IFB              Assemble If Argument Is Blank                 MASM Directive

IFB 
statements
[ELSE
statements]
ENDIF

Initiates a conditional block, causing assembly of statements up to
the optional ELSE directive, provided argument is blank.

Notes:     IFB and IFNB are usually used for conditional assembly
of macros based on the presence or absence of parameters
in the macro call.

Note that the angle brackets, <>, are required around
the argument.  The argument can be any name, number, or
expression.

See also: IFNB

IFB Assemble If Argument Is Blank