Echelon Neuron - Manual (Page 155)

IFNDEF (Conditional Assembly)
The Neuron assembler provides the following directives for conditional control of
the assembly of blocks of source lines: IF, IFDEF, IFNDEF, ELSE, and
ENDIF.
A conditional assembly block begins with the IF, IFDEF, or IFNDEF directive,
and ends with a matching ENDIF directive. A conditional block can contain at
most one matching ELSE directive.
Conditional assembly directives can be nested. Thus, a group of source-code lines
between an IF directive and a matching ELSE or ENDIF can contain additional
IF, IFDEF, or IFNDEF directives, and ELSE directives, as long as there are an
identical number of matching ENDIF directives. The maximum number of
nested directives is five. The total maximum number of conditional IF, IFDEF,
and IFNDEF directives for a single source file is 256.
Source-code lines that the assembler skips because of conditional assembly must
still conform to the Neuron assembler syntax.
Syntax:
The IFNDEF directive requires a symbol as its argument and cannot have a
label. The IFNDEF directive must be followed by a matching ENDIF directive.
IFNDEF symbol
If the symbol is not defined, the Neuron assembler processes the lines following
the directive, up to a matching ELSE directive, if any. If the symbol is defined,
the Neuron assembler skips all of the lines following the directive, up to a
matching ELSE directive, if any, or to the matching ENDIF directive.
You can define symbols anywhere in the assembly source file using the EQU
directive; see EQU (Equate Symbol). You can also provide a symbol to the
assembler using the --define command line on the console; see NAS Command
Switches.
Neuron Assembly Language Reference 145