The given bug report involves an assertion failure during the execution of specific RISC-V instructions within QEMU. The error message points to a problem in `decode_save_opc`, specifically when `ctx->insn_start` is null. This indicates an issue with how QEMU decodes or handles certain opcodes.

Since this relates to instruction decoding and not system calls or general runtime errors, the appropriate category is:

**instruction**

Answer: **instruction**