vcd files, see emulator/common/Makefile.include to add the "-v${vcdfilename}" flag to the emulator-debug binary.

RISC-V fesvr allows you to use elf as input to sodor cores so no need to generate the hex files

Have fun!

The riscv-test Collection

Sodor includes a submodule link to the "riscv-tests" repository. To help Sodor users, the tests and benchmarks have been pre-compiled and placed in the ./install directory.

Building a RV32I Toolchain

If you would like to compile your own tests, you will need to build an RV32I compiler. Set $RISCV to where you would like to install RISC-V related tools, and make sure that $RISCV/bin is in your path.

git clone
cd riscv-gnu-toolchain
mkdir build; cd build
../configure --prefix=$RISCV --with-arch=rv32i
make install

This will install a compiler named riscv32-unknown-elf-gcc, complete with newlib libraries that will only emit integer instructions. More advanced users will want to consult the riscv-gnu-toolchain README regarding multilib support for different base ISAs.

Compiling the tests yourself

    cd riscv-tests/isa

This will compile ALL RISC-V assembly tests (32b and 64b). Sodor only supports the rv32ui-p (user-level) and rv32mi-p (machine-level) physical memory tests.

    cd riscv-tests/benchmarks

You will need to modify the Makefile in riscv-tests/benchmarks to compile RV32I binaries. By default, it will compile RV64G. If you compiled a pure RV32I compiler, then you may only need to change the name of the compiler used (riscv32-unknown-elf-gcc). If your toolchain supports multiple ISAs, then you may need to specify "-m32 --with-arch=RV32I" for the compiler and linker flags as appropriate.

Running tests on the ISA simulator

If you would like to run tests yourself, you can use the Spike ISA simulator (found in riscv-tools on the webpage). By default, Spike executes in RV64G mode. To execute RV32I binaries, for example:

cd ./install
spike --ISA=RV32I rv32ui-p-simple
spike --ISA=RV32I dhrystone.riscv

The generated assembly code looks too complex!

For Sodor, the assembly tests rely on macros that can be found in the riscv-tests/env/p directory. You can simplify these macros as desired.


What is the goal of these cores?

First and foremost, to provide a set of easy to understand cores that users can easily modify and play with. Sodor is useful both as a quick introduction to the RISC-V ISA and to the hardware construction language Chisel3.

Are there any diagrams of these cores?

Diagrams of some of the processors can be found either in the Sodor Github wiki, in doc/, or in doc/lab1.pdf. A more comprehensive write-up on the micro-code implementation can be found at the CS152 website.

How do I generate Verilog code for use on a FPGA?

Chisel3 outputs verilog by default which can be generated by

cd emulator/rv32_1stage
make generated-src/Top.v

I want to help! Where do I go?

You can participate in the Sodor conversation on gitter. Downstream development is also taking place at Librecores. Major milestones will be pulled back here. Check it out! We also accept pull requests here!


Here is an informal list of things that would be nice to get done. Feel free to contribute!

  • Reduce the port count on the scratchpad memory by having the HTIF port share one of the cpu ports.
  • Provide a Verilog test harness, and put the 3-stage on a FPGA.
  • Add support for the ma_addr, ma_fetch ISA tests. This requires detecting misaligned address exceptions.
  • Greatly cleanup the common/csr.scala file, to make it clearer and more understandable.
  • Refactor the stall, kill, fencei, and exception logic of the 5-stage to be more understandable.
  • Update the u-code to properly handle illegal instructions (rv32mi-p-illegal) and to properly handle exceptions generated by the CSR file (rv32mi-p-csr).