po = 5
            # _ matches fields in table at:
            # https://libre-soc.org/openpower/sv/bitmanip/
            xo = 0b1_0010_110
            if 'w' in opcode:
                xo |= 0b100_000
            if 'i' in opcode:
                xo |= 0b1000_000
            Rc = 1 if '.' in opcode else 0
            rt = int(fields[0])
            ra = int(fields[1])
            rb_imm = int(fields[2])
            instr = po
            instr = (instr << 5) | rt
            instr = (instr << 5) | ra
            if opcode == 'grevi' or opcode == 'grevi.':
                assert 0 <= rb_imm < 64
                instr = (instr << 6) | rb_imm
                instr = (instr << 9) | xo
            else:
                assert 0 <= rb_imm < 32
                instr = (instr << 5) | rb_imm
                instr = (instr << 10) | xo
            instr = (instr << 1) | Rc
            asm = f"{opcode} {rt}, {ra}, {rb_imm}"
            yield f".4byte {hex(instr)} # {asm}"
            return