os

Study notes that go with this repo live in notes/:

• notes/RISC-V 101.md — primer on the 32-bit RISC-V ISA: assembly basics, registers, memory access, branches, function calls, and the stack.
• notes/INSTRUCTIONS.md — concise reference of the RISC-V instructions used throughout the project, grouped by purpose (arithmetic, memory, branches, function calls, stack).
• notes/BOOT.md — what happens before the kernel runs: OpenSBI / SBI, example QEMU boot output, monitor shortcuts, and the linker script.

1. lld deepdive
2. qemu deepdive
3. try riscv64 instead of riscv32
4. what are the all instructions in riscv32? (see llvm-objdump -d output)
5. what are all the registers in riscv32? (see llvm-objdump -d output)

• clang - C compiler, needs support for 32-bit RISC-V CPU
• lld - LLVM linker, bundles compiled object files into an executable
• llvm-objcopy - Object file editor, comes with the llvm package
• llvm-objdump - A disassembler, comes with the llvm package
• llvm-readelf - An ELF file reader, comes with the llvm package
• qemu-system-riscv32 - 32-bit RISC-V CPU emulator, it's part of qemu package
• Compiler Explorer - useful tool for learning assembly, as I type C code it shows the corresponding assembly code. By default it uses x86-64 CPU assembly. Specify RISC-V rv32gc clang (trunk) in the right pane to output 32-bit RISC-V assembly.
  • also we can specify options like -O0 (optimization off) or -O2 (optimization on) to see how the assembly code changes.

Apple's bundled clang does not include the riscv32 target,

$ clang --print-targets
  Registered Targets:
    aarch64    - AArch64 (little endian)
    aarch64_32 - AArch64 (little endian ILP32)
    aarch64_be - AArch64 (big endian)
    arm        - ARM
    arm64      - ARM64 (little endian)
    arm64_32   - ARM64 (little endian ILP32)
    armeb      - ARM (big endian)
    thumb      - Thumb
    thumbeb    - Thumb (big endian)
    x86        - 32-bit X86: Pentium-Pro and above
    x86-64     - 64-bit X86: EM64T and AMD64

so install Homebrew's LLVM (which is built with all targets) plus lld and qemu:

brew install llvm lld qemu

Verify the RISC-V target is available:

$(brew --prefix llvm)/bin/clang -print-targets | grep riscv
    riscv32     - 32-bit RISC-V
    riscv32be   - 32-bit big endian RISC-V
    riscv64     - 64-bit RISC-V
    riscv64be   - 64-bit big endian RISC-V

Use the Homebrew toolchain by full path in run.sh (don't shadow Apple's clang globally):

/opt/homebrew/opt/llvm/bin/clang        # Apple Silicon
/usr/local/opt/llvm/bin/clang           # Intel Mac

If a shell session needs llvm-objcopy etc. on PATH:

export PATH="$(brew --prefix llvm)/bin:$PATH"

./run.sh

Even though it does not exist in the real world, it's simple and very similar to real devices. I can emulate on it for free, no need to buy a physical hardware. When I encounter debugging issues, I can read QEMU's source code, or attach a debugger to the QEMU process to investigate what's wrong.

QEMU documentation

QEMU console: Ctrl-A then c; type quit to exit.