Rust-GB

Compile Rust code to GBZ80 (Work in Progress)
My first language is not English. It is always welcome to point out the awkward parts of this README.

^ filltest example of GBDK-2020, ported to Rust.

How is this possible?

1. The Rust compiler can generate LLVM-IR for the ATMega328 processor. (which powers Arduino)
2. LLVM-IR can be converted to C code using llvm-cbe.
3. The C code can then be compiled to Z80 Assembly using sdcc.
4. Z80 Assembly can be assembled into GBZ80 object code with sdasgb.
5. The GBZ80 object code can be linked with GBDK libraries and built into a Game Boy ROM using lcc.

I referred to z80-babel for steps 1–3, and used gbdk-2020 for steps 4–5.

Why use Rust instead of C or ASM?

1. Rust provides higher-level and better grammer than C.
2. Rust's memory stability and strict types help you avoid to write incorrect code (even on a small device).
3. Putting everything aside, it's fun!

Goal

My goal is to develop a Game Boy Development Kit that enables the creation of Game Boy games using Rust.

Thanks to GBDK, Z80 Assembly generated from Rust can call GBDK's low-level library functions (such as delay, waitpad, etc.).

My task is to wrap these functions in high-level Rust abstractions.

Dependencies

• rust
• avr-gcc
• avr-libc
• llvm
• llvm-cbe
• sdcc

This project is still a work in progress, and I haven't tested it outside of my development environment.

Dependencies may change as the project evolves.

Build

I do not recommend that you build this project now. because this is WIP and I'm still testing many things.

But if you want to do it, Here is the description below.

1. Install all dependencies in your linux (Use WSL for Windows)
2. Build llvm-cbe in ./ext directory. follow llvm-cbe's instruction to build it.
   the path is ./ext/llvm-project/llvm/build/bin/llvm-cbe
3. Execute cargo build --release in ./ext/rust-deps
4. Now, cargo build-rom will build your GB ROM to ./out/main.gb

# build GB ROM from Rust code
cargo build-rom

# build GB ROM from LLVM-IR
cargo llvm-rom
# ... from C code
cargo c-rom
# ... from ASM code
cargo asm-romm

Can do and Can't do

Can do

• Call inline GB ASM functions in Rust
• Call the GBDK library in Rust
• Use core or alloc libraries of Rust

Can't do

• Some Rust functions use fast calling convention which not supported in LLVM-CBE. (alloc::Vec or assert_eq! etc. I'm finding a solution for it.
• External crates are likely not supported.

Build chain Description

Rust bundling

Rust codes in ./source bundled in one .rs file by rust-bundler-cp

Rust -> LLVM-IR

Bundled Rust code is compiled to target avr-unknown-gnu-atmega328.
This will provide 8-bit compatibility for z80.

LLVM-IR post-processing

Currently, nigtly Rust's LLVM version is 19. and LLVM-CBE's LLVM version is 17.

Keep compatibility through the replacement of new function or polyfill.

LLVM-IR -> C

LLVM-CBE compile LLVM-IR to C code.
Unfortunetly, LLVM-CBE do not support some calling convention generated by Rust.
So, Some features from rust (like alloc::Vec) can not be used.

Also, I'm considering If it can be replaced with llvm-gbz80

C post-processing

The generated C code is for GCC. Therefore, it goes through post-processing before it is entered into SDCC.

It's a simple string replacement right now, but it can be more complicated.

C -> ASM

SDCC compile C code for GBZ80 (sm83)

ASM -> ROM

I used GBDK's build chain for this. GBDK's lcc link ASM with GBDK libraries and build a complete Gameboy ROM file.