Echelon Neuron - Manual (Page 153)

IF (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 IF directive requires a constant expression as its argument and cannot have
a label. The IF directive must be followed by a matching ENDIF directive.
IF const_expr
If the expression evaluates to a non-zero value, the Neuron assembler processes
the lines following the directive, up to a matching ELSE directive, if any. If the
expression evaluates to zero, 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.
Neuron Assembly Language Reference 143