IFNDEF           Assemble If Name Is Not Defined               MASM Directive

IFNDEF 
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.

See also: IFDEF

IFNDEF Assemble If Name Is Not Defined