custom instruction not working in 13.1.3

Thanks, probably easier for you to raise the the bug than for us to try to do so.

Adding 'readrb' makes the RTL viewer show some reasonable contents for my instruction.

That 'add immediate' instruction I wrote as a quick example isn't entirely stupid.

Although you'd probably want to add an extra 1 (adding 32 is more useful than adding 0) and maybe sign extend to give -16..+16.