RISC-V bare metal assembly with xmake

Small "Hello World" echo program, written in bare metal (no BIOS) RISC-V machine mode assembly for use with the QEMU virt device. Provided as a reference for getting started with RISC-V assembly.

Features

• UART input and output, works with UTF-8 (backspace and ANSI escapes passthrough)
• Supports both 32 and 64bit via helper macros and conditional compilation, no C preprocessor required
• Clean exit from QEMU with return code indicating success or failure (on checked overflow)
• Instructions for debugging with GDB

Usage

1. Install xmake, gcc-riscv64-unknown-elf toolchain
2. xmake build hello
3. xmake run hello
4. To debug, run xmake run gdb hello in one terminal and xmake run attach hello in another
   • Optionally, install gdb-multiarch to get register ABI names instead of canonical ones
5. Switch between configurations with xmake config --arch=rv32g and --arch=rv64g

References

1. noteed/riscv-hello-asm - toolchain, harts
2. Benjamin-Davies/spark-minimal-uart - UART I/O with status registers
3. rust-embedded/qemu-exit - clean exit from QEMU
4. QEMU GDB usage - attaching debugger