if !isAmount {
			return errors.New("invalid register extension")
		}
		switch ext {
		case "UXTB":
			if a.Type == obj.TYPE_MEM {
				return errors.New("invalid shift for the register offset addressing mode")
			}
			a.Reg = arm64.REG_UXTB + Rnum
		case "UXTH":
			if a.Type == obj.TYPE_MEM {
				return errors.New("invalid shift for the register offset addressing mode")
			}