we are doing this - a lot - i.e. in nearly every single Function Unit:

        # RA (or RC)
        reg3_ok = yield dec2.e.read_reg3.ok
        reg1_ok = yield dec2.e.read_reg1.ok
        assert reg3_ok != reg1_ok
        if reg3_ok:
            data1 = yield dec2.e.read_reg3.data
            res['a'] = sim.gpr(data1).value
        elif reg1_ok:
            data1 = yield dec2.e.read_reg1.data
            res['a'] = sim.gpr(data1).value

if instead the CSV files *specified* that RS was in position (read_reg1)
*already*, this would no longer be necessary.

the same thing applies to ShiftRot - except this time RS and RB should
be actually *swapped over*.

this is quite a big change as it affects every single Function Unit,
unit test, and formal proof.