C Hardware Access for RISC-V

Example for accessing low level RISC-V hardware with C.

The code will initialize the C run-time environment and jump to main, enable and handle timer interrupts, and initiate and handle ecall exceptions.

More details at either:

• https://five-embeddev.com/articles/2023/03/20/c-hardware-access/
• https://philmulholland.medium.com/

Files

Source Code

• src/startup.c - Entry/Startup/Runtime
• src/main.c - Main Program, Interrupt Handlers, Exception Handler
• src/timer.c / include/timer.h - Timer Driver
• src/vector_table.c / include/vector_table.h - Interrupt Vector Table
• include/riscv-csr.h / include/riscv-abi.h / include/riscv-interrupts.h - RISC-V Hardware Support

Platform IO:

• platformio.ini - IDE Configuration
• post_build.py - Example post build to create disassembly file

CMake Build Environment:

• build_cmake.sh
• cmake/riscv.cmake
• src/CMakeLists.txt

GitHub/Docker CI

• Dockerfile
• docker_entrypoint.sh
• action.yml
• .github/workflows/main.yml

Source Code Check/Formatting

• check.sh - Run clang-tidy and cppcheck
• format - Run clang-format

Building

Platform IO or CMake is used to build the project locally.

Docker is used to build the project on github.

PlatformIO

Platform IO will download the required toolchain to build and QEMU to run the target locally. This is using q quite an old GCC (8.3.0).

platformio run --target build

CMake

To build with CMake a RISC-V cross compiler must be installed. It will look for one of:

• riscv-none-embed-gcc
• riscv-none-elf-gcc
• riscv32-unknown-elf-gcc

The tools can be found at: https://xpack.github.io/dev-tools/riscv-none-elf-gcc/

cmake \
        -G "Unix Makefiles" \
        -DCMAKE_TOOLCHAIN_FILE=../cmake/riscv.cmake \
        -B build \
        -S src 
cmake --build build

Docker

The included dockerfile installs the xpack RISC-V GCC toolchain and uses CMake to compile the project. This is using a recent GCC (12.2).

To build locally using the docker file use these commands:

docker build --tag=c_hw_access_riscv:latest .
docker run \
       -it \
       -v `pwd`:/work \
       c_hw_access_riscv:latest \
       /work \
       ./src