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IFNDEF           Assemble If Name Is Not Defined               MASM Directive

  IFNDEF <name>
  statements
  [ELSE
  statements]
  ENDIF

    Initiates a conditional block, causing assembly of statements up to
    the optional ELSE directive, provided name is a label, variable, or
    symbol that has not yet been defined.

       Notes: The name argument can be any valid name.

                  If the name is defined as a forward reference, IFNDEF
                  will be true on Pass 1 IFNDEF, but false on Pass 2.