Illegal Opcode Traps
Trap illegal instructions to an OS handler which can
- Interpret them in software or
- Print a humane error report.
On bad opcode in RF stage
- Select ILL OP address as next <PC>
- Annul instruction in IF stage
- Substitute BNE(r31, 0, XP) for bad instruction
- Need bypass path to make XP usable by code at ILL OP address.
… ILL OP: ST(r0, savereg, r31)