CS override in protected mode

The current instruction requires a CS override, and you are assembling
instructions for the processor in protected mode. For example,

       P386
       CODESEG
   CVAL DW   ? 
       mov  [CVAL],1      ;generates CS override

The /P command-line option enables this warning. When running in protected
mode, instructions with CS overrides won't work without you taking special
measures.