/p        Checks for impure protected mode code

          The /p option specifies that you want to be warned about any
          instructions that generate "impure" code in protected mode.
          Instructions that move data into memory by using a CS: override
          in protected mode are considered impure because they might not
          work correctly unless you take special measures.

          You only need to use this option if you are writing a program
          that runs on the 80286 or 80386+ in protected mode.