The RISC-V Foundation publish the official specification.
- Online viewer: https://riscv.org/specifications/
- GitHub source: https://github.com/riscv/riscv-isa-manual
I find HTML easier as a reference. These are un-official conversions to HTML from the GitHub source:
ABI References:
- Standard ABI: https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md
- Embeddded EABI: https://github.com/riscv/riscv-eabi-spec/blob/master/EABI.adoc
Other Resources:
- A “green card” summary of the ISA:
- A machine readable set of opcodes is available on GitHub: https://github.com/riscv/riscv-opcodes