For what it's worth, the RISC-V unprivileged spec says: > The behavior upon decoding a reserved instruction is UNSPECIFIED. > Some platforms may require that

For what it's worth, the RISC-V unprivileged spec says: > The behavior upon decoding a reserved instruction is UNSPECIFIED. > Some platforms may require that opcodes reserved for standard use raise an illegal-instruction exception. Other platforms may permit reserved opcode space be used for non-conforming extensions. The RVA23 says: (https://github.com/riscv/riscv-profiles/blob/main/src/rva23-profile.adoc https://github.com/riscv/riscv-profiles/blob/main/src/rva23-...) > Implementations are strongly recommended to raise illegal-instruction exceptions when attempting to execute unimplemented opcodes or access unimplemented CSRs. and has an optional extension "Ssstrict": > Ssstrict No non-conforming extensions are present. Attempts to execute unimplemented opcodes or access unimplemented CSRs in the standard or reserved encoding spaces raises an illegal instruction exception that results in a contained trap to the supervisor-mode trap handler.