def effective_address(self,
                          mode: str,
                          length: int,
                          *args):
        if mode in {"Rm"  , "Rn"  }:
            # the effective address is the register
            assert False, mode
        if mode in {"@Rm" , "@Rn" }:
            n_m, = args
            return self.reg[n_m]
        if mode in {"@Rm+", "@Rn+"}:
            n_m, = args
            value = self.reg[n_m]
            self.reg[n_m] += length
            return value
        if mode in {"@-Rm-", "@-Rn"}:
            n_m, = args
            self.reg[n_m] -= length
            return self.reg[n_m]
        if mode in {"@(disp,Rn)", "@(disp,Rm)"}:
            disp, n_m = args
            return self.reg[n_m] + (disp * length)
        if mode in {"@(R0,Rn)"  , "@(R0,Rm)"  }:
            n_m, = args
            return self.reg[n_m] + self.reg[0]
        if mode in {"@(disp,GBR)"}:
            disp, = args
            assert length in {1, 2, 4}, length
            return self.gbr + (disp * length)
        if mode in {"@(R0,GBR)"}:
            assert args == [], args
            return self.regs[0] + self.gbr
        if mode in {"@(disp,PC)"}:
            disp, = args
            assert length in {2, 4}, length
            mask = 0xffffffff & (~((2 ** length) - 1))
            return (self.pc & mask) + (disp * length)
        if mode in {"label"}:
            disp, = args
            assert length == 2, length
            return self.pc + (disp * length)
        if mode in {"#imm"}:
            # immediate should be calculated in another function
            assert False, mode
        # otherwise, assert False
        assert False, mode