RISC-V Assembler

A RISC-V 32-bit assembler written from scratch in C. It translates RV32I+M assembly into ELF32 executables that can be run by the companion riscv-emulator.

Designed for Unix systems, or WSL on Windows.

Pipeline

Source is processed in stages, each with explicit ownership of its allocations through an arena allocator:

1. Lexer turns source text into tokens
2. Parser builds instructions, directives, and labels
3. Expander decomposes pseudoinstructions into base instructions
4. First pass builds the symbol table and assigns addresses
5. Second pass resolves symbols and encodes the binary
6. ELF writer emits an ELF32 executable

Supported Features

Instruction sets: RV32I and the M extension

Pseudoinstructions: nop, mv, not, neg, seqz, snez, sltz, sgtz, beqz, bnez, blez, bgez, bltz, bgtz, bgt, ble, bgtu, bleu, j, jal, jr, jalr, ret, call, tail, li, la, lla

Directives: .text, .data, .bss, .rodata, .byte, .half, .word, .string/.asciz, .ascii, .zero/.space, .equ/.set, .align/.balign/.p2align

Dependencies

• cmake 3.21 or newer
• gcc 13 or newer
• clangd (optional, for editor tooling)

Building

cmake -B build/release -DCMAKE_BUILD_TYPE=Release
cmake --build build/release

Other build types: Debug (default, AddressSanitizer), Test (builds the unit tests), and Profile.

Usage

riscv-assembler [-o <output>] <file.s>

The output defaults to a.out. To assemble and run a program:

riscv-assembler -o hello.elf hello.s
riscv-emulator run hello.elf

Running Tests

cmake -B build/test -DCMAKE_BUILD_TYPE=Test
cmake --build build/test
ctest --test-dir build/test --output-on-failure

Known Limitations

• Operands cannot contain operator expressions (for example li x1, (1 << 4) - 1) or symbol+offset
• Numeric local labels (1:, 1f, 1b) are not supported
• fence and fence.i are not supported, even though the emulator supports them