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Ferris Fencing

An eternal duel between programmable crabs with swords.

Ferris Fencing is a live tournament in which player-programmed bots combat each other on a RISC-V virtual machine.

It is a showcase of CKB-VM, a simple implementation of the RISC-V instruction set, written in the Rust programing language.


Ferris Fencing is in early development. It does not yet run in the cloud, and the final rules are not yet determined.

For now, it is possible to write bots in Rust, compile them to RISC-V, and run them locally with the Ferris Fencing runtime.

Soon, players will be able to upload their bots to the Ferris Fencing server to challenge others' bots.

The Rules

See for the game rules.

Building and running

For simplicity, we suggest building off the code in the brson/ferris-fencing workspace, which contains the runtime, example bots, command line tools, and the web server.

This project uses a nightly toolchain, because bots are running on (virtual) bare-metal RISC-V, and that requires some nightly features. The exact toolchain is listed in the rust-toolchain file in the repo, and will be used automatically.

For building bots this project requires the riscv32-imac-unknown-none-elf compiler target.

The following commands will set you up:

git clone
cd ferris-fencing
rustup target add riscv32imac-unknown-none-elf

The repository is a cargo workspace that contains the following projects, in the src directory, each of which can be build or run with the cargo -p flag:

  • ckb_vm_glue - A basic bot runtime library containing the boilerplate necessary to run main. Think of it as std for the Ferris Fencing platform.
  • ckb_vm_syscall - Assembly-language trampolines for calling RISC-V syscalls.
  • example_bot - A working Ferris Fencing bot.
  • ff_local - The CLI for running a local match between two bots.
  • ff_rt - The Ferris Fencing platform runtime. The game logic.
  • ff_web - The Ferris Fencing web API.
  • ff_web_common - Support library for the website.
  • ff_web_json - Runs a match and emits json.

It also contains two demo_* projects. These were used in the Rust.Tokyo 2019 talk for which Ferris Fencing was made. On their own they are relatively uninteresting.

Note that the projects in this workspace are a mixture of standard desktop projects and RISC-V-specific projects. Because of this, at the moment, running cargo with the --all flag will fail. By default cargo build and cargo run will build ff_local.

Building the example bot

cargo build -p example_bot --target=riscv32imac-unknown-none-elf

This will put a binary in


Running a match

export EXAMPLE_BOT=target/riscv32imac-unknown-none-elf/debug/ff-example-bot
cargo run -p ff_local -- $EXAMPLE_BOT $EXAMPLE_BOT

(The export here is just to make the tabove more readable. You can just type the paths out).

Adding RUST_LOG=debug will log some useful info about what is happening in the VM and the Ferris Fencing runtime.

Building your own bot

Either edit example_bot in place or copy it elsewhere as a template to work off of.

The RISC-V gcc toolchain

Having a RISC-V gcc toolchain may be useful for debugging, assembly, dissassamebly, and writing C. The following commands will build and install them to $HOME/riscv-gcc.

git clone --recursive
cd riscv-gnu-toolchain
mkdir build && cd build
../configure --prefix=$HOME/riscv-gcc --with-arch=rv32imac --with-abi=ilp32
make install

For example, to decompile the example bot:

 ~/riscv-gcc/bin/riscv32-unknown-elf-objdump -d target/riscv32imac-unknown-none-elf/debug/ff-example-bot


Here's a vague description of the MVP:

  • Players can upload bots via the ff_upload command.
  • Player bots consist of an elf exe, a single-grapheme name (emoji encouraged), and a 128-bit random identifier.
  • The website is static but contains a live.html frame that contains all the dynamic logic. This frame can be embedded elsewhere as needed.
  • On load, live.html requests a random match; the server generates it and responds; live.html interprets the results by moving Ferris, energy bars, move indicators, and the scoreboard.
  • After a match is complete, live.html requests another.
  • The number of bots is capped to prevent abuse, and are "garbage collected" in FIFO order.
  • The game rules are improved to be interesting.

There is extensive potential beyond the MVP, but this first.


Contributions for bug fixes, and toward the MVP, welcome.




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